Solar system a beginners guide1

extra-solar planets Neptune has been used as a metonym: discovered bodies of similar
mass are often referred to as “Neptunes” just as astronomers refer to various extra-solar
“Jupiters.”

Composition

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions
of the atmosphere at −218 °C (55 K). Deeper inside the layers of gas, however, the
temperature rises steadily. It is thought that this may be leftover heat generated by
infalling matter during the planet’s birth, now slowly radiating away into space.

The internal structure resembles that of Uranus. There is likely to be a core consisting of
molten rock and metal, surrounded by a mixture of rock, water, ammonia, and methane.
There is no solid surface and the atmosphere, extending perhaps 10 to 20 percent of the
way towards the center, is mostly hydrogen and helium at high altitudes (80% and 19%,
respectively). Increasing concentrations of methane, ammonia, and water are found as the
dark, hotter and lower regions atmosphere approaches and finally blends into the
superheated liquid interior. The pressure at the center of Neptune is millions of times
more than that on the surface of Earth. Comparing its rotational speed to its degree of
oblateness indicates that it has its mass less concentrated towards the center than does
Uranus.

Magnetic field

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted
relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres)
from the planet’s physical center. Comparing the magnetic fields of the two planets,
scientists think the extreme orientation may be characteristic of flows in the interior of
the planet and not the result of Uranus’ sideways orientation.

Weather

Great Dark Spot (top), Scooter (middle white cloud), and the Wizard’s eye (bottom).

One difference between Neptune and Uranus is the level of meteorological activity.
Uranus is visually quite bland, while Neptune’s high winds come with notable weather
phenomena. Neptune’s atmosphere has the highest wind speeds in the solar system,

thought to be powered by the flow of internal heat, and its weather is characterized by
extremely violent hurricanes, with winds reaching up to 2000 km/h.

In 1989, the Great Dark Spot, a cyclonic storm system the size of Eurasia, was discovered
by NASA’s Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter.
However, on November 2, 1994 the Hubble Space Telescope did not see the Great Dark
Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the
planet’s northern hemisphere. The reason for the Great Dark Spot’s disappearance is
unknown. Many scientists believe heat transfer from the planet’s core disrupted the
atmospheric equilibrium and disrupted existing circulation patterns. The Scooter is
another storm described as a white cloud south of the Great Dark Spot. The Wizard’s eye
(Great Dark Spot 2) is a southern hurricane, the second most intensive hurricane on the
planet.

Unique among the gas giants is the presence of high clouds casting shadows on the
opaque cloud deck below. Though Neptune’s atmosphere is much more dynamic than that
of Uranus, both planets are made of the same gases and ices. Uranus and Neptune are not
strictly gas giants similar to Jupiter and Saturn, but are rather ice giants, meaning they
have a larger solid core and are also made of ices. Neptune is very cold, with temperatures
as low as -224°C (-372°F) recorded at the cloud tops in 1989.

Exploration of Neptune

Voyager 2 image of Neptune

The closest approach of Voyager 2 to Neptune occurred on August 25, 1989. Since this
was the last major planet the spacecraft could visit, it was decided to make a close flyby
of the moon Triton, regardless of the consequences to the trajectory, similarly to what
was done for Voyager 1’s encounter with Saturn and its moon Titan.

The probe also discovered the Great Dark Spot, which has since disappeared, according
to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it
was later postulated to be a hole in the visible cloud deck.

Neptune turned out to have the strongest winds of all the solar system’s gas giants. In the
outer regions of the solar system, where the Sun shines over 1000 times fainter than on
Earth (still very bright with a magnitude of -21), the last of the four giants defied all
expectations of the scientists.
One might expect that the farther one gets from The Sun, the less energy there would be
to drive the winds around. The winds on Jupiter were already hundreds of kilometres per
hour. Rather than seeing slower winds, the scientists found faster winds (over 1600 km/h)
on more distant Neptune.
Scientists now know why this is the case —if enough energy is produced, turbulence is
created, which slows the winds down (like those of Jupiter). At Neptune however, there
is so little energy, that once winds are started, they meet very little resistance, and are
able to maintain extremely high velocities.

Planetary rings

Neptune has a faint planetary ring system of unknown composition. The rings have a
peculiar “clumpy” structure, the cause of which is not currently understood but which
may be due to the gravitational interaction with small moons in orbit near them.

Neptune’s rings

Evidence that the rings are incomplete first arose in the mid-1980s, when stellar
occultation experiments were found to occasionally show an extra “blink” just before or
after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the
ring system was found to contain several faint rings. The outermost ring, Adams, contains
three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and
Fraternity). The existence of arcs is very difficult to understand because the laws of
motion would predict that arcs spread out into a uniform ring over very short timescales.

The gravitational effects of Galatea, a moon just inward from the ring, are now believed
to confine the arcs.

Several other rings were detected by the Voyager cameras. In addition to the narrow
Adams Ring 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km
and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the
Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring
at 57,000 km.

New Earth-based observations announced in 2005 appeared to show that Neptune’s rings
are much more unstable than previously thought. In particular, it seems that the Liberté
ring might disappear in as little as one century. The new observations appear to throw our
understanding of Neptune’s rings into considerable confusion.

Name of ring Radius (km) Width (km) Notes

1989 N3R (‘Galle’) 41,900 15 Named after Johann Galle

1989 N2R (‘Leverrier’) 53,200 15 Named after Urbain Le Verrier

1989 N4R (‘Lassell’) 55,400 6 Named after William Lassell

Arago Ring 57,600 - Named after François Arago

Liberté Ring Arc 62,900 - “Leading” arc

Égalité Ring Arc 62,900 - “Equidistant” arc

Fraternité Ring Arc 62,900 - “Trailing” arc

Courage Ring Arc 62,900 -

1989 N1R (‘Adams’) 62,930 <50 Named after John Couch Adams

Natural satellites

Neptune has 13 known moons. The largest by far, and the only one massive enough to be
spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of
Neptune itself. Unlike all other large planetary moons, Triton has a retrograde orbit,
indicating that it was captured, and probably represents a large example of a Kuiper Belt
object (although clearly no longer in the Kuiper Belt). It is close enough to Neptune to be
locked into a synchronous orbit, and is slowly spiraling inward and eventually will be
torn apart when it reaches the Roche limit. Triton is the coldest object that has been
measured in the solar system, with temperatures of 38.15K (-235°C, -392°F).

Triton, compared to Earth’s Moon

Name Diameter Mass Orbital radius Orbital period
(km) (kg) (km) (days)
(Pronunciation
key) 354,800 -5.877
(90% Luna) (20% Luna)
trye’-tən 2700 2.15×1022
Triton < traɪtən (80% Luna) (30% Luna)

Neptune’s second known satellite, the irregular moon Nereid, has one of the most
eccentric orbits of any satellite in the solar system.

From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these,
the irregularly shaped Proteus is notable for being as large as a body of its density can be
without being pulled into a spherical shape by its own gravity. Although the second most
massive Neptunian moon, it is only one quarter of one percent of the mass of Triton.
Neptune’s innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close
enough to be within Neptune’s rings. The next farthest out, Larissa was originally
discovered in 1981 when it had blocked a star. This was attributed to ring arcs, but when
Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon.
Five new irregular moons discovered between 2002 and 2003 were announced in 2004.
As Neptune was the Roman god of the sea, the planet’s moons have been named after
lesser sea gods.

Appearance and visibility from Earth

Neptune is never visible with the naked eye. The brightness of Neptune is between
magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With
the use of a telescope it appears as a small blue-green disk, similar in appearance to
Uranus; the blue-green colour comes from the methane in its atmosphere. Its small

apparent size has made it almost impossible to study visually; even observatory data was
fairly poor until the advent of adaptive optics.

With an orbital period of 165 years, Neptune will soon return to the approximate position
in the sky where Galle discovered it. This will happen three different times. These are
April 11, 2009, when it will be in prograde motion; July 17, 2009, when it will be in
retrograde motion; and finally for the last time for the next 165 years, on February 7,
2010. This is explained by the concept of retrogradation. Like all planets in the solar
system beyond Earth, Neptune undergoes retrogradation at certain points during its
synodic period. In addition to the start of retrogradation, other events within the synodic
period include astronomical opposition, the return to prograde motion, and conjunction to
the Sun.

In its orbit around the Sun, Neptune will return to its original point of discovery in
August 2011.

Pluto

Pluto

Discovery

Discovered by Clyde W. Tombaugh

Discovered on February 18, 1930

Orbital characteristics (Epoch J2000)

Semi-major axis 5,906,376,272 km
39.481 686 77 AU

Orbital circumference 36.530 Tm
244.186 AU

Eccentricity 0.248 807 66

Perihelion 4,436,824,613 km
29.658 340 67 AU

Aphelion 7,375,927,931 km
49.305 032 87 AU

Orbital period 90,613.3055 d
(248.09 a)

Synodic period 366.73 d

Avg. Orbital Speed 4.666 km/s

Max. Orbital Speed 6.112 km/s

Min. Orbital Speed 3.676 km/s

Inclination 17.141 75°
(11.88° to Sun’s equator)

Longitude of the 110.303 47°
ascending node

Argument of the 113.763 29°
perihelion

Number of satellites 3

Physical characteristics

Diameter 2390 km
Surface area (19% of Earth, or
Volume 1485 mi)
Mass
1.795×107 km²
(0.033 Earths)

7.15×109 km³
(0.0066 Earths)

(1.305±0.007)×1022 kg
(0.0021 Earths)

Mean density 2.03±0.06 g/cm³

Equatorial gravity 0.58 m/s²
(0.059 gee)

Escape velocity 1.2 km/s

Rotation period −6.387230 d
(6 d 9 h 17 m 36 s)

Rotation velocity 47.18 km/h (at the equator)

Axial tilt 119.59° (to orbit)
112.78° (to the ecliptic)

Right ascension 133.045±0.02°
of North pole (8 h 52 min 11 s)

Declination -6.145±0.02°

Albedo 0.49–0.66 (varies by 35%)

Surface temp. min mean max
33 K 44 K 55 K

Adjective Plutonian

Atmospheric characteristics

Atmospheric pressure 0.30 pascals (summer maximum)

Composition nitrogen, methane
.

Pluto (IPA: / plu təʊ/), designated (134340) Pluto in the Minor Planet Center
catalogue, is the second-largest known dwarf planet in the solar system. It orbits between
29 and 49 AU from the Sun, and was the first Kuiper Belt Object to be discovered.
Approximately one-fifth the mass of the Earth’s Moon, Pluto is primarily composed of
rock and ice. It has an eccentric orbit that is highly inclined with respect to the planets
and takes it closer to the Sun than Neptune during a portion of its orbit. Pluto and its
largest satellite, Charon, could be considered a binary system because they are closer in
size than any of the other known planetoid/moon combinations in the solar system, and
because the barycentre of their orbits does not lie within either body. However, the
International Astronomical Union (IAU) has yet to formalize a definition for binary
dwarf planets, so Charon is regarded as a moon of Pluto. Two smaller moons, Nix and
Hydra, were discovered in 2005. Pluto is smaller than several of the natural satellites or
moons in our solar system (see the list of solar system objects by radius).

From its discovery by Clyde Tombaugh in 1930, Pluto was considered the ninth planet
from the Sun. In the late 20th and early 21st century, many similar objects were discovered
in the outer solar system, most notably the Trans-Neptunian object Eris which is slightly
larger than Pluto. In August 2006 the IAU redefined the term “planet”, and classified
Pluto, Ceres, and Eris as dwarf planets. Pluto is also classified as the prototype of a family
of trans-Neptunian objects. After the reclassification, Pluto was added to the list of
minor planets and given the number 134340.

Discovery

Discovery photographs of Pluto

In 1930 Clyde Tombaugh was working on a project searching for a ninth planet at Lowell
Observatory. Tombaugh’s work was to systematically take pictures of the celestial sky in
pairs, one to two weeks apart, then look for objects that had moved between images. On
February 18, 1930, Tombaugh discovered a possible moving object on photographic
plates taken on January 23 and January 29 of that year. A lesser-quality photo taken on
January 20 helped confirm the movement. After the observatory worked to obtain further
confirmatory photographs, news of the discovery was telegraphed to the Harvard College
Observatory on March 13, 1930. Pluto would later be found on photographs dating back
to March 19, 1915.

Relations to Neptune and Uranus

The history of how Pluto was discovered is intertwined with the discoveries of Neptune
and Uranus. In the 1840s, using Newtonian mechanics, Urbain Le Verrier, and John
Couch Adams had correctly predicted the position of the then-undiscovered planet
Neptune after analysing perturbations in the orbit of Uranus. Theorizing the perturbations
were caused by the gravitational pull of another planet, Johann Gottfried Galle discovered
Neptune on September 23, 1846.

Observations of Neptune in the late 19th century had astronomers starting to speculate that
Neptune’s orbit too was also being disturbed by another planet in a similar manner that
Neptune was disturbing Uranus. By 1909, William H. Pickering and Percival Lowell had
suggested several possible celestial coordinates for such a planet. In May 1911, the
Bulletin of the Astronomical Society of France published calculations by Indian
astronomer V.B. Ketakar which predicted a location for an undiscovered planet.

Percival Lowell’s influence

Percival Lowell would have significant influence on Pluto’s discovery. In 1905, Lowell
Observatory (founded by Lowell in 1894) started an extensive project in search of a

possible ninth planet. The work continued after Lowell’s death in 1916. Lowell was
searching for a theoretical Planet X to match observations seen in Uranus and Neptune.

Pluto is too small to have the effect on Neptune’s orbit that initiated the search. After the
flyby of Neptune by Voyager 2 in 1989, it was conclusively demonstrated that the
discrepancies in Neptune’s orbit observed by 19th century astronomers were due instead to
inaccurate estimates of Neptune’s mass. Once found, Pluto’s faintness and lack of a
visible disk cast doubt on the idea that it could be Percival Lowell’s Planet X. Lowell had
made a prediction of Pluto’s position in 1915 which was fairly close to its actual position
at that time; however, Ernest W. Brown concluded almost immediately that this was a
coincidence, and this view is still held today. Tombaugh’s discovery is therefore even
more surprising, given that Pluto’s proximity to the region predicted by Pickering,
Lowell, and Ketakar was likely a mere coincidence.

Naming

Venetia Burney, the girl who named Pluto

The right to name the new object belonged to the Lowell Observatory and its director,
Vesto Melvin Slipher. Tombaugh urged Slipher to suggest a name quickly for the new
object before someone else did. Name suggestions poured in from all over the world.
Constance Lowell, Percival Lowell’s widow, proposed Zeus, then Lowell, and finally her
own first name, none of which met with any enthusiasm. Mythological names, such as
Cronus and Minerva, were high on a list of considered names.

The name Pluto was first suggested by Venetia Phair (née Burney), at the time an eleven-
year-old girl from Oxford, England. Venetia, who was interested in Classical mythology
as well as astronomy, suggested the name, the Roman equivalent of Hades, in a
conversation to her grandfather Falconer Madan, a former librarian of Oxford
University’s Bodleian Library. Madan passed the suggestion to Professor Herbert Hall
Turner, Turner then cabled the suggestion to colleagues in America. After favourable
consideration which was almost unanimous,[citation needed] the name Pluto was officially
adopted and an announcement made by Slipher on March 5, 1930.

The name retained for the object is that of the Roman god Pluto, and it is also intended to

evoke the initials of the astronomer Percival Lowell. In the Chinese, Japanese, and

Korean languages, the name was translated as death king star ( ), suggested by

Houei Nojiri in 1930. China started official use of the name in 1933. Japan used the

pronunciation (purūtō); later Tokyo Observatory decided to adopt it, and it

became the official name in Japan in 1943. In Vietnamese it is named after Yama (Sao

Diêm Vương), the Guardian of Hell in Buddhist mythology.

Symbol

Pluto’s astronomical symbol is a P-L monogram, . This represents both the first two
letters of the name Pluto and the initials of Percival Lowell, who had searched
extensively for a ninth planet and who had founded Lowell Observatory, the observatory
from which Tombaugh discovered Pluto. Besides its astronomical symbol Pluto also has

an astrological symbol. Pluto’s astrological symbol resembles that of Neptune ( ), but

has a circle in place of the middle prong of the trident ( ).

Physical characteristics

Diagram of Pluto (top left) and its moons (top right) compared in size, albedo and color
index with the largest plutinos: Orcus (bottom left) and Ixion (bottom right).

Many details about Pluto remain unknown, mainly due to the fact that it has not been
visited up close by spacecraft. Pluto’s distance from Earth makes in-depth investigation
difficult.

Appearance

Pluto’s apparent magnitude is fainter than 14 m and therefore a telescope is required for
observation. To see it, a telescope of around 30 cm aperture is desirable. It looks star-like
even in very large telescopes because its angular diameter is only 0.15. The color of Pluto
is light brown with a very slight tint of yellow.

Charon’s discovery resulted in the calculation of Pluto’s albedo’s being revised upward;
since Pluto was now seen as being far smaller than originally estimated, its capacity to
reflect light must be greater than formerly believed. Current estimates place Pluto’s
albedo as marginally less than that of Venus, which is fairly high.

Distance and limits on telescope technology make it currently impossible to directly
photograph surface details on Pluto. Images from the Hubble Space Telescope barely
show any distinguishable surface definitions or markings. The best images of Pluto
derive from brightness maps created from close observations of eclipses by its largest
moon, Charon. Using computer processing, observations are made in brightness factors
as Pluto is eclipsed by Charon. For example, eclipsing a bright spot on Pluto makes a
bigger total brightness change than eclipsing a gray spot. Using this technique, one can
measure the total average brightness of the Pluto-Charon system and track changes in
brightness over time.

Mass and size

Pluto’s volume is about 0.66% that of Earth’s

Pluto’s diameter and mass were incorrectly overestimated for many decades after its
discovery. Initially it was thought to be relatively large, with a mass comparable to Earth,
but over time the estimates were revised sharply downward as observations were refined.

The discovery of its satellite Charon in 1978 enabled a determination of the mass of the
Pluto-Charon system by application of Newton’s formulation of Kepler’s third law.
Originally it was believed that Pluto was larger than Mercury but smaller than Mars, but
that calculation was based on the premise that a single object was being observed.Once it
was realized that there were two objects instead of one, the estimated size of Pluto was
revised downward. Observations were able to determine Pluto’s diameter when it is at
occultation with Charon, and its shape can be resolved by telescopes using adaptive
optics.

Pluto (bottom right) compared in size to the largest moons in the solar system (from left
to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton.

Among the objects of the Solar System, Pluto is not only smaller and much less massive
than any planet, but at less than 0.2 lunar masses it is also smaller and less massive than
seven of the moons: Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton. Pluto is
more than twice the diameter and a dozen times the mass of Ceres, a dwarf planet in the
asteroid belt. However, it is smaller than trans-Neptunian Kuiper belt object Eris,
discovered in 2005. See List of solar system objects by mass and List of solar system
objects by radius.

Atmosphere

Pluto does not have a significant atmosphere. It has a thin envelope of gas that is most
likely made up of nitrogen, methane, and carbon monoxide, that develops in equilibrium
with solid nitrogen and carbon monoxide ices on the surface as it approaches the Sun. As
Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere
freezes and falls to the ground. When it returns to a closer proximity to the Sun, the
temperature of Pluto’s solid surface will increase, causing the nitrogen ice to sublimate
into gas—creating an anti-greenhouse effect. Much as sweat evaporating from the surface
of human skin, this sublimation has a cooling effect and scientists have recently
discovered, by use of the Submillimeter Array, that Pluto’s temperature is 10 kelvins less
than they expected.

Pluto was found to have an atmosphere from an occultation observation in 1985 (IAU
Circ. 4097; MNRAS 276, 571); the finding was confirmed and significantly strengthened
by extensive observations of another occultation in 1988. When an object with no
atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star
dimmed out gradually. From the rate of dimming, the atmosphere was determined to have
a pressure of 0.15 Pa, roughly 1/700,000 that of Earth.

In 2002, another occultation of a star by Pluto was observed and analyzed by teams led
by Bruno Sicardy of the Paris Observatory and by Jim Elliot of MIT and Jay Pasachoff of
Williams College. Surprisingly, the atmosphere was estimated to have a pressure of 0.3
Pa, even though Pluto was further from the Sun than in 1988, and hence should be colder
and have a less dense atmosphere. The current best hypothesis is that the south pole of

Pluto came out of shadow for the first time in 120 years in 1987, and extra nitrogen
sublimated from a polar cap. It will take decades for the excess nitrogen to condense out
of the atmosphere.

The MIT-Williams College team of Elliot and Pasachoff and a Southwest Research
Institute team led by Leslie Young observed a further occultation of a star by Pluto on 12
June 2006 from sites in Australia.

Composition

The surface of Pluto is remarkably heterogeneous, as evidenced by its lightcurve, maps of
its surface constructed from Hubble Space Telescope observations, and by periodic
variations in its infrared spectra. The face of Pluto oriented toward Charon has more
methane ice, while the opposite face has more ices of nitrogen and carbon monoxide.
This makes Pluto the second most contrasted body in the Solar System after Iapetus.

Orbit

Orbit of Pluto – ecliptic view. This ‘side view’ of Pluto’s orbit (in red) shows how
steeply inclined the orbit is in comparison to Neptune’s more normal orbit (in blue)

Pluto’s orbit is very unusual in comparison to the planets of the solar system. The planets
orbit the Sun close to an imaginary flat plane called the plane of the ecliptic, and have
nearly circular orbits. In contrast, Pluto’s orbit is highly inclined above the ecliptic (up to
17° above it) and very eccentric (non-circular). Owing to the orbit’s inclination, Pluto’s
perihelion is well above (~8.0 AU) the ecliptic. The high eccentricity means that part of
Pluto’s orbit is closer to the Sun than Neptune’s.

Heliocentric distance

Orbit of Pluto – polar view. This ‘view from above’ shows how Pluto’s orbit (in red) is
less circular than Neptune’s (in blue), and also shows how Pluto is sometimes closer to
the Sun than Neptune. The darker halves of both orbits show where they pass below the
plane of the ecliptic. The positions of both are marked as of April 16, 2006; in April 2007
they will have changed by about 1 pixel.

Near perihelion, Pluto gets closer to the Sun than Neptune; the most recent occurrence of
this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical
calculations indicate that the previous occurrence lasted only fourteen years from July 11,
1735 to September 15, 1749. However, the same calculations indicate that Pluto was
closer to the Sun than Neptune between April 30, 1483 and July 23, 1503, which is almost
exactly the same length as the 1979 to 1999 period. Recent studies suggest each
crossing of Pluto to inside Neptune’s orbit lasts alternately for approximately thirteen and
twenty years with minor variations.

Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto
from behind their gravity starts to pull on each other slightly, resulting in an interaction
between their positions in orbit of the same sort that produces Trojan points. Since the
orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always
passes Pluto when they are almost farthest apart. Half a Pluto orbit later, when Pluto is
nearing its closest approach, it initially seems as if Neptune is about to catch up with
Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead
of Neptune, and pulls ahead until they meet again on the other side of Pluto’s orbit.

Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered, and a
certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this
orbital resonance are named “plutinos”, after Pluto.

Trans-Neptunian object

This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected
dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance
to facilitate comparison. The closest approach is in 1896.

Pluto’s orbit is often described as ‘crossing’ that of Neptune. In fact, Pluto’s nodes (the
points at which the orbit crosses the ecliptic) are both situated outside Neptune’s orbit and
are separated by a distance of 6.4 AU (that is, over six times the distance of the Earth
from the Sun). Furthermore, due to the orbital resonance between them, Pluto executes 2
full cycles while Neptune makes 3; this means that when Neptune reaches the ‘closest’
point on the orbit, Pluto remains far behind and when Pluto in turn reaches that point,
Neptune is far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an
orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this point in
its orbit.

The actual closest approach between Neptune and Pluto occurs at the opposite part of the
orbit, some 30 years after Pluto’s aphelion (its last aphelion was in 1866) when Neptune
catches up with Pluto (i.e. Neptune and Pluto have similar longitudes). The minimum
distance was 18.9 AU in June 1896. In other words, Pluto never approaches Neptune
much closer than it approaches Saturn.

Comet comparison

The Kuiper belt is believed to be the source for all short-period comets, and Pluto, like
other Kuiper Belt objects, shares features in common with comets. The solar wind is
gradually blowing Pluto’s surface into space, in the manner of a comet. If Pluto were
placed near the Sun, it would develop a tail, like comets do.

Moons

Main article: Pluto’s natural satellites

Pluto and its three known moons. Pluto and Charon are the bright objects in the center,
the two smaller moons are at the right and bottom, farther out.

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer
James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.

Charon

The Pluto-Charon system is noteworthy for being the largest of the few binary systems in
the solar system, i.e. the barycenter lies above the primary’s surface (617 Patroclus is a
smaller example). This and the large size of Charon relative to Pluto lead some
astronomers to call it a dwarf double planet. The system is also unusual among planetary
systems in that they are both tidally locked to each other: Charon always presents the
same face to Pluto, and Pluto also always presents the same face to Charon.

Some researchers have theorized that Pluto and Charon were moons of Neptune that were
knocked out of Neptunian orbit when Triton was captured.[citation needed] Triton, the largest
moon of Neptune, which shares many atmospherical and geological composition
similarities with Pluto, may once have been a Kuiper belt object in a solar orbit. Today it
is widely accepted that Pluto never orbited Neptune. [citation needed]

Pluto and Charon, compared to Earth’s Moon

Name Diameter Mass Orbital Orbital
(Pronunciation key) (km) (kg) radius (km) period
(days)

Pluto ploo’-toe 2306 1.3×1022 2390 6.3872
Charon
/ plu təʊ/ (65% Moon) (18% Moon) (0.6% Moon) (25% Moon)

shair’-ən 1205 1.5×1021 19,570
(5% Moon)
(35% Moon) (2% Moon)

/ ʃɛərən/

Nix and Hydra

Diagram of the Plutonian system. P 1 is Hydra, and P 2 is Nix.

Two additional moons of Pluto were imaged by astronomers working with the Hubble
Space Telescope on May 15, 2005, and received provisional designations of S/2005 P 1
and S/2005 P 2. The International Astronomical Union officially christened Pluto’s
newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra
(Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three times the distance of
Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometers from the barycenter of
the system. They have nearly circular prograde orbits in the same orbital plane as Charon,
and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with
Charon.

Observations of Nix and Hydra are ongoing to determine individual characteristics.
Hydra is sometimes brighter than Nix, speculating that it either is larger in dimension or
different parts of its surface may vary in brightness. Sizes are estimated from albedos.
The moons’ spectral similarity with Charon suggests a 35% albedo similar to Charon’s;
this results in diameter estimates of 46 kilometers for Nix and 61 kilometers for brighter
Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of
the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km
respectively. At the larger end of this range, the inferred masses are less than 0.3% of
Charon’s mass, or 0.03% of Pluto’s.

With the discovery of the two small moons, Pluto may possess a variable ring system.
Small body impacts can create debris that can form into a ring system. Data from a deep
optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope

suggests that no ring system is present. If such a system exists, it is either tenuous like the
Rings of Jupiter, or it is tightly confined to less than 1000km in width.

Distribution

Artist’s concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on
left).
The distribution of Plutonian moons is highly unusual compared to other observed
systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill
sphere radius (stable gravitational zone of influence) of 6.0 million kilometers. In simple
terms, an imaginary sphere is drawn around an object to represent the potential of an
object to have other objects orbit it stably. For example, Psamathe orbits Neptune at 40%
of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be
occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be
“highly compact and largely empty.”

Additional moons?

In imaging the Plutonian system, observations from Hubble placed limits on any
additional moons. With 90% confidence, no additional moons larger than 12 km (or a
maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds
from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence
level the limit is 8 kilometers.

Exploration of Pluto

Main article: New Horizons

Photo of New Horizons, the first probe to Pluto, launched on January 19, 2006 (it is
planned to reach Pluto in July 2015)

Pluto presents significant challenges for space craft because of its small mass and great
distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for
a close flyby of Saturn’s moon Titan, which resulted in a trajectory incompatible with a
Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. In 2000,
NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch
vehicle delays.

The first spacecraft to visit Pluto will be NASA’s New Horizons, launched on January 19,
2006. The craft will benefit from a gravity assist from Jupiter, and the closest approach to
Pluto will be on July 14, 2015. Observations of Pluto will begin 5 months prior to closest
approach and will continue for at least a month after the encounter.

New Horizons will use a remote sensing package that includes imaging instruments and a
radio science investigation tool, as well as spectroscopic and other experiments, to
characterize the global geology and morphology of Pluto and its moon Charon, map their
surface composition and characterize Pluto’s neutral atmosphere and its escape rate. New
Horizons will also photograph the surfaces of Pluto and Charon. The ashes of Pluto’s
discoverer, Clyde W. Tombaugh, are aboard the spacecraft.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe.
With the relatively low escape velocity of Nix and Hydra, collisions with Kuiper belt
debris may produce a tenuous dusty ring. Were New Horizons to fly through such a ring
system, there would be an increased potential for micrometeorite damage that could
damage or disable the probe.

Planetary status controversy

Pluto’s official status as a planet has been a constant subject of controversy, fueled by the
past lack of a clear definition of planet, since at least as early as 1992, when the first
Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further discoveries
intensified the debate in the 21st century.

Omission from museum models

Museum and planetarium directors occasionally created controversy by omitting Pluto
from planetary models of the solar system. Some omissions were intentional; the Hayden
Planetarium reopened after renovation in 2000 with a model of 8 planets without Pluto.
The controversy made headlines in the media at the time.

Commemoration as a planet

Pluto is shown as a planet on the Pioneer plaque, an inscription on the space probes
Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give
information about the origin of the probes to any alien civilization that might in the future
encounter the vehicles, includes a diagram of our solar system, showing nine planets.
Similarly, an analog image contained within the Voyager Golden Record included on the
probes Voyager 1 and Voyager 2 (also launched in the 1970s) includes data regarding
Pluto and again shows it as the ninth planet.
Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after
the planets Uranus, Neptune, and Pluto.

New discoveries ignite debate

Pluto compared to Eris, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, and Varuna
compared to Earth (artist’s impressions; no detailed photographs exist).

Continuing advances in telescope technology allowed for further discoveries of Trans-
Neptunian objects in the 21st century, some of comparable size to that of Pluto. In 2002,
50000 Quaoar was discovered, with a 1,280 kilometers diameter, making it a bit more
than half the size of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit
of 1,800 kilometers on its diameter, near Pluto’s diameter of 2,320 kilometers.

On July 29, 2005, a Trans-Neptunian object called Eris was announced, which on the
basis of its magnitude and simple albedo considerations is assumed to be slightly larger
than Pluto. This was the largest object discovered in the solar system since Neptune in
1846. Discoverers and media initially called it the “tenth planet”, although there was no
official consensus at the time on whether to call it a planet. Others in the astronomical
community considered the discovery to be the strongest argument for reclassifying Pluto
as a minor planet.

The last remaining distinguishing feature of Pluto was now its large moon, Charon, and
its atmosphere; these characteristics are probably not unique to Pluto: several other
Trans-Neptunian objects have satellites; and Eris’ spectrum suggests that it has a similar
surface composition to Pluto, as well as a moon, Dysnomia, discovered in September
2005. Trans-Neptunian object 2003 EL61 (nicknamed “Santa”) has two moons (one of
which is nicknamed “Rudolph”) and is the fourth largest TNO behind Eris, Pluto, and
2005 FY9 (nicknamed “Easterbunny”).

IAU Decision

There are three main conditions for an object to be called a ‘planet’, according to the IAU
resolution passed August 24, 2006.

1. The object must be in orbit around the Sun.
2. The object must be massive enough to be a sphere by its own gravitational force.

More specifically, its own gravity should pull it into a shape of hydrostatic
equilibrium.
3. It must have cleared the neighborhood around its orbit.

Pluto fails to meet the third condition.

The IAU further resolved that Pluto be classified in the simultaneously created dwarf
planet category, and that it act as prototype for a yet-to-be-named category of trans-
Neptunian objects, in which it would be separately, but concurrently, classified.

Prior to this decision several other definitions had been proposed, some of which might
have ruled out planetary status for Earth or Mercury or may have classified several of the
asteroids as planets. This version was democratically chosen in a successful attempt at
avoiding these non-traditional results.

Impact of the IAU decision

There has been resistance amongst the astronomical community towards the
reclassification. Alan Stern, principal investigator with NASA’s “New Horizons” mission
to Pluto, has publicly derided the IAU resolution, stating that “the definition stinks” albeit
“for technical reasons.” Stern’s current contention is that by the terms of the new
definition Earth, Mars, Jupiter and Neptune, all of which share their orbits with asteroids
would be excluded. However, his own published writing has supported the new list of
planets, as “our solar system clearly contains” eight planets that have cleared their
neighborhoods. Others have supported the IAU. Mike Brown, the astronomer who
discovered Eris, said “through this whole crazy circus-like procedure, somehow the right
answer was stumbled on. It’s been a long time coming. Science is self-correcting
eventually, even when strong emotions are involved.”

Among the general public, reception is mixed amidst widespread media coverage. Some
have accepted the reclassification, while some are seeking to overturn the decision, with
online petitions urging the IAU to consider reinstatement. A resolution introduced by
some members of the California state assembly light-heartedly denounces the IAU for
“scientific heresy,” among other crimes. Others reject the change for sentimental reasons,
citing that they have always known Pluto as a planet and will continue to do so regardless
of the IAU decision

Eris (dwarf planet)

Eris

Eris (center) and Dysnomia (right of center).
Keck Observatory.

Official name 136199 Eris

Discovery A

Discoverers M. E. Brown,
C. A. Trujillo,
Discovery date D. L. Rabinowitz
Alternate
October 21, 2003
designations B
2003 UB313

Category Trans-Neptunian object
(Scattered disc object)

Orbital elements C

Epoch March 6, 2006 (JD 2453800.5)

Semi-major axis (a) 67.6681 AU (10.12 Tm)

Eccentricity (e) 0.44177

Perihelion (q) 37.77 AU (5.65 Tm)

Aphelion (Q) 97.56 AU (14.60 Tm)

Orbital period (P) 203,500 d (557 a)

Mean orbital speed 3.436 km/s

Inclination (i) 44.187°

Longitude of the 35.8696°
ascending node (Ω)

Argument of 151.4305°
perihelion (ω)

Mean anomaly (M) 197.63427°

Physical characteristics D

Diameter 2400 km ± 100 km
Mass (1500 mi ± 60 mi,
or 19% of Earth)

?×10? kg

Density ? g/cm³

Surface gravity ? m/s²

Escape velocity ? km/s

Rotation period > 8 h?

Spectral class ?
Absolute magnitude −1.12 ± 0.01

Albedo (geometric) 0.86 ± 0.07

Mean surface ~30 K
temperature

Eris (IPA pronunciation / iɹɪs/ or / ɛɹɪs/), also designated (136199) Eris or 136199
Eris (See Minor planet names), is the largest known dwarf planet in the solar system. It is
a trans-Neptunian object (TNO), orbiting the Sun in a region of space known as the

scattered disc, just beyond the Kuiper belt, and accompanied by at least one moon,
Dysnomia. Mike Brown, who led the Mount Palomar-based discovery team, announced
in April 2006 that the Hubble Telescope has measured Eris’s diameter to be 2400 km,
slightly larger than that of Pluto.

Eris’ size resulted in its discoverers and NASA labelling it the solar system’s tenth
planet. This, along with the prospect of other similarly sized objects being discovered in
the future, stimulated the International Astronomical Union (IAU) to define the term
“planet” more precisely. Under a new definition approved on August 24, 2006, Eris was
designated a “dwarf planet” along with Pluto and Ceres. Brown has since stated his
approval of the new “dwarf planet” label.

Discovery

Animation showing the movement of Eris on the images used to discover it. Eris is
located on the left side, slightly above the middle of the image. The three frames were
taken over a period of three hours.

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on
January 5, 2005, from images taken on October 21, 2003. The discovery was announced
on July 29, 2005, the same day as two other large TNOs, (136108) 2003 EL61 and
(136472) 2005 FY9. The search team has been systematically scanning for large outer
solar system bodies for several years, and had previously been involved in the discovery
of several other very large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377
Sedna.

Routine observations were taken by the team on October 21, 2003, using the 48-inch
(122 cm) Samuel Oschin reflecting telescope at Mount Palomar Observatory, California,
but the object captured on the images was not discovered at that point due to its very slow
motion across the sky: the team’s automatic image-searching software excluded all objects
moving at less than 1.5 arcseconds per hour to reduce the number of false positives
returned. When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light
of that the team reanalyzed their old data with a lower limit on the angular motion, sorting
through the previously excluded images by eye. In January 2005, the re-analysis revealed
Eris’ slow motion against the background stars.

Follow-up observations were then carried out to make a preliminary determination of its
orbit, which allowed its distance and size to be estimated. The team had planned to delay
announcing their discovery until further observations had been made which would have
allowed more accurate determinations of the body’s size and mass, but were forced to
bring forward the announcement when the discovery of another object they had been
tracking (2003 EL61) was announced by another group in Spain.
Yet more observations released in October 2005 revealed that the object had a moon,
Dysnomia, nicknamed “Gabrielle” at the time. Scientists plan to use this information to
determine the mass of Eris.

Classification

Distribution of trans-Neptunian Objects.

Eris is classified as a dwarf planet and a scattered disk object (SDO).The latter is a
category of the TNOs that are believed to have been “scattered” from the Kuiper belt into
more distant and unusual orbits following gravitational interactions with Neptune as the
solar system was forming. Although its high orbital inclination is unusual among the
known SDOs, theoretical models suggest that objects that were originally near the inner
edge of the Kuiper belt are scattered into orbits with higher inclinations than objects from
the outer belt. Inner-belt objects are expected to be generally more massive than outer-
belt objects, and so astronomers expect to discover more large objects like Eris in high-
inclination orbits.

As Eris is larger than Pluto, it was initially described as the “tenth planet” by NASA and
in media reports of its discovery. In response to the uncertainty over its status, and
because of continuing debate over whether Pluto should be classified as a planet, the IAU
delegated a group of astronomers to develop a new definition of the term planet. This
definition was clarified under the new IAU definition of a planet, adopted on 24 August
2006. Eris has been termed a dwarf planet by the IAU. It may also be under consideration

as a member of “a new class of trans-Neptunian objects” yet to be defined by that body. It
is not, however, considered to be a planet.

Name

Eris (Athenian painting, circa 550 BCE)

136199 Eris is named after the goddess Eris (Greek Ἔρις), a personification of strife and
discord. This name was assigned on September 13, 2006 following an unusually long
period in which it was best known by the provisional designation 2003 UB313, which was
granted automatically by the IAU under their naming protocols for minor planets.

Nicknames

Before the name Eris was granted, two nicknames were used for the planet by the popular
media.

• “Xena” was an informal name used by the discovery team. It was inspired by
eponymous heroine of the television series Xena: Warrior Princess. The
discovery team had reportedly saved the nickname ‘Xena’ for the first body they
discovered that was larger than Pluto. Their only stated reason was that “We have
always wanted to name something Xena” (apparently implying that the name was
chosen without any reference to Planet X).

• The nickname “Lila” has also been used, but this is a misunderstanding of
planetlila, part of the URL of the discovery web page; the web page’s name is
derived from Mike Brown’s daughter Lilah.

Choosing an official name

The delay in assigning a name was due to uncertainty over whether the object was
classified as a planet or a minor planet; different nomenclature procedures apply to these
different classes of object. The decision on a name had to wait until after the August 24,
2006 IAU ruling defining the object as a dwarf planet.

Brown had previously speculated that Persephone would be a good name for the object.
However, this was not possible once the object was classified as a dwarf planet, because

there is already an asteroid with that name (399 Persephone). Since IAU regulations
demand a name from a creation mythology for objects with orbital stability beyond
Neptune’s orbit, the team had also been considering such possibilities.
The discovery team proposed ‘Eris’ on 6 September 2006, and on 13 September 2006, it
was accepted as the official name by the IAU. The name in part reflects the discord in
the astronomical community caused by the debate over the object’s nature.

Orbit

The diagram illustrates the orbit of Eris (blue) compared to those of Pluto and the three
outermost planets (white/grey). The segments of orbits below the ecliptic are plotted in
darker colours, and the red dot is the Sun. The diagram on the left is a polar view while
the diagrams on the right are different views from the ecliptic.
Eris has an orbital period of 556.7 years, and currently lies at almost its maximum
possible distance from the Sun (aphelion). It is currently the most distant known solar
system object from the Sun at a distance of roughly 97 astronomical units. Its semimajor
axis is 67.669 AU, its perihelion distance is 37.78 AU, and its aphelion distance is 97.56
AU. Approximately forty known TNOs (most notably 2000 OO67 and Sedna), while
currently closer to the Sun than Eris, have greater average orbital distances.

Its orbit is highly eccentric, and brings it to within 37.8 AU of the Sun (a typical
perihelion for scattered objects), still safe from direct interaction with Neptune (at ~30
AU). For comparison, Pluto, like other plutinos, follows a less inclined and less eccentric
orbit and, protected by orbital resonance, it can cross Neptune’s orbit. Unlike the
terrestrial planets and gas giants, whose orbits all lie roughly in the same plane as the
Earth’s, Eris’ orbit is very inclined — it is tilted at an angle of about 44 degrees to the
ecliptic.

The object currently has an apparent magnitude of about 19, making it bright enough to
be detectable in some amateur telescopes. A telescope with an 8” lens or mirror and a
CCD can detect Eris under favorable conditions. The reason it had not been noticed until
now is because of its steep orbital inclination: most searches for large outer solar system
objects concentrate on the ecliptic plane, in which most solar system material is found.

Eris is now in the constellation Cetus. It was in Sculptor until 1929, and will enter Pisces
in 2036. Because the orbit of Eris is highly inclined, it only passes through a few
constellations of the traditional Zodiac.

Size

Optical measurement from HST pictures

The diameter of Eris has been measured to be 2400 km using images from the Hubble
Space Telescope. The brightness of an object depends both on its size and the amount of
light it reflects (its albedo). At a distance of 67 AU, an object with a radius of 3000 km
would have an angular size of 40 milliarcseconds, which is directly measurable with
HST; although resolving such small objects is at the very limit of Hubble’s capabilities,
sophisticated image processing techniques such as deconvolution can be used to measure
such angular sizes fairly accurately.

Eris compared to Pluto, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, Varuna, and Earth.

This revised estimate of the diameter makes Eris only 4% larger than Pluto According to
Hubble, Eris’ diameter measures 2,397 km (1,490 mi), give or take 100 km (60 mi). Pluto
is about 2,306 km (1,433 mi) across. It also indicates that the albedo is 0.86, higher than
any other large body in the solar system other than Enceladus. It is speculated that the
high albedo is due to the surface ices being replenished due to temperature fluctuations as
Eris’ eccentric orbit takes it closer and farther from the Sun.

Thermal measurement

Previous observations of the thermal emission of Eris at a wavelength of 1.2 mm, where
the object’s brightness depends only on temperature and surface area, indicated a
diameter of 3000+270 -100 km, about a third larger than Pluto. If the object rotates quickly,

resulting in a more even heat distribution and a temperature of 23 to 24 kelvins, a likely
diameter would be in the higher portion of the range (best fit 3090 km); if it rotates
slowly, the visible surface would be warmer (about 27 K) and a likely diameter would be
in the smaller end of the range (best fit 2860 km). The 2860 km figure implies a Pluto-
like albedo of 60%, consistent with its Pluto-like spectral signature.

Possible explanation of the inconsistent results

The apparent inconsistence of the HST PSF results (2400 ± 100 km) with the above
IRAM results (3000 ± 370 km) will be certainly studied in more length. Brown explains
it by a slightly lower absolute magnitude than the one assumed by Bertoldi (−1.12 ± 0.01
versus −1.16 ± 0.1, resulting by itself in almost 100 km difference in diameter).
Assuming further the highest diameter (2500 km) and pole-on position of the object the
difference between the results would appear consistent with 1.1-σ error margin.

Another possible explanation for the IRAM results is offered by the Max-Planck-Institut
für Radioastronomie. The ratio between the bolometric albedo (representing the total
reflected energy and used in the thermal method) and the geometric albedo (representing
the reflection in some visual wavelength and used to calculate the diameter from HST
pictures) is not known with high precision and depends on many factors. By itself, this
uncertainty could bridge the gap between the two measures.

Surface

The infrared spectrum of Eris, compared to that of Pluto, shows the marked similarities
between the two bodies. Arrows denote methane absorption lines.

The discovery team followed up their initial identification of Eris with spectroscopic
observations made at the 8 m Gemini North Telescope in Hawaiʻi on January 25, 2005.
Infrared light from the object revealed the presence of methane ice, indicating that the
surface of Eris is rather similar to Pluto, which was the only TNO already known to show
the presence of methane. Neptune’s moon Triton is probably related to Kuiper Belt
objects, and also has methane on its surface.

Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. Pluto’s
reddish color is believed to be due to deposits of tholins on its surface, and where these
deposits darken the surface, the lower albedo leads to higher temperatures and the
evaporation of methane deposits. In contrast, Eris is far enough away from the Sun that
methane can condense onto its surface even where the albedo is low. The condensation of
methane uniformly over the surface reduces any albedo contrasts and would cover up any
deposits of red tholins.

Methane is very volatile and its presence shows either that Eris has always resided in the
distant reaches of the solar system where it is cold enough for methane ice to persist, or
that it has an internal source of methane to replenish gas that escapes from its
atmosphere. This contrasts with observations of another recently discovered TNO, 2003
EL61, which reveal the presence of water ice but not methane.

Atmosphere

Even though Eris can be up to three times further from the Sun than Pluto, it approaches
close enough that some of the various ices that exist on the surface might become warm
enough to sublimate and form a fine atmosphere; however, it is unclear whether this
actually happens on Eris.

Due to its orbit, surface temperatures vary between about −232 and −248 degrees Celsius.

Moon

During 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out
observations of the four brightest TNOs (Pluto, 2005 FY9, 2003 EL61, and Eris), using the
newly commissioned laser guide star adaptive optics system. Observations taken on
September 10 revealed a moon in orbit around Eris, which received its name (Dysnomia)
at the same time as its primary. In keeping with the “Xena” nickname already in use for
Eris, the moon was previously nicknamed Gabrielle by its discoverers, after the television
warrior princess’s sidekick. The name Dysnomia is taken from a mythological dem
on of lawlessness who was Eris’ daughter. This is also an acknowledgement of the
former nicknames, as the character of Xena was played by Lucy Lawless

Asteroid

253 Mathilde, a C-type asteroid.

Asteroids, also called minor planets or planetoids, are a class of astronomical object. The
term asteroid is generally used to indicate a diverse group of small celestial bodies that
drift in the solar system in orbit around the Sun. Asteroid (Greek for “star-like”) is the
word used most in the English literature for minor planets, which has been the term
preferred by the International Astronomical Union; some other languages prefer
planetoid (Greek: “planet-like”), because it more accurately describes what they are. In
late August 2006, the IAU introduced the term “small solar system bodies” (SSSBs),
which includes most objects thusfar classified as minor planets, as well as comets. At the
same time they introduced the term dwarf planet for the largest minor planets. This
article deals specifically with the minor planets that orbit in the inner solar system
(roughly up to the orbit of Jupiter). For other types of objects, such as comets, Trans-
Neptunian objects, and Centaurs, see Small solar system body.

The first asteroid to be discovered in the Solar System, Ceres, is the largest asteroid
known to date and is now classified as a dwarf planet. All others are currently classified
as small solar system bodies. The vast majority of asteroids are found within the main
asteroid belt, with elliptical orbits between those of Mars and Jupiter. It is thought that
these asteroids are remnants of the protoplanetary disc, and in this region the
incorporation of protoplanetary remnants into the planets was prevented by large
gravitational perturbations induced by Jupiter during the formative period of the solar
system. Some asteroids have moons or are found in pairs known as binary systems.

Asteroids in the solar system

Left to right: 4 Vesta, 1 Ceres, Earth’s Moon.
Hundreds of thousands of asteroids have been discovered within the solar system and the
present rate of discovery is about 5000 per month. As of September 17, 2006, from a total
of 342,358 registered minor planets, 136,563 have orbits known well enough to be given
permanent official numbers. Of these, 13,479 have official names. The lowest-numbered
but unnamed minor planet is (3708) 1974 FV1.; the highest-numbered named minor
planet (other than the dwarf planet 136199 Eris) is 135268 Haignere.

Location of the Main Belt asteroids

Current estimates put the total number of asteroids above 1 km in diameter in the solar
system to be between 1.1 and 1.9 million. The largest asteroid in the inner solar system is
1 Ceres, with a diameter of 900-1000 km. Two other large inner solar system belt
asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. Vesta is the only
main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions,
a near-Earth asteroid may be visible without technical aid; see 99942 Apophis).
The mass of all the asteroids of the Main Belt is estimated to be about 3.0-3.6×1021 kg, or
about 4% of the mass of our moon. Of this, 1 Ceres comprises 0.95×1021 kg, some 32%
of the total. Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%),
and 10 Hygiea (3%), bring this figure up to 51%; while the three after that, 511 Davida

(1.2%), 704 Interamnia (1.0%), and 3 Juno (0.9%), only add another 3% to the total mass.
The number of asteroids then increases rapidly as their individual masses decrease.

Asteroid classification

Asteroids are commonly classified into groups based on the characteristics of their orbits
and on the details of the spectrum of sunlight they reflect.

Orbit groups and families

Many asteroids have been placed in groups and families based on their orbital
characteristics. It is customary to name a group of asteroids after the first member of that
group to be discovered. Groups are relatively loose dynamical associations, whereas
families are much “tighter” and result from the catastrophic break-up of a large parent
asteroid sometime in the past.

For a full listing of known asteroid groups and families, see minor planet and asteroid
family.

Spectral classification

This picture of 433 Eros shows the view looking from one end of the asteroid across the
gouge on its underside and toward the opposite end. Features as small as 35 m across can
be seen.

In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was
developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are
thought to correspond to the composition of the asteroid’s surface material. Originally,
they classified only three types of asteroids:

• C-type asteroids - carbonaceous, 75% of known asteroids
• S-type asteroids - silicaceous, 17% of known asteroids
• M-type asteroids - metallic, most of the remaining asteroids

This list has since been expanded to include a number of other asteroid types. The
number of types continues to grow as more asteroids are studied. See Asteroid spectral
types for more detail or Category:Asteroid spectral classes for a list.

Note that the proportion of known asteroids falling into the various spectral types does
not necessarily reflect the proportion of all asteroids that are of that type; some types are
easier to detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid’s composition:

• C - Carbonaceous
• S - Silicaceous
• M - Metallic

However, the correspondence between spectral class and composition is not always very
good, and there are a variety of classifications in use. This has led to significant
confusion. While asteroids of different spectral classifications are likely to be composed
of different materials, there are no assurances that asteroids within the same taxonomic
class are composed of similar materials.

At present, the spectral classification based on several coarse resolution spectroscopic
surveys in the 1990s is still the standard. Scientists have been unable to agree on a better
taxonomic system, largely due to the difficulty of obtaining detailed measurements
consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral
data such as densities would be very useful).

Asteroid discovery

243 Ida and its moon Dactyl, the first satellite of an asteroid to be discovered.

Historical discovery methods

Asteroid discovery methods have drastically improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24
astronomers to search the sky for the “missing planet” predicted at about 2.8 AU from the
Sun by the Titius-Bode law, partly as a consequence of the discovery, by Sir William
Herschel in 1781, of the planet Uranus at the distance “predicted” by the law. This task

required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to
an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again
and any moving object would, hopefully, be spotted. The expected motion of the missing
planet was about 30 seconds of arc per hour, readily discernable by observers.

Ironically, the first asteroid, 1 Ceres, was not discovered by a member of the group, but
rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in
Sicily. He discovered a new star-like object in Taurus and followed the displacement of
this object during several nights. His colleague, Carl Friedrich Gauss, used these
observations to determine the exact distance from this unknown object to the Earth.
Gauss’ calculations placed the object between the planets Mars and Jupiter. Piazzi named
it after Ceres, the Roman goddess of agriculture.

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few
years, with Vesta found in 1807. After eight more years of fruitless searches, most
astronomers assumed that there were no more and abandoned any further searches.

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in
1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also
found 6 Hebe less than two years later. After this, other astronomers joined in the search
and at least one new asteroid was discovered every year after that (except the wartime
year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de
Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson,
Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J.
Palisa, Paul Henry and Prosper Henry and Auguste Charlois.

In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids,
which appeared as short streaks on long-exposure photographic plates. This drastically
increased the rate of detection compared with previous visual methods: Wolf alone
discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than
300 had been discovered up to that point. Still, a century later, only a few thousand
asteroids were identified, numbered and named. It was known that there were many
more, but most astronomers did not bother with them, calling them “vermin of the skies”.

Modern discovery methods

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky
was photographed by a wide-field telescope (usually an Astrograph). Pairs of
photographs were taken, typically one hour apart. Multiple pairs could be taken over a
series of days. Second, the two films of the same region were viewed under a
stereoscope. Any body in orbit around the Sun would move slightly between the pair of
films. Under the stereoscope, the image of the body would appear to float slightly above
the background of stars. Third, once a moving body was identified, its location would be
measured precisely using a digitizing microscope. The location would be measured
relative to known star locations.

These first three steps do not constitute asteroid discovery: the observer has only found
an apparition, which gets a provisional designation, made up of the year of discovery, a
letter representing the week of discovery, and finally a letter and a number indicating the
discovery’s sequential number (example: 1998 FJ74).

The final step of discovery is to send the locations and time of observations to Brian
Marsden of the Minor Planet Center. Dr. Marsden has computer programs that compute
whether an apparition ties together previous apparitions into a single orbit. If so, the
object gets a number. The observer of the first apparition with a calculated orbit is
declared the discoverer, and he gets the honour of naming the asteroid (subject to the
approval of the International Astronomical Union) once it is numbered.

Latest technology: detecting hazardous asteroids

2004 FH is the centre dot being followed by the sequence; the object that flashes by
during the clip is a satellite.

There is increasing interest in identifying asteroids whose orbits cross Earth’s orbit, and
that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three
most important groups of near-Earth asteroids are the Apollos, Amors, and the Atens.
Various asteroid deflection strategies have been proposed.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s
brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862
Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of
the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the level of alarm: the increasing acceptance of
Walter Alvarez’ theory of dinosaur extinction being due to an impact event, and the 1994
observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also
declassified the information that its military satellites, built to detect nuclear explosions,
had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10
metres across.

All of these considerations helped spur the launch of highly efficient automated systems
that consist of Charge-Coupled Device (CCD) cameras and computers directly connected
to telescopes. Since 1998, a large majority of the asteroids have been discovered by such
automated systems. A list of teams using such automated systems includes:

• The Lincoln Near-Earth Asteroid Research (LINEAR) team
• The Near-Earth Asteroid Tracking (NEAT) team
• Spacewatch
• The Lowell Observatory Near-Earth-Object Search (LONEOS) team
• The Catalina Sky Survey (CSS)
• The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
• The Japanese Spaceguard Association
• The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 67,820 asteroids as of June 13, 2006. Between
all of the automated systems, 4076 near-Earth asteroids have been discovered including
over 600 more than 1 km in diameter.

Naming asteroids

Overview: naming conventions

A newly discovered asteroid is given a provisional designation consisting of the year of
discovery and an alphanumeric code (such as 2002 AT4). Once its orbit has been
confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The
formal naming convention uses parentheses around the number (e.g. (433) Eros), but
dropping the parentheses is quite common. Informally, it is common to drop the number
altogether, or to drop it after the first mention when a name is repeated in running text.

Asteroids that have been given a number but not a name keep their provisional
designation, e.g. (29075) 1950 DA. As modern discovery techniques are discovering vast
numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to
be left unnamed was (3360) 1981 VA. On rare occasions, an asteroid’s provisional
designation may become used as a name in itself: the still unnamed (15760) 1992 QB1
gave its name to a group of asteroids which became known as cubewanos.

Numbering asteroids

Asteroids are awarded with an official number once their orbits are confirmed. With the
increasing rapidity of asteroid discovery, asteroids are currently being awarded six-figure
numbers. The switch from five figures to six figures arrived with the publication of the
Minor Planet Circular (MPC) of October 19, 2005, which saw the highest numbered
asteroid jump from 99947 to 118161. This change caused a small “Y2k”-like crisis for

various automated data services, since only five digits were allowed in most data formats
for the asteroid number. Most services have now widened the asteroid number field. For
those which did not, the problem has been addressed in some cases by having the
leftmost digit (the ten-thousands place) use the alphabet as a digit extension. A=10,
B=11,…, Z=35, a=36,…, z=61. A high number such as 120437 is thus cross-referenced
as C0437 on some lists.

Sources for names

The first few asteroids were named after figures from Graeco-Roman mythology, but as
such names started to run out, others were used —famous people, literary characters, the
names of the discoverer’s wives, children, and even television characters.

The first asteroid to be given a non-mythological name was 20 Massalia, named after the
city of Marseilles. For some time only female (or feminized) names were used;
Alexander von Humboldt was the first man to have an asteroid named after him, but his
name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago
was named; even then, oddly feminised names show up in the list for years afterward.

As the number of asteroids began to run into the hundreds, and eventually the thousands,
discoverers began to give them increasingly frivolous names. The first hints of this were
482 Petrina and 483 Seppina, named after the discoverer’s pet dogs. However, there was
little controversy about this until 1971, upon the naming of 2309 Mr. Spock (which was
not even named after the Star Trek character, but after the discoverer’s cat who
supposedly bore a resemblance to him). Although the IAU subsequently banned pet
names as sources, eccentric asteroid names are still being proposed and accepted, such as
6042 Cheshirecat, 9007 James Bond, or 26858 Misterrogers.

Special naming rules

Asteroid naming is not always a free-for-all: there are some types of asteroid for which
rules have developed about the sources of names. For instance Centaurs (asteroids
orbiting between Saturn and Neptune) are all named after mythological centaurs, Trojans
after heroes from the Trojan War, and trans-Neptunian objects after underworld spirits.

Another well-established rule is that comets are named after their discoverer(s), whereas
asteroids are not. One way to “circumvent” this rule has been for astronomers to
exchange the courtesy of naming their discoveries after each other. A particular exception
to this rule is 96747 Crespodasilva, which was named after its discoverer, Lucy
d’Escoffier Crespo da Silva, because she sadly died shortly after the discovery, at age 22.

Asteroid symbols

The first few asteroids discovered were assigned symbols like the ones traditionally used
to designate Earth, the Moon, the Sun and planets. The symbols quickly became

ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids,
each (except one) with its own symbol. The first four’s main variants are shown here:

1 Ceres
2 Pallas
3 Juno
4 Vesta

Johann Franz Encke made a major change in the Berliner Astronomisches Jahrbuch
(BAJ, “Berlin Astronomical Yearbook”) for 1854. He introduced encircled numbers
instead of symbols, although his numbering began with Astraea, the first four asteroids
continuing to be denoted by their traditional symbols. This symbolic innovation was
adopted very quickly by the astronomical community. The following year (1855),
Astraea’s number was bumped up to 5, but Ceres through Vesta would be listed by their
numbers only in the 1867 edition. A few more asteroids (28 Bellona, 35 Leukothea, and
37 Fides) would be given symbols as well as using the numbering scheme.

The circle would become a pair of parentheses, and the parentheses sometimes omitted
altogether over the next few decades.

Asteroid exploration

Until the age of space travel, asteroids were merely pinpricks of light in even the largest
telescopes and their shapes and terrain remained a mystery.

The first close-up photographs of asteroid-like objects were taken in 1971 when the
Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are
probably captured asteroids. These images revealed the irregular, potato-like shapes of
most asteroids, as did subsequent images from the Voyager probes of the small moons of
the gas giants.

951 Gaspra, the first asteroid to be imaged in close up.

The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed
in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe
en route to Jupiter.

The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253
Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its
surface in 2001.

Other asteroids briefly visited by spacecraft en route to other destinations include 9969
Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in
detail and will return samples of its surface to earth. Following that, the next asteroid
encounters will involve the European Rosetta probe (launched in 2004), which will study
2867 Šteins and 21 Lutetia in 2008 and 2010.

NASA is planning to launch the Dawn Mission in 2007, which will orbit 1 Ceres and 4
Vesta in 2011-2015, with its mission possibly then extended to 2 Pallas.

It has been suggested that asteroids might be used in the future as a source of materials
which may be rare or exhausted on earth (asteroid mining).

Asteroids in fiction

A common depiction of asteroids (and less often, of Comets) in fiction is as a threat,
whose impact on Earth could result with incalculable damage and loss of life. This has a
basis in scientific hypotheses regarding such impacts in the distant past as responsible for
the extinction of the Dinosaurs and other past catastrophes —though, as they seem to
occur within tens of millions of years of each other, there is no special reason (other than
creating a dramatic story line) to expect a new such impact at any close millennium.

Another way in which asteroids could be considered a source of danger is by depicting
them as a hazard to navigation, especially threatening to ships travelling from Earth to the
outer parts of the Solar System and thus needing to pass the Asteroid Belt (or make a
time- and fuel-consuming detour around it). In this context, asteroids serve the same role
in space travel stories as reefs and underwater rocks in the older genre of sea-faring
adventure stories. And like reefs and rocks in the ocean, asteroids as navigation hazards
can also be used by bold outlaws to avoid pursuit. Representations of the Asteroid Belt in
film tend to make it unrealistically cluttered with dangerous rocks. In reality asteroids,
even in the main belt, are spaced extremely far apart.

Before colonization of the asteroids became an attractive possibility, a main interest in
them was theories as to their origin - specifically, the theory that the asteroids are
remnants of an exploded planet. This naturally leads to SF plotlines dealing with the
possibility that the planet had been inhabited, and if so - that the inhabitants caused its
destruction themselves, by war or gross environmental mismanagement. A further
extension is from the past of the existing asteroids to the possible future destruction of
Earth or other planets and their rendering into new asteroids.

When the theme of interplanetary colonization first entered SF, the Asteroid Belt was
quite low on the list of desirable real estate, far behind such planets as Mars and Venus
(often conceived as a kind of paradise planet, until probes in the 1960s revealed the
appalling temperatures and conditions under its clouds). Thus, in many stories and books
the Asteroid Belt, if not a positive hazard, is still a rarely-visited backwater in a colonized
Solar System.

The prospects of colonizing the Solar System planets became more dim with increasing
discoveries about conditions on them. Conversely, the potential value of the asteroids
increased, as a vast accumulation of mineral wealth, accessible in conditions of minimal
gravity, and supplementing Earth’s dwindling resources. Stories of asteroid mining
became more and more numerous since the late 1940s, with the next logical step being
depictions of a society on terraformed asteroids —in some cases dug under the surface, in
others having dome colonies and in still others provided with an atmosphere which is
kept in place by an artificial gravity. An image developed and was carried from writer to
writer, of “Belters” or “Rock Rats” as rugged and independent-minded individuals,
resentful of all Authority (in some books and stories of the military and political power of
Earth-bound nation states, in others of the corporate power of huge companies). As such,
this sub-genre proved naturally attractive to writers with Libertarian tendencies.
Moreover, depictions of the Asteroid Belt as The New Frontier clearly draw (sometimes
explicitly) on the considerable literature of the Nineteenth-Century Frontier and the Wild
West.

Meteoroid

A meteoroid is a relatively small (sand- to boulder-sized) fragment of debris in the Solar
System. When entering a planet’s atmosphere, the meteoroid is heated up by friction and
partially or completely vaporizes. The gas along the path of the meteoroid becomes
ionized and glows. The trail of glowing vapor is called a meteor, or a shooting star. If any
portion of the meteoroid survives to reach the ground, it is then referred to as a meteorite.

The current International Astronomical Union (IAU) definition dates back to the XIth
General Assembly, held in 1961:

“A solid object moving in interplanetary space, of a size considerably smaller than an
asteroid and considerably larger than an atom or molecule.”

As a result of the inexorable progress of instrumentation, this definition is now deemed
by many as unacceptably vague. The most common definition was proposed in 1995 and
sets the size limits of meteoroids to between 100 µm and 10 m across. Larger than that,
the object is an asteroid; smaller than that, it is interplanetary dust. Oddly enough,

because it is thought than an object must be about 50 m across before it can survive
atmospheric entry, this means terrestrial meteorites never come from meteoroids.

Comet

Comet Hale-Bopp

A comet is a small body in the solar system that orbits the Sun and (at least occasionally)
exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar
radiation upon the comet’s nucleus, which itself is a minor body composed of rock, dust,
and ices. Comets’ orbits are constantly changing: their origins are in the outer solar
system, and they have a propensity to be highly affected (or perturbed) by relatively
close approaches to the major planets. Some are moved into sungrazing orbits that
destroy the comets when they near the Sun, while others are thrown out of the solar
system forever.
Most comets are believed to originate in a cloud (the Oort cloud) at large distances from
the Sun consisting of debris left over from the condensation of the solar nebula; the outer
edges of such nebulae are cool enough that water exists in a solid (rather than gaseous)
state. Asteroids originate via a different process, but very old comets which have lost all
their volatile materials may come to resemble asteroids.
The word comet came to the English language through Latin cometes. From the Greek
word komē, meaning “hair of the head,” Aristotle first used the derivation komētēs to
depict comets as “stars with hair.”

Physical characteristics

Long-period comets are believed to originate in a distant cloud known as the Oort cloud
(after the astronomer Jan Hendrik Oort who hypothesised its existence). They are
sometimes perturbed from their distant orbits by gravitational interactions, falling into
extremely elliptical orbits that can bring them very close to the Sun. One theory says that
as a comet approaches the inner solar system, solar radiation causes part of its outer
layers, composed of ice and other materials, to melt and evaporate, but this has not been
proven. The streams of dust and gas this releases form a very large, extremely tenuous
atmosphere around the comet called the coma, and the force exerted on the coma by the
Sun’s radiation pressure and solar wind cause an enormous tail to form, which points
away from the sun. The streams of dust and gas each form their own distinct tail, each
pointed in slightly different directions. The tail made of dust is left behind in the comet’s
orbit in such a manner that it often forms a curved tail. At the same time, the ion tail,
made of gases, always pointing directly away from the Sun, as this gas is more strongly
affected by the solar wind than dust is, following magnetic field lines rather than an
orbital trajectory. While the solid body of comets (called the nucleus) is generally less
than 50km across, the coma may be larger than the Sun, and the ion tails have been
observed to extend 150 million km (1 Astronomical unit) or more.

Both the coma and tail are illuminated by the Sun and may become visible from the Earth
when a comet passes through the inner solar system, the dust reflecting sunlight directly,
and the gases glowing from ionization. Most comets are too faint to be visible without the
aid of a telescope, but a few each decade become bright enough to be visible with the
naked eye. Before the invention of the telescope, comets seemed to appear out of
nowhere in the sky and gradually vanish out of sight. They were usually considered bad
omens of deaths of kings or noble men, or coming catastrophes. From ancient sources,
such as Chinese oracle bones, it is known that their appearance have been noticed by
humans for millennia. One very famous old recording of a comet is the appearance of
Halley’s Comet on the Bayeux Tapestry, which records the Norman conquest of England
in 1066.

Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar
system. The Giotto probe found that Comet Halley’s nucleus reflects approximately 4%
of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly’s surface
reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of
the light that falls on it. It is thought that complex organic compounds are the dark
surface material. Solar heating drives off volatile compounds leaving behind heavy long-
chain organics that tend to be very dark, like tar or crude oil. The very darkness of
cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

In 1996, comets were found to emit X-rays. These X-rays surprised researchers, because
their emission by comets had not previously been predicted. The X-rays are thought to be
generated by the interaction between comets and the solar wind: when highly charged
ions fly through a cometary atmosphere, they collide with cometary atoms and molecules.
In these collisions, the ions will capture one or more electrons leading to emission of X-
rays and far ultraviolet photons.

Orbital characteristics

Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity of the orbit and
more rapid motion when closer to the Sun.

Histogram of the aphelia of the 2005 comets, showing the giant planet comet families.
The abscissa is the natural logarithm of the aphelion expressed in AUs.
Comets are classified according to their orbital periods. Short period comets have orbits
of less than 200 years, while Long period comets have longer orbits but remain
gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt.
Single-apparition comets have parabolic or hyperbolic orbits which will cause them to
permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than
could be accounted for by perturbations from Jupiter. If comets pervaded interstellar
space, they would be moving with velocities of the same order as the relative velocities of
stars near the Sun (a few tens of kilometres per second). If such objects entered the solar
system, they would have positive total energies, and would be observed to have

genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic
comets per century, within Jupiter’s orbit, give or take one and perhaps two orders of
magnitude.

On the other extreme, the short period Comet Encke has an orbit which never places it
farther from the Sun than Jupiter. Short-period comets are thought to originate in the
Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A
variety of mechanisms have been proposed to explain why comets get perturbed into
highly elliptical orbits, including close approaches to other stars as the Sun follows its
orbit through the Milky Way Galaxy; the Sun’s hypothetical companion star Nemesis; or
an unknown Planet X.

Because of their low masses, and their elliptical orbits which frequently take them close
to the giant planets, cometary orbits are often perturbed. Short period comets display a
strong tendency for their aphelia to coincide with a giant planet’s orbital radius, with the
Jupiter family of comets being the largest, as the histogram shows. It is clear that comets
coming in from the Oort cloud often have their orbits strongly influenced by the gravity
of giant planets as a result of a close encounter. Jupiter is the source of the greatest
perturbations, being more than twice as massive as all the other planets combined, in
addition to being the swiftest of the giant planets.

A number of periodic comets discovered in earlier decades or previous centuries are now
“lost.” Their orbits were never known well enough to predict future appearances.
However, occasionally a “new” comet will be discovered and upon calculation of its orbit
it turns out to be an old “lost” comet. An example is Comet 11P/Tempel-Swift-LINEAR,
discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It
was not found again until accidentally rediscovered by LINEAR in 2001.

Comet nomenclature

The names given to comets have followed several different conventions over the past two
centuries. Before any systematic naming convention was adopted, comets were named in
a variety of ways. Prior to the early 20th century, most comets were simply referred to by
the year in which they appeared, sometimes with additional adjectives for particularly
bright comets; thus, the “Great Comet of 1680” (Kirch’s Comet), the “Great September
Comet of 1882,” and the “Daylight Comet of 1910” (“Great January Comet of 1910”).
After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the
same body and successfully predicted its return in 1759, that comet became known as
Comet Halley. Similarly, the second and third known periodic comets, Comet Encke and
Comet Biela, were named after the astronomers who calculated their orbits rather than
their original discoverers. Later, periodic comets were usually named after their
discoverers, but comets that had appeared only once continued to be referred to by the
year of their apparition.

In the early 20th century, the convention of naming comets after their discoverers became
common, and this remains so today. A comet is named after up to three independent

discoverers. In recent years, many comets have been discovered by instruments operated
by large teams of astronomers, and in this case, comets may be named for the instrument.
For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS
satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when
multiple comets were discovered by the same individual, group of individuals, or team,
the comets’ names were distinguished by adding a numeral to the discoverers’ names;
thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by
some instruments (in August 2005, SOHO discovered its 1000th comet) has rendered this
system impractical, and no attempt is made to ensure that each comet has a unique name.
Instead, the comets’ systematic designations are used to avoid confusion.

Until 1994, comets were first given a provisional designation consisting of the year of
their discovery followed by a lowercase letter indicating its order of discovery in that
year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the
comet had been observed through perihelion and its orbit had been established, the comet
was given a permanent designation of the year of its perihelion, followed by a Roman
numeral indicating its order of perihelion passage in that year, so that Comet Bennett
1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in
1970)

Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the
International Astronomical Union approved a new naming system. Comets are now
designated by the year of their discovery followed by a letter indicating the half-month of
the discovery and a number indicating the order of discovery (a system similar to that
already used for asteroids), so that the fourth comet discovered in the second half of
February 2006 would be designated 2006 D4. Prefixes are also added to indicate the
nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic
comet, X/ indicating a comet for which no reliable orbit could be calculated, D/
indicating a comet which has broken up or been lost, and A/ indicating an object that was
mistakenly identified as a comet, but is actually a minor planet. After their second
observed perihelion passage, periodic comets are also assigned a number indicating the
order of their discovery. So Halley’s Comet, the first comet to be identified as periodic,
has the systematic designation 1P/1682 Q1. Comet Hale-Bopp’s designation is C/1995
O1.

There are only four objects that are cross-listed as both comets and asteroids: 2060
Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus
(174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).

History of comet study

Early observations and thought

Historically, comets were thought to be unlucky, or even interpreted as attacks by
heavenly beings against terrestrial inhabitants. Some authorities interpret references to