Solar system a beginners guide1

Solar System

Major features of the Solar System (not to scale): The Sun, the eight planets, the asteroid
belt containing the dwarf planet Ceres, outermost there is the dwarf planet Pluto (the
dwarf planet Eris not shown), and a comet.
The Solar System or solar system comprises the Sun and the retinue of celestial objects
gravitationally bound to it: the eight planets, their 162 known moons, three currently
identified dwarf planets and their four known moons, and thousands of small bodies. This
last category includes asteroids, meteoroids, comets, and interplanetary dust.

The principal component of the Solar System is the Sun or Sol, (astronomical symbol );
a main sequence G2 star that contains 99.86% of the system’s known mass and
dominates it gravitationally. Because of its large mass, the Sun has an interior density
high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of
which is radiated into space in the form of electromagnetic radiation, including visible
light. The Sun’s two largest orbiting bodies, Jupiter and Saturn, account for more than
90% of the system’s remaining mass. (The currently hypothetical Oort cloud, should its
existence be confirmed, would also hold a substantial percentage).
In broad terms, the charted regions of the Solar System consist of the Sun, four rocky
bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas
giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. In order of
their distances from the Sun, the planets are Mercury ( ), Venus ( ), Earth ( ), Mars (

), Jupiter ( ), Saturn ( ), Uranus ( ), and Neptune ( ). All planets but two are in
turn orbited by natural satellites (usually termed “moons” after Earth’s Moon), and every
planet past the asteroid belt is encircled by planetary rings of dust and other particles. The

planets, with the exception of Earth, are named after gods and goddesses from Greco-
Roman mythology.

From 1930 to 2006, Pluto ( ), the largest known Kuiper belt object, was considered the
Solar System’s ninth planet. However, in 2006 the International Astronomical Union
(IAU) created an official definition of the term “planet”. Under this definition, Pluto is
reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition
to Pluto, the IAU currently recognizes two other dwarf planets: Ceres ( ) , the largest
object in the asteroid belt, and Eris, which lies beyond the Kuiper belt in a region called
the scattered disc. Of the known dwarf planets, only Ceres has no moons.

For many years, the Solar System was the only known example of planets in orbit around
a star. The discovery in recent years of many extrasolar planets has led to the term “solar
system” being applied generically to all the newly discovered systems. Technically,
however, it should strictly refer to Earth’s system only, as the word “solar” is derived
from the Sun’s Latin name, Sol. Other such systems are usually referred to by the names
of their parent star: “the Alpha Centauri system” or “the 51 Pegasi system”.

Layout

The ecliptic viewed in sunlight from behind the Moon in this Clementine image. From
left to right: Mercury, Mars, Saturn

Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic,
which is roughly parallel to the Sun’s equator. The planets lie very close to the ecliptic,
while comets and kuiper belt objects often lie at significant angles to it. All of the planets,
and most other objects, also orbit with the Sun’s rotation in a counter-clockwise direction
as viewed from a point above the Sun’s north pole. There is a direct relationship between
how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the
smallest orbital circumference, travels the fastest, while Neptune, being much farther
from the Sun, travels more slowly.

A planet’s distance from the Sun varies in the course of its year. Its closest approach to
the Sun is known as its perihelion, while its farthest point from the Sun is called its
aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to

their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly
elliptical orbits, with large differences between perihelion and aphelion.
Astronomers most often measure distances within the solar system in astronomical units,
or AU. One AU is the average distance between the Earth and the Sun, or roughly 149
598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun, while Jupiter lies at
roughly 5.2 AU.
Informally, the Solar System is sometimes divided into separate “zones”; the first zone,
known as the inner Solar System, comprises the inner planets and the main asteroid
belt. The outer solar system is sometimes defined as everything beyond the asteroids;
however, it is also the name often given to the region beyond Neptune, with the gas
giants as a separate “middle zone.”

The orbits of the bodies in the solar system to scale (clockwise from top left)
One common misconception with regards to the Solar System is that the orbits of the
major objects (planets, Pluto, and asteroids) are equidistant. Because of the vast distances
involved, many representations of the Solar System tend to simplify these orbits, with
equal spacing between each object. However, with certain exceptions, it can generally be
stated that the farther a planet or belt is from the Sun, the greater the distance between it
and the previous orbit. For example, Venus is approximately 0.33 AU farther out than
Mercury, whereas Jupiter lies 1.9 AU from the farthest extent of the asteroid belt, and
Neptune’s orbit is roughly 20 AU farther out than that of Uranus. Attempts have been
made to determine a correlation between these distances (see Bode’s Law) but to date
there is no accepted theory that explains the respective orbital distances.

Planets, dwarf planets, and small solar system bodies

Planets and Dwarf Planets of the solar system. While the size is to scale, the relative
distances from the Sun are not.

In a decision passed by the International Astronomical Union General Assembly on
August 24, 2006, the objects in the Solar System were divided into three separate groups:
planets, dwarf planets and small solar system bodies.

Under this classification, a planet is any body in orbit around the Sun that a) has enough
mass to form itself into a spherical shape and b) has cleared its immediate neighborhood
of all smaller objects. Eight objects in the Solar System currently meet this definition;
they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet is a newly defined classification for astronomical objects. The key
difference between planets and dwarf planets is that while both are required to orbit the
Sun and be of large enough mass that their own gravity pulls them into a nearly round
shape, dwarf planets are not required to clear their neighborhood of other celestial bodies.
Three objects in the solar system are currently included in this category; they are Pluto
(formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The
IAU will begin evaluating other known objects to see if they fit within the definition of
dwarf planets. The most likely candidates are some of the larger asteroids and several
Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar.

The remainder of the objects in the Solar System were classified as small solar system
bodies. A small solar system body (SSSB) is a term defined in 2006 by the International
Astronomical Union to describe Solar System objects which are neither planets nor dwarf
planets.

All other objects ... orbiting the Sun shall be referred to collectively as “Small Solar
System Bodies” .... These currently include most of the Solar System asteroids, most
Trans-Neptunian Objects (TNOs), comets, and other small bodies.

As of 2006, the IAU considers the following bodies to be SSSB’s:

1. all asteroids except Ceres

2. all centaurs
3. all trans-Neptunian objects, including Kuiper belt & Scattered disc objects, with

the exception of Pluto and Eris
4. all comets

Formation

Artist’s conception of a protoplanetary disc

Using radiometric dating, scientists can estimate that the solar system is 4.6 billion years
old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are
rare, as the Earth’s surface is constantly being reshaped by erosion, volcanism and plate
tectonics. To estimate the age of the solar system scientists must use meteorites, which
were formed during the early condensation of the solar nebula. The oldest meteorites
(such as the Canyon Diablo meteorite) are found to have an age of 4.6 billion years,
hence the solar system must be at least 4.6 billion years old.
The current hypothesis of Solar System formation is the nebular hypothesis, first
proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon
Laplace.The nebular theory holds that the Solar System was formed from the
gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of
100 AU and was 2–3 times the mass of the Sun. Over time, a disturbance (possibly a
nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces
overcame the internal gas pressure and it began to collapse. As the nebula collapsed,
conservation of angular momentum meant that it spun faster, and became warmer. As the
competing forces associated with gravity, gas pressure, magnetic fields, and rotation
acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk
with a gradually contracting protostar at the center.

From this cloud and its gas and dust, the various planets formed. The inner solar system
was too warm for volatile molecules like water and methane to condense, and so the
planetesimals which formed there were relatively small (comprising only 0.6% the mass
of the disc) and composed largely of compounds with high melting points, such as
silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther
out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects
present to come together, leaving behind the asteroid belt. Farther out still, beyond the
frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune
captured much less gas and are known as ice giants because their cores are believed to be
made mostly of ice, that is, hydrogen compounds.

The gas giants were massive enough to retain a “primary atmosphere” of hydrogen and
helium captured from the surrounding solar nebula. The terrestrial planets eventually lost
their retained hydrogen and helium, and subsequently generated their own “secondary
atmospheres” via volcanism, comet impacts, and, also in Earth’s case, the evolution of
life.

After 100 million years, the pressure and density of hydrogen in the centre of the
collapsing nebula became great enough for the protosun to begin thermonuclear fusion,
which increased until hydrostatic equilibrium was achieved. The young Sun’s solar wind
then cleared away all the gas and dust in the protoplanetary disk, blowing it into
interstellar space, thus ending the growth of the planets.

Sun

The Sun as seen from Earth.

The Sun is the Solar System’s parent star, and far and away its chief component. It is
classed as a moderately large yellow dwarf. However, this name is misleading, as on the
scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based
on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness
of stars against their surface temperature. Generally speaking, the hotter a star is, the
brighter it is. Stars which follow this pattern are said to be on the main sequence, and the
Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun
as “average;” however, stars brighter and hotter than it are rare, whereas stars dimmer

and cooler than it are common. The vast majority of stars are dim red dwarfs, though they
are under-represented in star catalogues as we can observe only those few that are very
near the Sun in space.

The Sun’s position on the main sequence means, according to current theories of stellar
evolution, that it is in the “prime of life” for a star, in that it has not yet exhausted its store
of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more
inefficient elements such as helium and carbon. The Sun is growing increasingly bright as
it ages. Early in its history, it was roughly 75 percent as bright as it is today. Calculations
of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through
its life cycle, and will eventually begin moving off the main sequence, becoming larger,
brighter and redder, until, about five billion years from now, it too will become a red
giant.

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been
born in the later stages of the universe’s evolution. As such, it contains far more elements
heavier than hydrogen and helium (“metals” in astronomical parlance) than older
population II stars such as those found in globular clusters. Since elements heavier than
hydrogen and helium were formed in the cores of ancient and exploding stars, the first
generation of stars had to die before the universe could be enriched with them. For this
reason, the very oldest stars contain very little “metal”, while stars born later have more.
This high “metallicity” is thought to have been crucial in the Sun’s developing a
planetary system, because planets form from accretion of metals.

The heliospheric current sheet

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind,
ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very
tenuous “atmosphere” (the heliosphere), that permeates the solar system for at least
100 AU. This environment is known as the interplanetary medium. Small quantities of
cosmic dust (some of it arguably interstellar in origin) are also present in the
interplanetary medium and are responsible for the phenomenon of zodiacal light. The
influence of the Sun’s rotating magnetic field on the interplanetary medium creates the
largest structure in the solar system, the heliospheric current sheet.

Earth’s magnetic field protects its atmosphere from interacting with the solar wind.
However, Venus and Mars do not have magnetic fields, and the solar wind causes their
atmospheres to gradually bleed away into space.

Inner planets

The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)

The four inner or terrestrial planets are characterised by their dense, rocky composition,
few or no moons, and lack of ring systems. They are composed largely of minerals with
high melting points such as silicates to form the planets’ solid crusts and semi-liquid
mantles, and metallic dust grains such as iron, which forms their cores. Three of the four
inner planets have atmospheres. All have impact craters, and all but one possess tectonic
surface features, such as rift valleys and volcanoes. The term inner planet should not be
confused with inferior planet, which designates those planets which are closer to the Sun
than the Earth is (i.e. Mercury and Venus).

The four inner planets are:

Mercury

Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the planets,
at only 0.055 Earth masses. Mercury has a very thin atmosphere consisting of atoms
blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly
escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable,
Mercury’s atmosphere is constantly being replenished. Mercury is surrounded by an
extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of
gases is so thin that the greatest possible atmospheric pressure (force exerted by the
weight of gases) on Mercury would be about 0.000000000002 kgf/cm² (0.00000000003
psi or 0.2 µPa). The atmospheric pressure on the Earth is about 1.03 kgf/cm² (14.7 psi or
101 kPa). It has no natural satellite, and, to date, no observed geological activity save that
produced by impacts. Its relatively large iron core and thin mantle have not yet been
adequately explained. Hypotheses include that its outer layers were stripped off by a giant
impact, and that it was prevented from fully accreting by the Sun’s gravity. The
MESSENGER probe should aid in resolving this issue when it arrives in Mercury’s orbit
in 2011.

Venus

Venus (0.7 AU), the first truly terrestrial planet, is of comparable mass to the Earth
(0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron
core, as well as a substantial atmosphere and evidence of one-time internal geological
activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90
times as dense and is composed overwhelmingly (96.5%) of carbon dioxide. Unlike
Earth, evidence suggests that Venus’s crust is not divided into tectonic plates but instead
comprises a single very thick rind. Venus has no natural satellite. It is the hottest planet,
despite being farther from the sun than Mercury, with temperatures reaching more than
400 degrees Celsius. This is most likely because of the amount of greenhouse gases in the
atmosphere.

Earth

The largest and densest of the inner planets, Earth (1 AU) is also the only one to
demonstrate unequivocal evidence of current geological activity. Earth is the only planet
known to have life. Its liquid hydrosphere, unique among the terrestrials, is probably the
reason Earth is also the only planet where multi-plate tectonics has been observed,
because water acts as a lubricant for subduction. Its atmosphere is radically different from
the other terrestrials, having been altered by the presence of life to contain 21 percent free
oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit
with its partner, because its orbit around the Sun never actually loops back on itself when
observed from above. The Moon possesses many features in common with other
terrestrial planets, though it lacks an iron core.

Mars

Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It
possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast
volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically
active and recent evidence suggests this may have been true until very recently. Mars
possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.

Asteroid belt

Image of the main asteroid belt and the Trojan asteroids.

Asteroids are mostly small solar system bodies that are composed in significant part of
rocky, non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and
3.3 AU from the Sun. It is thought to be the remnants of a small terrestrial planet that
failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in
size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres,
are classified as small solar system bodies; however, a number of other asteroids, such as
Vesta and Hygeia, could potentially be reclassed as dwarf planets if it can be
conclusively shown that they are spherical. The asteroid belt contains tens of thousands -
and potentially millions - of objects over one kilometre in diameter. However, despite
their large numbers, the total mass of the main belt is unlikely to be more than a
thousandth of that of the Earth. In contrast to its various depictions in science fiction, the
main belt is very sparsely populated; spacecraft routinely pass through without incident.
Asteroids with a diameter of less than 50 m are called meteoroids.

Ceres

Ceres

Ceres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known
dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for
its own gravity to pull it into a spherical shape. Ceres was considered a planet when it

was discovered in the nineteenth century, but was reclassified as an asteroid as further
observation revealed additional asteroids. It has since been again reclassified as a dwarf
planet.

Asteroid groups

Asteroids in the main belt are subdivided into asteroid groups and families based on their
specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids.
They are not as clearly distinguished as planetary moons, sometimes being almost as
large as their partners. The asteroid belt also contains main-belt comets which may have
been the source of Earth’s water.
Trojan asteroids are located in either of Jupiter’s L4 or L5 points, (gravitationally stable
regions leading and trailing a planet in its orbit) though the term is also sometimes used
for asteroids in any other planetary Lagrange point as well.
The inner solar system is also dusted with rogue asteroids, many of which cross the orbits
of the inner planets.

Outer planets

Gas giant

From top to bottom: Neptune, Uranus, Saturn, and Jupiter (sizes not to scale).
The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they
collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn

are true giants, at 318 and 95 Earth masses, respectively, and composed largely of
hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14
and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of
hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and
methane. For this reason some astronomers suggested that they belong in their own
category, “Uranian planets,” or “ice giants.” All four of the gas giants exhibit orbital
debris rings, although only the ring system of Saturn is easily observable from Earth. The
term outer planet should not be confused with superior planet, which designates those
planets which lie outside Earth’s orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put
together. Its composition of largely hydrogen and helium is not very different from that
of the Sun, and the planet has been described as a “failed star”. Jupiter’s strong internal
heat creates a number of semi-permanent features in its atmosphere, such as cloud bands
and the Great Red Spot. The four largest of its 63 satellites, Ganymede, Callisto, Io, and
Europa (the Galilean satellites) share elements in common with the terrestrial planets,
such as volcanism and internal heating. Ganymede, the largest satellite in the Solar
System, has a diameter larger than Mercury.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common
with Jupiter, including its atmospheric composition, though it is far less massive, being
only 95 Earth masses. Two of its 56 moons, Titan and Enceladus, show signs of
geological activity, though they are largely made of ice. Titan, like Ganymede, is larger
than Mercury; it is also the only satellite in the solar system with a substantial
atmosphere, similar in composition to that of the atmosphere of the early Earth.

Uranus

Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely
among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to
the ecliptic. Its core is remarkably cold (compared with the other gas giants; it is still
several thousand degrees Celsius) and radiates very little heat into space. Uranus has 27
satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, it is denser and slightly more
massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much
as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense “arcs” of
material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically
active, with geysers of liquid nitrogen, and is the only large satellite to revolve around its
host planet in a prograde (clockwise) motion.

Kuiper belt

Artist’s rendering of the Kuiper Belt and hypothetical Oort cloud.

The area beyond Neptune, often referred to as the outer solar system or simply the “trans-
Neptunian region”, is still largely unexplored.

This region’s first formation, which actually begins inside the orbit of Neptune, is the
Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of
ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region
is thought to be the place of origin for short-period comets, such as Halley’s comet.
Though it is composed mainly of small solar system bodies, many of the largest Kuiper
belt objects could soon be reclassified as dwarf planets. There are estimated to be over
100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass
of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth. Many
Kuiper belt objects have multiple satellites and most have orbits that take them outside
the plane of the ecliptic.

The Kuiper belt can be roughly divided into two regions: the “resonant” belt, consisting
of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance,
three times for every two Neptune orbits, or twice for every one), which actually begins
within the orbit of Neptune itself, and the “classical” belt, consisting of objects that don’t
have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.

Pluto and Charon

Pluto, and its three known moons

Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously
accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf
planet by the Astronomers Congress organized by the International Astronomers Union
(IAU). Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and
ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to

AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of
Pluto, but in light of the redefinition it is unclear whether Charon will continue to be
classified as a moon of Pluto or as a dwarf planet itself. Charon does not exactly orbit
Pluto in a traditional sense; Charon is about one-tenth the mass of Pluto and the center of
gravity of the pair is not within Pluto. Both bodies orbit a barycenter of gravity above the
surface of Pluto (in empty space), making Pluto-Charon a binary system. Two much
smaller moons, Nix and Hydra, orbit Pluto and Charon.

Those Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune
(ie, they orbit twice for every three Neptunian orbits) are called Plutinos. Other Kuiper
belt objects have different resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly.
The remaining Kuiper belt objects, in more “classical” orbits, are classified as
Cubewanos, after the first of their kind to be discovered, 1992 QB1.

Comets

Comet Hale-Bopp

Comets are small solar system bodies (usually only a few kilometres across) composed
largely of volatile ices, which possess highly eccentric orbits, generally having a
perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a
comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a
coma; a long tail of gas and dust which is often visible with the naked eye.

There are two basic types of comet: short-period comets, with orbits less than 200 years,
and long-period comets, with orbits lasting thousands of years. Short-period comets are
believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp
(pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic
orbits may originate outside the solar system. Old comets that have had most of their
volatiles driven out by solar warming are often categorized as asteroids.

Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in
the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron,
has been called a comet since it has been shown to develop a coma just as comets do
when they approach the sun.

Scattered disc

Black: scattered disc; blue: classical Kuiper belt; green: resonant KBOs inc. Pluto.
Overlapping the Kuiper belt but extending much further outwards is the scattered disc.
Scattered disc objects are believed to have been originally native to the Kuiper belt, but
were ejected into erratic orbits in the outer fringes by the gravitational influence of
Neptune’s outward migration. Most scattered disc objects have perihelia within the
Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly
inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers,
such as Kuiper belt co-discoverer David Jewitt, consider the scattered disc to be merely
another region of the Kuiper belt, and describe scattered disc objects as “scattered Kuiper
belt objects.”

Eris

Eris and its moon Dysnomia

Eris (68 AU average) is the largest known scattered disc object and was the cause of the
most recent debate about what constitutes a planet since it is at least 5% larger than Pluto
with an estimated diameter of 2400 km (1500 mi). It is now the largest of the known
dwarf planets. It has one moon, Dysnomia.

The object has many similarities with Pluto: its orbit is highly eccentric, with a perihelion
of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and is
steeply inclined to the ecliptic plane, at 44 degrees, more so than any known object in the
solar system except the newly-discovered object 2004 XR190 (also known as “Buffy”) and
is believed to consist largely of rock and ice.

Farthest regions

The point at which the solar system ends and interstellar space begins is not precisely
defined, since its outer boundaries are delineated by two separate forces: the solar wind
and the Sun’s gravity. The solar wind extends to a point roughly 130 AU from the Sun,
whereupon it surrenders to the surrounding environment of the interstellar medium. It is
generally accepted, however, that the Sun’s gravity holds sway to the Oort cloud. This
great mass of up to a trillion icy objects, currently hypothetical, is believed to be the
source for all long-period comets and to surround the solar system like a shell from
50,000 to 100,000 AU beyond the Sun, or almost a quarter the distance to the next star
system. The vast majority of the solar system, therefore, is completely unknown;
however, recent observations of both the solar system and other star systems have led to
an increased understanding of what is or may be lying at its outer edge.

An artist’s conception of Sedna

Sedna

Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes
it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to
complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part
of the scattered disc or the Kuiper Belt as it has too distant a perihelion to have been
affected by Neptune’s migration. He and other astronomers consider it to be the first in an
entirely new population, one which also may include the object 2000 CR105, which has a
perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years. Sedna is
very likely a dwarf planet, though its shape has yet to be determined with certainty.

Heliopause

The Voyagers entering the heliosheath

The heliosphere expands outward in a great bubble to about 95 AU, or three times the
orbit of Pluto. The edge of this bubble is known as the termination shock; the point at
which the solar wind collides with the opposing winds of the interstellar medium. Here
the wind slows, condenses and becomes more turbulent, forming a great oval structure
known as the heliosheath that looks and behaves very much like a comet’s tail; extending
outward for a further 40 AU at its stellar-windward side, but tailing many times that
distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the
point at which the solar wind finally terminates, and one enters the environment of
interstellar space.[34] Beyond the heliopause, at around 230 AU, lies the bow shock, a
plasma “wake” left by the Sun as it travels through the Milky Way.[35]

Galactic context

Artist’s conception of the Local Bubble

The solar system is located in the Milky Way galaxy, a barred spiral galaxy with a
diameter estimated at about 100,000 light years containing approximately 200 billion
stars. Our Sun resides in one of the Milky Way’s outer spiral arms, known as the Orion
Arm or Local Spur.The immediate galactic neighborhood of the solar system is known as
the Local Fluff, an area of dense cloud in an otherwise sparse region known as the Local
Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years
across. The bubble is suffused with high-temperature plasma that suggests it is the
product of several recent supernovae.

Estimates place the solar system at between 25,000 and 28,000 light years from the
galactic center. Its speed is about 220 kilometres per second, and it completes one
revolution every 226 million years. The apex of solar motion—that is, the direction in
which the Sun is heading—is near the current location of the bright star Vega.[38] At the
galactic location of the solar system, the escape velocity with regard to the gravity of the
Milky Way is about 1000 km/s.

Presumed location of the solar system within our galaxy

The solar system appears to have a very remarkable orbit. It is both extremely close to
being circular, and at nearly the exact distance at which the orbital speed matches the
speed of the compression waves that form the spiral arms. The solar system appears to
have remained between spiral arms for most of the existence of life on Earth. The
radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces,
preventing the formation of large animal life on land. By remaining out of the spiral
arms, Earth may be unusually free to form large animal life on its surface. The solar
system also lies well outside the star-crowded environs of the galactic centre. The
opposing gravitational tugs from so many close stars within the galactic centre would
have prevented planets from forming.

Recent studies of Extrasolar systems neighboring Earth’s have shown that our system’s
configuration might not be common, as the vast majority so far discovered have been
found to be markedly different. For instance, many extrasolar planetary systems contain a
“hot Jupiter”; a planet of comparable size to Jupiter that nonetheless orbits very close to
its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in
these systems formed in the same place as the gas giants in Earth’s solar system did,
some sort of migration took place which resulted in the giant planet spiralling in towards

the parent star. Any terrestrial planets which had previously existed would presumably
either be destroyed or ejected from the system. On the other hand, the apparent
prevalence of hot Jupiters could result from a sampling error, as planets of similar size at
greater distances from their stars are more difficult to detect.

Discovery and exploration

For many thousands of years, people, with a few notable exceptions, did not believe the
solar system existed. The Earth was believed not only to be stationary at the centre of the
universe, but to be categorically different from the divine or ethereal objects that moved
through the sky. The conceptual advances of the 17th century, led by Nicolaus
Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton, led gradually to the
acceptance of the idea not only that Earth moved round the Sun, but that the planets were
governed by the same laws that governed the Earth, and therefore could be similar to it.

Telescopic observations

Galileo’s telescope

The first exploration of the solar system was conducted by telescope, with astronomers
learning that the Moon and other planets possessed such Earthlike features as craters, ice
caps, and seasons.
Galileo Galilei was the first to discover physical details about the individual bodies of the
Solar System. He discovered that the Moon was cratered, that the Sun was marked with
sunspots, and that Jupiter had four satellites in orbit around it.[41] Christiaan Huygens
followed on from Galileo’s discoveries by discovering Saturn’s moon Titan and the shape

of the rings of Saturn. [42] Giovanni Domenico Cassini later discovered four more moons
of Saturn, the Cassini division in Saturn’s rings, and the Great Red Spot of Jupiter.[43]

In 1682, Edmund Halley realised that repeated sightings of a comet were in fact
recording the same object, returning regularly once every 75-6 years. This proved once
and for all that comets were not atmospheric phenomena, as had been previously thought,
and was the first evidence that anything other than the planets orbited the Sun.[44]

In 1781, William Herschel was looking for binary stars in the constellation of Taurus
when he observed what he thought was a new comet. In fact, its orbit revealed that it was
a new planet, Uranus, the first ever discovered.[45]

In 1801, Giuseppe Piazzi discoverd Ceres, a small world between Mars and Jupiter that
was initially considered a new planet. However, subsequent discoveries of thousands of
other small worlds in the same region led to their eventual separate reclassification:
asteroids.

In 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be
tugging at it from farther out. Urbain Le Verrier’s calculations eventually led to the
discovery of Neptune.

Further discrepancies in the orbits of the planets led Percival Lowell to conclude yet
another planet, “Planet X” must still be out there. After his death, his Lowell Observatory
conducted a search, which ultimately led to Clyde Tombaugh’s discovery of Pluto in
1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer
planets, and its discovery was therefore coincidental. Like Ceres, it was initially
considered to be a planet, but after the discovery of many other similarly sized objects in
its vicinity it was eventually reclassified as a Kuiper belt object.

In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu of the
Massachusetts Institute of Technology discovered 1992 QB1, the first object found
beyond Neptune in 62 years. This object proved to be the first of a new population, which
came to be known as the Kuiper Belt; an icy analogue to the asteroid belt of which such
objects as Pluto and Charon were deemed a part. Many of the largest of these objects,
such as Chaos, Quaoar, Varuna and Ixion, where discovered by astronomer Mike Brown.[

In 2005, Mike Brown announced the discovery of Eris, a Scattered disc object larger than
Pluto and the largest object discovered in the solar system since Neptune.

Observations by spacecraft

The Pale Blue Dot photo, a photo of Earth as a tiny dot (taken 4 billion miles from Earth
by Voyager 1 at the edge of the solar system)

Since the start of the space age, a great deal of exploration has been performed by
unmanned space missions that have been organized and executed by various space
agencies. The first probe to land on another solar system body was the Soviet Union’s
Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant
planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the
asteroid 433 Eros in 2001, and Saturn’s moon Titan in 2005. Spacecraft have also made
close approaches to other planets: Mariner 10 passed Mercury in 1973.

The planned Phoenix Mars lander

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in
1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a
grand tour of the outer planets following their launch in 1977, with both probes passing
Jupiter in 1979 and Saturn in 1980 – 1981. Voyager 2 then went on to make close
approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far
beyond Neptune’s orbit, and astronomers anticipate that they will encounter the
heliopause which defines the outer edge of the solar system in the next few years.

All planets in the solar system have now been visited to varying degrees by spacecraft
launched from Earth, the last being Neptune in 1989. Through these unmanned missions,
humans have been able to get close-up photographs of all of the planets and, in the case
of landers, perform tests of the soils and atmospheres of some.

No Kuiper belt object has been visited by a man-made spacecraft. Launched in 19
January 2006, the New Horizons is currently enroute to becoming the first man-made
spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in
July 2015. Should it prove feasible, the mission will then be extended to observe a
number of other Kuiper belt objects

Planetary system

An artist’s concept of a protoplanetary disc.

A planetary system consists of the various non-stellar objects orbiting a star such as
planets, moons, asteroids, meteoroids, comets, and cosmic dust. The Sun and its planetary
system, which includes Earth, is known as the Solar System.

Origin and evolution

Planetary systems around sun-like stars are generally believed to form as part of the same
process which results in star formation. Some early theories involved another star passing
extremely close to the sun, and drawing material out from it which then coalesced to form
the planets. However, the probability of such a near collision is now known to be far too
low to make this a viable model. Accepted theories today argue that planetary systems
form from a solar nebula.

Some planetary systems are very unlike our own, however: planetary systems around
pulsars have been inferred from slight variations in the period of the pulses of
electromagnetic radiation. Pulsars are formed in violent supernova explosions, and a
normal planetary system could not possibly survive such a blast - planets would either
evaporate, or the sudden loss of most of the mass of the central star would see them
escape the gravitational hold of the star. One theory is that existing stellar companions
were almost entirely evaporated by the supernova blast, leaving behind planet-sized
bodies. Alternatively, planets may somehow form in the accretion disk surrounding
pulsars.

Extrasolar planet

An extrasolar planet, or exoplanet, is a planet that is beyond the Solar System. As of 9
October 2006, 210 extrasolar planets have been discovered

Known exoplanets are members of planetary systems that orbit a star. There have also
been unconfirmed reports of free-floating planetary-mass objects (that is, ones that do not

orbit any star). Since the International Astronomical Union has currently left open the
question as to whether such objects fall within the definition of “planet”, and none are
known, they will not be discussed in this article. For more information, see interstellar
planet.

Infrared image of 2M1207 (blue) and its planet 2M1207b, as viewed by the Very Large
Telescope. As of September 2006 this is the only confirmed extrasolar planet to have
been directly imaged.
For centuries, extrasolar planets were a subject of speculation. Astronomers generally
supposed that some existed, but it was a mystery how common they were and how
similar they were to the planets of the Solar System. The first confirmed detections were
finally made in the 1990s. Since 2002, more than twenty have been discovered every year.
It is now estimated that at least 10% of sunlike stars have planets, and the true proportion
may be much higher. The discovery of extrasolar planets raises the question of
whether some might support extraterrestrial life.

History of detection

Claims have been made for the detection of exoplanets going back many decades. Some
of the earliest involve the binary star 70 Ophiuchi. In 1855 Capt. W. S. Jacob, working at
the Madras Observatory of the East India Company reported that orbital anomalies made
it “highly probable” that there was a “planetary body” in this system. In the 1890s,
Thomas J. J. See of the University of Chicago and the United States Naval Observatory
claimed that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi
system, with a 36 year period around one of the stars. But shortly afterward Forest Ray
Moulton published a paper proving that a three-body system with those orbital parameters
would be highly unstable. During the 1950s and 1960s, Peter van de Kamp of
Swarthmore College made another prominent series of detection claims, this time for

planets orbiting Barnard’s Star. Astronomers now generally regard all these early
“detections” as erroneous.

Our solar system compared with the system of 55 Cancri

The first published discovery to have received subsequent confirmation was made in
1988 by the Canadian astronomers Bruce Campbell, G.A.H Walker, and S. Yang. Their
radial-velocity observations suggested that a planet orbited the star Gamma Cephei (also
known as Alrai). They remained cautious about claiming a true planetary detection, and
widespread skepticism persisted in the astronomical community for several years about
this and other similar observations. Mainly that was because the observations were at the
very limits of instrumental capabilities at the time. Another source of confusion was that
some of the possible planets might instead have been brown dwarfs, objects intermediate
in mass between planets and stars.
The following year, additional observations were published that supported the reality of
the planet orbiting Gamma Cephei. But subsequent work in 1992 raised serious doubts.
Finally, in 2003, improved techniques allowed the planet’s existence to be confirmed.
In 1991, Andrew Lyne, M. Bailes and S.L. Shemar claimed to have discovered a pulsar
planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly
received intense attention, but Lyne and his team soon retracted it.

Our inner solar system superimposed behind the orbits of the planets HD 179949 b, HD
164427 b, Epsilon Reticuli ab, and Mu Arae b (all parent stars are in the center)

In early 1992, the Polish astronomer Aleksander Wolszczan (with Dale Frail) announced
the discovery of planets around another pulsar, PSR 1257+12. This discovery was
quickly confirmed, and is generally considered to be the first definitive detection of
exoplanets. These pulsar planets are believed to have formed from the unusual remnants
of the supernova that produced the pulsar, in a second round of planet formation, or else
to be the remaining rocky cores of gas giants that survived the supernova and then
spiralled in to their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva
announced the first definitive detection of an exoplanet orbiting an ordinary main-
sequence star (51 Pegasi). This discovery ushered in the modern era of exoplanetary
discovery. Technological advances, most notably in high-resolution spectroscopy, led to
the detection of many new exoplanets at a rapid rate. These advances allowed
astronomers to detect exoplanets indirectly by measuring their gravitational influence on
the motion of their parent stars. Several extrasolar planets were eventually also detected
by observing the variation in a star’s apparent luminosity as a planet passed in front of it.

As of October 9, 2006, 210 exoplanets have been found, including a few that were
confirmations of controversial claims from the late 1980s. Many of these discoveries
were made by a team led by Geoffrey Marcy at the University of California’s Lick and
Keck Observatories. The first system to have more than one planet detected was υ
Andromedae. Twenty such multiple-planet systems are now known. Among the known
exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of
circumstellar dust disks also suggest the existence of millions of comets in several
extrasolar systems.

Detection methods

Any planet is an extremely faint light source compared to its parent star. In addition to
the intrinsic difficulty of detecting such a faint light source, the light from the parent star
causes a glare that washes it out. Therefore astronomers have generally had to resort to
indirect methods to detect extrasolar planets. At the present time, direct imaging and six
different indirect methods have yielded success:

This diagram shows how a smaller object orbiting a larger produces changes in the
position and velocity of the latter.

• Astrometry Consists of precisely measuring a star’s position in the sky and
observing how that position changes over time. If the star has a planet, then the
gravitational influence of the planet will cause the star itself to move in a tiny
circular or elliptical orbit.

• Radial velocity Also known as the “Doppler method” or “wobble method”.
Variations in the speed with which the star moves towards or away from Earth —
i.e. variations in the radial velocity of the star with respect to Earth — can be
deduced from the displacement in the parent star’s spectral lines due to the
Doppler effect. This has been by far the most productive technique used by planet
hunters.

• Pulsar timing Pulsars (the small, ultradense remnant of a star that has exploded
as a supernova) emit radio waves extremely regularly as they rotate. Slight
anomalies in the timing of its observed radio pulses can be used to track changes
in the pulsar’s motion caused by the presence of planets.

• Transit method If a planet crosses (transits) in front of its parent star’s disk, then
the observed visual brightness of the star drops a small amount. The amount the
star dims depends on its size and on the size of the planet.

• Gravitational microlensing Occurs when the gravitational field of a star acts like
a lens, magnifying the light of a distant background star. If the foreground lensing
star has a planet, then that planet’s own gravitational field can make a detectable
contribution to the lensing effect.

• Circumstellar disks Disks of space dust surround many stars, which can be
detected because it absorbs ordinary starlight and re-emits it as infrared radiation.
Features in dust disks sometimes suggest the presence of full-sized planets.

• Direct imaging In a few unusual cases current telescopes may be capable of
directly imaging planets. Specifically, this may be possible when the planet is
especially large (considerably larger than Jupiter), widely separated from its
parent star, and young (so that it is hot and emits intense infrared radiation).

For the future, several space missions are planned that will employ already proven planet-
detection methods. Astronomical measurements done from space can be more sensitive
than measurements done from the ground, since the distorting effect of the Earth’s
atmosphere is removed, and the instruments can view in infrared wavelengths that do not

penetrate the atmosphere. Some of these space probes should be capable of detecting
planets similar to our own Earth. Huge proposed ground telescopes may also be able to
directly image extrasolar planets.

Nomenclature

A lower case letter is placed after the star name, starting with “b” for the first planet
found in the system (e.g. 51 Pegasi b), with the next planet being for example “51 Pegasi
c”, then “51 Pegasi d”... (The letter “a” is not used because it might be interpreted as
referring to the star itself.)

Planet naming conventions are based on discovery date - for example, the first planet
detected will be designated with the letter “b.” Any additional planets will be given
additional letters regardless of position. A real world example is the Gliese 876 system:
that latest discovered planet is Gliese876d, which is the closest orbiting planet.

Before the discovery of 51 Pegasi b in 1995, extrasolar planets were named differently.
The first extrasolar planets found around pulsar PSR 1257+12 were named with capital
letters: PSR 1257+12 B and PSR 1257+12 C. When a new, closer-in exoplanet was found
around the pulsar, it was named PSR 1257+12 A, not D.

Several extrasolar planets also have unofficial nicknames. For example, HD b is
unofficially called “Osiris”, and 51 Pegasi b is called “Bellerophon.”

General properties of exoplanets

All extrasolar planets discovered by radial velocity (blue dots), transit (red) and
microlensing (yellow) to 31 August 2004. Also shows detection limits of forthcoming
space- and ground-based instruments.

Most known exoplanets orbit stars roughly similar to our own Sun—that is, main-
sequence stars of spectral categories F, G, or K. One reason is simply that planet search
programs have tended to concentrate on such stars. But even after taking that into
account, statistical analysis suggests that lower-mass stars (red dwarfs, of spectral
category M) are either less likely to have planets or have planets that are themselves of
lower mass. Recent observations by the Spitzer Space Telescope indicate that planetary
formation does not occur within the vicinity of an O class star due to the Photo
evaporation effect.

All stars are composed mainly of the light elements hydrogen and helium. They also
contain a small fraction of heavier elements such as iron; astronomers refer to that
fraction as a star’s metallicity. Stars of higher metallicity are much more likely to have
planets, and the planets they have tend to be more massive than those of lower-metallicity
stars.

The vast majority of exoplanets found so far have high masses. Ninety percent of them
have more than 10 times the mass of Earth. Many are considerably more massive than
Jupiter, our own Solar System’s largest planet. However, these high masses are in large

part an observational selection effect: All detection methods are much more likely to
discover massive planets. That observational selection effect makes statistical analysis
difficult, but it appears that lower-mass planets are actually more common than higher-
mass ones, at least within a broad mass range that includes all giant planets. Also, the fact
that astronomers have found several planets only a few times more massive than Earth,
despite the great difficulty of detecting them, indicates that such planets are fairly
common.

It is believed that the vast majority of known exoplanets are in substantial part gaseous,
like the giant planets of our own Solar System. That has only been confirmed, however,
for the exoplanets that have been studied with the transit method. A few of the smallest
exoplanets are suspected to be rocky, like Earth and the other inner planets of our Solar
System.

Many exoplanets orbit much closer around their parent star than any planet in our own
Solar System orbits around the Sun. Again, that is mainly an observational selection
effect. The radial-velocity method is most sensitive to planets with such small orbits.
Astronomers were initially very surprised by these “hot Jupiters,” but it is now clear that
most exoplanets (or at least, most high-mass exoplanets) have much larger orbits. It
appears plausible that in most exoplanetary systems, there are one or two giant planets
with orbits comparable in size to those of Jupiter and Saturn in our own Solar System.

This planetary habitability chart shows where life might exist on extrasolar planets based
on our own solar system and life on Earth.

The eccentricity of an orbit is a measure of how elliptical (elongated) it is. Most known
exoplanets have quite eccentric orbits. This is not an observational selection effect, since
a planet can be detected about a star equally well regardless of how eccentric its orbit is.
The prevalence of elliptical orbits is a major puzzle, since current theories of planetary
formation strongly suggest planets should form with circular (non-eccentric) orbits. One
possible theory is that small companions such as T dwarfs (methane bearing brown
dwarfs) can hide in such solar systems and can cause the orbits of planets to be extreme.
This is also an indication that our own Solar System may be unusual, since all of its
planets do follow basically circular orbits.

Many unanswered questions remain about the properties of exoplanets, such as details of
their composition and how likely they are to have moons. One of the most intriguing
questions about them is whether they might support life. Several planets do have orbits in
their parent star’s habitable zone, where it should be possible for Earth-like conditions to
prevail. All of those planets are giant planets more similar to Jupiter than to Earth, so if
they have large moons perhaps those would be the most plausible abode of life. Detection
of life (other than an advanced civilization) at interstellar distances, however, is a
tremendously challenging technical task that will not be feasible for many years, even if
such life is commonplace.

Notable extrasolar planets

Artist’s impression from 2005 of the planet HD 69830 d, with the star HD 69830’s
asteroid belt in the background

Artist’s impression of the pulsar planet PSR B1620-26c (discovered in 2003); it is over
12.5 billion years old, making it the oldest known extrasolar planet

Artist’s impression of a triple sunset on a conjectural moon orbiting HD 188753 Ab.

Artist’s conception of the planet OGLE-2005-BLG-390Lb (with surface temperature of
−220°C), orbiting its star 20,000 light years (117.5 quadrillion miles) from Earth; this
planet was discovered with gravitational microlensing

Artist’s conception of HAT-P-1b, the least dense known extrasolar planet

There have been a number of milestones in the discovery of extrasolar planets, beginning
in 1992, when Wolszczan and Frail published results in Natureindicating that pulsar
planets existed around PSR B1257+12. Wolszczan had discovered the millisecond pulsar
in question in 1990 at the Arecibo radio observatory. These were the first exoplanets ever
verified, and they are still considered highly unusual in that they orbit a pulsar.

The first verified discovery of an exoplanet (51 Pegasi b) orbiting a main sequence star
(51 Pegasi) was announced by Michel Mayor and Didier Queloz in Nature on October 6,
1995. Astronomers were initially surprised by this “hot Jupiter” but soon set out to find
other similar planets with great success.

Since that time, other notable discoveries have included:

1999, HD 209458 b

This exoplanet, originally discovered with the radial-velocity method, became the
first exoplanet to be seen transiting its parent star. The transit detection conclusively
showed that the radial velocity measurements suspected to be planets actually were
planets.

2001, HD 209458 b

Astronomers using the Hubble Space Telescope announced that they had detected the
atmosphere of HD 209458 b. They found the spectroscopic signature of sodium in the
atmosphere, but at a smaller intensity than expected, suggesting that high clouds
obscure the lower atmospheric layers.

2003, PSR B1620-26c

On July 10, using information obtained from the Hubble Space Telescope, a team of
scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet yet. The
planet is located in the globular star cluster M4, about 5,600 light years from Earth in
the constellation Scorpius. This is the only planet known to orbit around a stellar
binary; one of the stars in the binary is a pulsar and the other is a white dwarf. The
planet has a mass twice that of Jupiter, and is estimated to be 13 billion years old.

2004, Mu Arae d and TrES-1

In August, a planet orbiting Mu Arae with a mass of approximately 14 times that of
the Earth was discovered with the ESO HARPS spectrograph. It is the third lightest
extrasolar planet orbiting a main sequence star to be discovered to date, and could be
the first terrestrial planet around a main sequence star found outside the solar system.
Further, a planet was discovered using the transit method with the smallest aperture
telescope to date, 4 inches. The planet was discovered by the TrES survey, and
provisionally named TrES-1, orbits the star GSC 02652-01324. The finding was
confirmed by the Keck Observatory, where planetary specifics were uncovered.

2005, Gliese 876 d

In June, a third planet orbiting the red dwarf star Gliese 876 was announced. With a
mass estimated at 7.5 times that of Earth, it is currently the second-lightest known

exoplanet that orbits an ordinary main-sequence star. It must almost certain be rocky
in composition. It orbits at 0.021 AU with a period of 1.94 days.

2005, HD 149026 b

In July a planet with the largest core ever was announced. The planet, HD 149026 b
orbits the star HD 149026, has a core that is estimated to be 70 Earth masses,
accounting for two thirds of the planet’s mass.

2005, HD 188753 Ab

In July, astronomers announced the discovery of a planet in a relatively tight triple
star system, a finding that challenges current theories of planetary formation. The
planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753
system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The
stellar trio (yellow, orange, and red) is about 149 light years away from Earth. The
planet orbits the main star (HD 188753 A) about once every 3.3 days, at a distance of
about a twentieth the distance between Earth and the Sun. The other two stars whirl
tightly around each other in 156 days, and circle the main star every 25.7 years at a
distance from the main star that would put them between Saturn and Uranus in our
own Solar system. The latter stars call into question the leading hot Jupiter formation
theory, which holds that these planets form at “normal” distances and then migrate
inward through some debatable mechanism. Such migration could not have occurred
here, since the outer star pair would have disrupted outer planet formation.

2006, OGLE-2005-BLG-390Lb

On January 25 the discovery of OGLE-2005-BLG-390Lb was announced. This is the
most distant and probably the coldest exoplanet yet found. It is believed to orbit a red
dwarf star around 21,500 light years away, towards the centre of our galaxy. It was
discovered using gravitational microlensing and is estimated to have a mass of 5.5
times that of Earth, making it the least massive known exoplanet to orbit an ordinary
main-sequence star. Prior to this discovery, the few known exoplanets with
comparably low masses had only been discovered on orbits very close to their parent
stars, but this planet is estimated to have a relatively wide separation of 2.6 AU from
its parent star.

2006, HAT-P-1b

Using a network of small automated telescopes known as HAT, Smithsonian
astronomers discovered a planet, since designated HAT-P-1b, that orbits one member
of a pair of distant stars 450 light-years away in the constellation Lacerta. The planet
has a radius 1.38 times that of Jupiter, but one-half the density, making it the least
dense planet on record. It remains unclear how such a planet could evolve, and it is
believed this object and HD 209458 b (also a low-density giant planet) could
ultimately provide insight on how planets form. According to Robert Noyes of the

Harvard-Smithsonian Center for Astrophysics (CfA), “We can’t dismiss HD 209458
b as a fluke. This new discovery suggests something could be missing in our theories
of how planets form.”

2006, SWEEPS-10

The planet candidate with the shortest orbital period yet found, named SWEEPS-10,
completes a full orbit of its star in just 10 hours. Located only 1.2 million kilometres
from its star (roughly three times the distance between the Earth and the Moon), the
planet is among the hottest ever detected, with an estimated temperature of about
1650 degrees C. “This star-hugging planet must be at least 1.6 times the mass of
Jupiter, otherwise the star’s gravitational muscle would pull the planet apart,” said
team leader Kailash Sahu of the Space Telescope Science Institute in Baltimore,
USA. Such ultra-short period planets (USPPs) seem to occur only around dwarf stars.
The smaller star’s relatively lower temperature allows the planet to exist. “USPPs
occur preferentially around normal red dwarf stars that are smaller and cooler than
our Sun,” Sahu said.

Sun

Mean diameter 1.3T9h2e×1S0u6nkm

s (109 Earth diameters)
area
4.373×106 km
(342 Earth diameters)

9×10−6

6.09×1012 km²
(11,900 Earths)

1.41×1018 km³
(1,300,000 Earths)

1.988 435×1030 kg

(332,946 Earths)

1.408 g/cm³

273.95 m s-2

Surface gravity Observation data

Mean distance (12479..96×g1) 06 km

fErsocmape velocity 6(9127..9554×k1m06/smi)
Efraormththe (8.31 minutes at the speed of light)
sVuirsfuaacle (−5256.E8amrths)
5785 K
Sburirgfhatcneess (V)

tATmeebmamsgponpeliuertrtuaeatduteurree of 4.8m

cSopreocntraal 5 MK
G2V
Ccloasrseification
temperature ~13.6 MK
Orbital characteristics

MLuemanindoissittaynce 3.827×1026 W
f(Lrom) the ~(2236.57,0×501×001-0127288k,0lmm00 light-years)

sol (~98 lm/W efficacy)

Milky Way core
GMaelaanctIinctpeenrsiiotyd 2.25-2.507×108 a-2 -1

(Isol) 2.009×10 W m sr
Velocity 217 km/s orbit around the center of
Obliquity RottahteioGnalcahxayr,a2c0tekrmis/tsicrselative to

7av.2e5ra°ge velocity of other stars in
s(ttoeltlhare neecilgiphtbico)rhood
Phy6s7i.c2a3l°characteristics
(to the galactic plane)

The Sun is the star of our Right ascension 286.13°
solar system. The Earth and of North pole (19 h 4 min 30 s)
other matter (including other
planets, asteroids, Declination +63.87°
meteoroids, comets and dust)
orbit the Sun, which by itself of North pole (63°52’ North)
accounts for more than 99%
of the solar system’s mass. Rotation period 25.3800 days
Energy from the Sun—in the
form of insolation from at equator (25 d 9 h 7 min 13 s)
sunlight—directly or
indirectly supports almost all Rotation 7174 km/h
life on Earth, and drives the velocity
Earth’s climate and weather. at equator

The Sun is sometimes Photospheric composition (by mass)
referred to by its Latin name
Sol or its Greek name Helios. Hydrogen 73.46 %
Its astrological and
astronomical symbol is a Helium 24.85 %
circle with a point at its
Oxygen 0.77 %
center: . Some ancient
peoples of the world Carbon 0.29 %
considered it a planet before
the acceptance of Iron 0.16 %
heliocentrism.
Neon 0.12 %
Overview
Nitrogen 0.09 %

Silicon 0.07 %

Magnesium 0.05 %

Sulfur 0.04 %

The sun as it appears through a camera lens from the surface of Earth

About 74% of the Sun’s mass is hydrogen, 25% is helium, and the rest is made up of
trace quantities of heavier elements. The Sun has a spectral class of G2V. “G2” means
that it has a surface temperature of approximately 5,500 K, giving it a white color, which
because of atmospheric scattering appears yellow. Its spectrum contains lines of ionized

and neutral metals as well as very weak hydrogen lines. The “V” suffix indicates that the
Sun, like most stars, is a main sequence star. This means that it generates its energy by
nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance,
neither contracting nor expanding over time. There are more than 100 million G2 class
stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter
than 85% of the stars in the galaxy, most of which are red dwarfs.

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 25,000
to 28,000 light years from the galactic center, completing one revolution in about 225–
250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400
years, and one AU every 8 days.

The Sun is a third generation star, whose formation may have been triggered by
shockwaves from a nearby supernova. This is suggested by a high abundance of heavy
elements such as gold and uranium in the solar system; these elements could most
plausibly have been produced by endergonic nuclear reactions during a supernova, or by
transmutation via neutron absorption inside a massive second-generation star.

Sunlight is the main source of energy near the surface of Earth. The solar constant is the
amount of power that the Sun deposits per unit area that is directly exposed to sunlight.
The solar constant is equal to approximately 1,370 watts per square meter of area at a
distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of
Earth is attenuated by the Earth’s atmosphere so that less power arrives at the surface—
closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun
is near the zenith. This energy can be harnessed via a variety of natural and synthetic
processes—photosynthesis by plants captures the energy of sunlight and converts it to
chemical form (oxygen and reduced carbon compounds), while direct heating or
electrical conversion by solar cells are used by solar power equipment to generate
electricity or to do other useful work. The energy stored in petroleum and other fossil
fuels was originally converted from sunlight by photosynthesis in the distant past.

Sunlight has several interesting biological properties. Ultraviolet light from the Sun has
antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has
other medical effects such as the production of Vitamin D. Ultraviolet light is strongly
attenuated by Earth’s atmosphere, so that the amount of UV varies greatly with latitude
because of the longer passage of sunlight through the atmosphere at high latitudes. This
variation is responsible for many biological adaptations, including variations in human
skin color in different regions of the globe.

Observed from Earth, the path of the Sun across the sky varies throughout the year. The
shape described by the Sun’s position, considered at the same time each day for a
complete year, is called the analemma and resembles a figure 8 aligned along a
North/South axis. While the most obvious variation in the Sun’s apparent position
through the year is a North/South swing over 47 degrees of angle (because of the 23.5-
degree tilt of the Earth with respect to the Sun), there is an East/West component as well.
The North/South swing in apparent angle is the main source of seasons on Earth.

The Sun is a magnetically active star; it supports a strong, changing magnetic field that
varies year-to-year and reverses direction about every eleven years. The Sun’s magnetic
field gives rise to many effects that are collectively called solar activity, including
sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carry
material through the solar system. The effects of solar activity on Earth include auroras at
moderate to high latitudes, and the disruption of radio communications and electric
power. Solar activity is thought to have played a large role in the formation and evolution
of the solar system, and strongly affects the structure of Earth’s outer atmosphere.

Although it is the nearest star to Earth and has been intensively studied by scientists,
many questions about the Sun remain unanswered, such as why its outer atmosphere has
a temperature of over a million K while its visible surface (the photosphere) has a
temperature of less than 6,000 K. Current topics of scientific inquiry include the sun’s
regular cycle of sunspot activity, the physics and origin of solar flares and prominences,
the magnetic interaction between the chromosphere and the corona, and the origin of the
solar wind.

Life cycle

The Sun’s current age, determined using computer models of stellar evolution and
nucleocosmochronology, is thought to be about 4.57 billion years.

Life-cycle of the Sun

The Sun is about halfway through its main-sequence evolution, during which nuclear
fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million
tonnes of matter are converted into energy within the Sun’s core, producing neutrinos and
solar radiation. The Sun will spend a total of approximately 10 billion years as a main
sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in 4-5 billion
years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the
core is consumed and the core contracts and heats up. Helium fusion will begin when the
core temperature reaches about 3×108 K. While it is likely that the expansion of the outer
layers of the Sun will reach the current position of Earth’s orbit, recent research suggests
that mass lost from the Sun earlier in its red giant phase will cause the Earth’s orbit to
move further out, preventing it from being engulfed. However, Earth’s water and most of
the atmosphere will be boiled away.

Following the red giant phase, intense thermal pulsations will cause the Sun to throw off
its outer layers, forming a planetary nebula. The only object that remains after the outer
layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a

white dwarf over many billions of years. This stellar evolution scenario is typical of low-
to medium-mass stars.

Structure

The Sun’s diameter is about 110 times that of the Earth.
While the Sun is an average-sized star, it contains approximately 99% of the total mass of
the solar system. The Sun is a near-perfect sphere, with an oblateness estimated at about
9 millionths, which means that its polar diameter differs from its equatorial diameter by
only 10 km. While the Sun does not rotate as a solid body (the rotational period is 25
days at the equator and about 35 days at the poles), it takes approximately 28 days to
complete one full rotation; the centrifugal effect of this slow rotation is 18 million times
weaker than the surface gravity at the Sun’s equator. Tidal effects from the planets do not
significantly affect the shape of the Sun, although the Sun itself orbits the center of mass
of the solar system, which is located nearly a solar radius away from the center of the Sun
mostly because of the large mass of Jupiter.

The Sun does not have a definite boundary as rocky planets do; the density of its gases
drops approximately exponentially with increasing distance from the center of the Sun.
Nevertheless, the Sun has a well-defined interior structure, described below. The Sun’s
radius is measured from its center to the edge of the photosphere. This is simply the layer

below which the gases are thick enough to be opaque but above which they are
transparent; the photosphere is the surface most readily visible to the naked eye. Most of
the Sun’s mass lies within about 0.7 radii of the center.

The solar interior is not directly observable, and the Sun itself is opaque to
electromagnetic radiation. However, just as seismology uses waves generated by
earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology
makes use of pressure waves (infrasound) traversing the Sun’s interior to measure and
visualize the Sun’s inner structure. Computer modeling of the Sun is also used as a
theoretical tool to investigate its deeper layers.

Core

The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has
a density of up to 150,000 kg/m3 (150 times the density of water on Earth) and a
temperature of close to 13,600,000 Kelvins (by contrast, the surface of the Sun is close to
5,785 Kelvins (1/2350th of the core)). Energy is produced by exothermic thermonuclear
reactions (nuclear fusion) that mainly convert hydrogen into helium, helium into carbon,
carbon into iron. The core is the only location in the Sun that produces an appreciable
amount of heat via fusion: the rest of the star is heated by energy that is transferred
outward from the core. All of the energy produced by fusion in the core must travel
through many successive layers to the solar photosphere before it escapes into space as
sunlight or kinetic energy of particles.

About 3.6×1038 protons (hydrogen nuclei) are converted into helium nuclei every second,
releasing energy at the matter-energy conversion rate of 4.3 million tonnes per second,
380 yottawatts (3.8×1026 W) or 9.1×1010 megatons of TNT per second. The rate of
nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-
correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up
more and expand slightly against the weight of the outer layers, reducing the fusion rate
and correcting the perturbation; and a slightly lower rate would cause the core to cool and
shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma and X-rays) released in fusion reactions take a long
time to reach the Sun’s surface, slowed down by the indirect path taken, as well as by
constant absorption and reemission at lower energies in the solar mantle. Estimates of the
“photon travel time” range from as much as 50 million years to as little as 17,000 years.
After a final trip through the convective outer layer to the transparent “surface” of the
photosphere, the photons escape as visible light. Each gamma ray in the Sun’s core is
converted into several million visible light photons before escaping into space. Neutrinos
are also released by the fusion reactions in the core, but unlike photons they very rarely
interact with matter, so almost all are able to escape the Sun immediately. For many years
measurements of the number of neutrinos produced in the Sun were much lower than
theories predicted, a problem which was recently resolved through a better understanding
of the effects of neutrino oscillation.

Radiation zone

From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that
thermal radiation is sufficient to transfer the intense heat of the core outward.

Convection zone

Structure of the Sun

From about 0.7 solar radii to the Sun’s visible surface, the material in the Sun is not dense
enough or hot enough to transfer the heat energy of the interior outward via radiation.
As a result, thermal convection occurs as thermal columns carry hot material to the
surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges
back downward to the base of the convection zone, to receive more heat from the top of
the radiative zone. Convective overshoot is thought to occur at the base of the convection
zone, carrying turbulent downflows into the outer layers of the radiative zone.

The thermal columns in the convection zone form an imprint on the surface of the Sun, in
the form of the solar granulation and supergranulation. The turbulent convection of this
outer part of the solar interior gives rise to a “small-scale” dynamo that produces
magnetic north and south poles all over the surface of the Sun.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun
becomes opaque to visible light. Above the photosphere visible sunlight is free to
propagate into space, and its energy escapes the Sun entirely. The change in opacity is
because of the decreasing overall particle density: the photosphere is actually tens to
hundreds of kilometers thick, being slightly less opaque than air on Earth. Sunlight has
approximately a black-body spectrum that indicates its temperature is about 6,000 K
(10,340°F / 5,727 °C), interspersed with atomic absorption lines from the tenuous layers
above the photosphere. The photosphere has a particle density of about 1023 m−3 (this is
about 1% of the particle density of Earth’s atmosphere at sea level).

During early studies of the optical spectrum of the photosphere, some absorption lines
were found that did not correspond to any chemical elements then known on Earth. In

1868, Norman Lockyer hypothesized that these absorption lines were because of a new
element which he dubbed “helium”, after the Greek Sun god Helios. It was not until 25
years later that helium was isolated on Earth.

Atmosphere

During a total solar eclipse, the sun’s atmosphere is more apparent to the eye.

The parts of the Sun above the photosphere are referred to collectively as the solar
atmosphere. They can be viewed with telescopes operating across the electromagnetic
spectrum, from radio through visible light to gamma rays, and comprise five principal
zones: the temperature minimum, the chromosphere, the transition region, the corona, and
the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere
of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a
sharp shock front boundary with the interstellar medium. The chromosphere, transition
region, and corona are much hotter than the surface of the Sun; the reason why is not yet
known.

The coolest layer of the Sun is a temperature minimum region about 500 km above the
photosphere, with a temperature of about 4,000 K. This part of the Sun is cool enough to
support simple molecules such as carbon monoxide and water, which can be detected by
their absorption spectra.

Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by
a spectrum of emission and absorption lines. It is called the chromosphere from the Greek
root chroma, meaning color, because the chromosphere is visible as a colored flash at
the beginning and end of total eclipses of the Sun. The temperature in the chromosphere
increases gradually with altitude, ranging up to around 100,000 K near the top.

Above the chromosphere is a transition region in which the temperature rises rapidly from
around 100,000 K to coronal temperatures closer to one million K. The increase is
because of a phase transition as helium within the region becomes fully ionized by the
high temperatures. The transition region does not occur at a well-defined altitude. Rather,
it forms a kind of nimbus around chromospheric features such as spicules and filaments,
and is in constant, chaotic motion. The transition region is not easily visible from Earth’s
surface, but is readily observable from space by instruments sensitive to the far ultraviolet
portion of the spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume
than the Sun itself. The corona merges smoothly with the solar wind that fills the solar
system and heliosphere. The low corona, which is very near the surface of the Sun, has a
particle density of 1014 m−3–1016 m−3. (Earth’s atmosphere near sea level has a particle

density of about 2×1025 m−3.) The temperature of the corona is several million kelvin.
While no complete theory yet exists to account for the temperature of the corona, at least
some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes
of the solar system. Its inner boundary is defined as the layer in which the flow of the
solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed
of Alfvén waves. Turbulence and dynamic forces outside this boundary cannot affect the
shape of the solar corona within, because the information can only travel at the speed of
Alfvén waves. The solar wind travels outward continuously through the heliosphere,
forming the solar magnetic field into a spiral shape, until it impacts the heliopause more
than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a
shock front that is thought to be part of the heliopause. Both of the Voyager probes have
recorded higher levels of energetic particles as they approach the boundary.

Solar activity

Sunspots and the solar cycle

Sunspot group 9393, one of the largest recorded in recent years

When observing the Sun with appropriate filtration, the most immediately visible features
are usually its sunspots, which are well-defined surface areas that appear darker than their
surroundings because of lower temperatures. Sunspots are regions of intense magnetic
activity where convection is inhibited by strong magnetic fields, reducing energy transport
from the hot interior to the surface. The magnetic field gives rise to strong
heating in the corona, forming active regions that are the source of intense solar flares and
coronal mass ejections. The largest sunspots can be tens of thousands of kilometers
across.

Measurements of solar cycle variation during the last 30 years

The number of sunspots visible on the Sun is not constant, but varies over a 10-12 year
cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and
occasionally none at all can be seen. Those that do appear are at high solar latitudes. As
the sunspot cycle progresses, the number of sunspots increases and they move closer to
the equator of the Sun, a phenomenon described by Spörer’s law. Sunspots usually exist
as pairs with opposite magnetic polarity. The polarity of the leading sunspot alternates
every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south
magnetic pole in the next.

History of the number of observed sunspots during the last 250 years, which shows the
~11 year solar cycle.

The solar cycle has a great influence on space weather, and seems also to have a strong
influence on the Earth’s climate. Solar minima tend to be correlated with colder
temperatures, and longer than average solar cycles tend to be correlated with hotter
temperatures. In the 17th century, the solar cycle appears to have stopped entirely for
several decades; very few sunspots were observed during this period. During this era,
which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold
temperatures. Earlier extended minima have been discovered through analysis of
tree rings and also appear to have coincided with lower-than-average global temperatures.

Effects on Earth

Solar activity has several effects on the Earth and its surroundings. Because the Earth has
a magnetic field, charged particles from the solar wind cannot impact the atmosphere
directly, but are instead deflected by the magnetic field and aggregate to form the Van
Allen belts. The Van Allen belts consist of an inner belt composed primarily of protons
and an outer belt composed mostly of electrons. Radiation within the Van Allen belts can
occasionally damage satellites passing through them.

The Van Allen belts form arcs around the Earth with their tips near the north and south
poles. The most energetic particles can ‘leak out’ of the belts and strike the Earth’s upper
atmosphere, causing auroras, known as aurorae borealis in the northern hemisphere and
aurorae australis in the southern hemisphere. In periods of normal solar activity, aurorae
can be seen in oval-shaped regions centered on the magnetic poles and lying roughly at a
geomagnetic latitude of 65°, but at times of high solar activity the auroral oval can
expand greatly, moving towards the equator. Aurorae borealis have been observed from
locales as far south as Mexico.

Theoretical problems

Solar neutrino problem

Extremely high resolution spectrum of the Sun showing thousands of elemental
absorption lines (Fraunhofer lines).

For many years the number of solar electron neutrinos detected on Earth was only a third
of the number expected, according to theories describing the nuclear reactions in the Sun.
This anomalous result was termed the solar neutrino problem. Theories proposed to
resolve the problem either tried to reduce the temperature of the Sun’s interior to explain
the lower neutrino flux, or posited that electron neutrinos could oscillate, that is, change
into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth.
Several neutrino observatories were built in the 1980s to measure the solar neutrino flux
as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande.
Results from these observatories eventually led to the discovery that neutrinos have a very
small rest mass and can indeed oscillate.. Moreover, the Sudbury Neutrino Observatory
was able to detect all three types of neutrinos directly, and found that the
Sun’s total neutrino emission rate agreed with the Standard Solar Model, although only
one-third of the neutrinos seen at Earth were of the electron type.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of
approximately 6,000 K. Above it lies the solar corona at a temperature of 1,000,000 K.
The high temperature of the corona shows that it is heated by something other than direct
heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion
in the convection zone below the photosphere, and two main mechanisms have been
proposed to explain coronal heating. The first is wave heating, in which sound,
gravitational and magnetohydrodynamic waves are produced by turbulence in the
convection zone. These waves travel upward and dissipate in the corona, depositing their
energy in the ambient gas in the form of heat. The other is magnetic heating, in which
magnetic energy is continuously built up by photospheric motion and released through
magnetic reconnection in the form of large solar flares and myriad similar but smaller
events.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves
except Alfven waves have been found to dissipate or refract before reaching the corona.
In addition, Alfvén waves do not easily dissipate in the corona. Current research focus
has therefore shifted towards flare heating mechanisms. One possible candidate to
explain coronal heating is continuous flaring at small scales, but this remains an open
topic of investigation.

Faint young sun problem

Theoretical models of the sun’s development suggest that 3.8 to 2.5 billion years ago,
during the Archean period, the Sun was only about 75% as bright as it is today. Such a
weak star would not have been able to sustain liquid water on the Earth’s surface, and
thus life should not have been able to develop. However, the geological record
demonstrates that the Earth has remained at a fairly constant temperature throughout its
history, and in fact that the young Earth was somewhat warmer than it is today. The
general consensus among scientists is that the young Earth’s atmosphere contained much
larger quantities of greenhouse gases (such as carbon dioxide and/or ammonia) than are
present today, which trapped enough heat to compensate for the lesser amount of solar
energy reaching the planet.

Magnetic field

The heliospheric current sheet extends to the outer reaches of the Solar System, and
results from the influence of the Sun’s rotating magnetic field on the plasma in the
interplanetary medium

All matter in the Sun is in the form of gas and plasma because of its high temperatures.
This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it

does at higher latitudes (about 35 days near its poles). The differential rotation of the
Sun’s latitudes causes its magnetic field lines to become twisted together over time,
causing magnetic field loops to erupt from the Sun’s surface and trigger the formation of
the Sun’s dramatic sunspots and solar prominences (see magnetic reconnection). This
twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic
activity as the Sun’s magnetic field reverses itself about every 11 years.

The influence of the Sun’s rotating magnetic field on the plasma in the interplanetary
medium creates the heliospheric current sheet, which separates regions with magnetic
fields pointing in different directions. The plasma in the interplanetary medium is also
responsible for the strength of the Sun’s magnetic field at the orbit of the Earth. If space
were a vacuum, then the Sun’s 10-4 tesla magnetic dipole field would reduce with the
cube of the distance to about 10-11 tesla. But satellite observations show that it is about
100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that
the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field,
induces electric currents which in turn generates magnetic fields, and in this respect it
behaves like an MHD dynamo.

History of solar observation

Early understanding of the Sun

The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an
important part of Nordic Bronze Age mythology.

Humanity’s most fundamental understanding of the Sun is as the luminous disk in the
heavens, whose presence above the horizon creates day and whose absence causes night.
In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other
supernatural phenomenon, and worship of the Sun was central to civilizations such as the
Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments
were constructed with solar phenomena in mind; for example, stone megaliths accurately
mark the summer solstice (some of the most prominent megaliths are located in Nabta
Playa, Egypt, and at Stonehenge in England); the pyramid of El Castillo at Chichén Itzá
in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at
the vernal and autumn equinoxes. With respect to the fixed stars, the Sun appears from
Earth to revolve once a year along the ecliptic through the zodiac, and so the Sun was

considered by Greek astronomers to be one of the seven planets (Greek planetes,
“wanderer”), after which the seven days of the week are named in some languages.

Development of modern scientific understanding

Comparison between the sun and the red supergiant Antares. The black circle is the size
of the orbit of Mars. Arcturus is also included in the picture for comparison.

The sun compared with the red supergiant VV Cephei A (The sun can only be seen when
image is clicked on twice)
One of the first people in the Western world to offer a scientific explanation for the sun
was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of
metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this
heresy, he was imprisoned by the authorities and sentenced to death (though later
released through the intervention of Pericles). Eratosthenes might have been the first
person to have accurately calculated the distance from the Earth to the Sun, in the 3rd
century BCE, as 149 million kilometers, roughly the same as the modern accepted figure.
Another scientist to challenge the accepted view was Nicolaus Copernicus, who in the
16th century developed the theory that the Earth orbited the Sun, rather than the other way
around. In the early 17th century, Galileo pioneered telescopic observations of the Sun,
making some of the first known observations of sunspots and positing that they were on
the surface of the Sun rather than small objects passing between the Earth and the Sun.
Isaac Newton observed the Sun’s light using a prism, and showed that it was made up of
light of many colors, while in 1800 William Herschel discovered infrared radiation

beyond the red part of the solar spectrum. The 1800s saw spectroscopic studies of the Sun
advance, and Joseph von Fraunhofer made the first observations of absorption lines in the
spectrum, the strongest of which are still often referred to as Fraunhofer lines.

In the early years of the modern scientific era, the source of the Sun’s energy was a
significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid
body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz
then proposed the Kelvin-Helmholtz mechanism to explain the energy output.
Unfortunately the resulting age estimate was only 20 million years, well short of the time
span of several billion years suggested by geology. In 1890 Joseph Lockyer, the
discoverer of helium in the solar spectrum, proposed a meteoritic hypothesis for the
formation and evolution of the sun. Another proposal was that the Sun extracted its
energy from friction of its gas masses.

It would be 1904 before a potential solution was offered. Ernest Rutherford suggested
that the energy could be maintained by an internal source of heat, and suggested
radioactive decay as the source. However it would be Albert Einstein who would provide
the essential clue to the source of a Sun’s energy with his mass-energy relation E=mc². In
1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the
Sun could produce a nuclear fusion reaction that merged hydrogen into helium, resulting
in a production of energy from the net change in mass. This theoretical concept was
developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans
Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear
reactions that power the Sun.

Finally, in 1957, a paper titled Synthesis of the Elements in Stars was published that
demonstrated convincingly that most of the elements heavier than hydrogen in the
universe had been created by nuclear reactions inside stars like the Sun.

Solar space missions

Solar “fireworks” in sequence as recorded in November 2000 by four instruments
onboard the SOHO spacecraft.