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Dwarf Planets and Small Solar-System Bodies

The words asteroid, minor planet and planetoid are (unofficial and) interchangable. They each refer to the small rocky bodies that are mainly confined to the asteroid belt between the planets Mars and Jupiter. The radius of the asteroid belt varies from 2.0 AU to 3.3 AU from the Sun with holes (called Kirkwood gaps) in the distribution of objects. These gaps coincide with orbital periods which are exact fractions of the period of Jupiter. However, these objects can be found elsewhere in the solar system. Some fly alarmingly close to the Earth whilst others inhabit the shadowy realms of the Kuiper Belt out beyond the planet Neptune.

On 24 August 2006, the International Astronomical Union defined a dwarf planet as a celestial body that

• is in orbit around the Sun,
• has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
• has not cleared the neighbourhood around its orbit, and
• is not a satellite.

Pluto, 1 Ceres and 136199 Eris are now classified as dwarf planets.

A small solar-system body is anything that isn't a planet, a dwarf planet, or a satellite. This includes most asteroids and comets.

The table below lists a few characteristics of some of the more interesting bodies. According to current policy, an official number is not given to a small solar-system body until a reliable orbit has been determined for it. Once the object has been numbered, the discoverer may name it.

Dwarf planets are marked with an asterisk * and listed first.

Some Interesting Bodies

Notes

Pluto was demoted from planet to dwarf planet status on 24 August 2006. It was the first trans-Neptunian object discovered and is one of the largest. It has three companions: Charon (discovered in 1978 by J. Christy) and Hydra and Nix (discovered in 2005 by the Hubble Space Telescope Pluto Companion Search Team).

1 Ceres is the largest of all the objects in the asteroid belt and is classified as a dwarf planet. Like Pluto, it was originally identified as a planet but was assigned a different classification when it became clear that it was just one of many such objects inhabiting the region between Mars and Jupiter.

2 Pallas has an unusually high orbital inclination.

4 Vesta is the brightest of the asteroids. It is the only one that can reach the limit of naked-eye visibility, sixth magnitude.

After a gap of 38 years, 5 Astraea was discovered.

243 Ida was photographed by the Galileo spacecraft in 1994. This historic encounter provided the first conclusive evidence that asteroids could have natural satellites when Dactyl was discovered accompanying its parent body in orbit about the Sun.

In June 1997, NEAR (Near Earth Asteroid Rendezvous) passed by 253 Mathilde on its way to a year 2000 Valentine's Day encounter with 433 Eros, an Earth-grazing asteroid and the first one discovered to come inside the orbit of Mars. One year later, NEAR made history by becoming the first spacecraft to touch down on an asteroid when it made an amazing soft landing on 433 Eros.

Numerical modelling suggests that members of the Baptistina asteroid group, named for 298 Baptistina, may be responsible not only for the large lunar crater Tycho but also the terrestrial impact site at Chicxulub which seems to be implicated in the extinction of the dinosaurs.

The Trojan asteroids are two families of asteroids stationed at the Lagrangian points in Jupiter's orbit. The first one discovered was 588 Achilles but the largest is 624 Hektor.

The last of the "lost" named and numbered asteroids, 719 Albert was recovered in May 2000, almost 89 years after its original discovery. With its recovery, the position of every numbered asteroid is now known.

944 Hidalgo has a highly elliptical orbit which carries it from the main asteroid belt between Mars and Jupiter to out beyond Saturn. Some think that 944 Hidalgo is actually an extinct comet.

In October 1991, the Galileo spacecraft took the first ever high-resolution photograph of an asteroid. The object concerned was 951 Gaspra, a member of the Flora group of asteroids.

1221 Amor is the prototype of the Amor group of asteroids. These objects come to perihelion between the orbits of Mars and Earth.

1566 Icarus has an orbit that carries it closer to the Sun than Mercury, the innermost planet in the solar system.

1862 Apollo is the prototype of the Apollo group of asteroids. These objects have perihelion distances of less than 1 AU so they cross the Earth's orbit.

2060 Chiron has an orbit entirely outside that of Jupiter. It has been observed to have a coma and may be a giant comet instead of an asteroid.

2062 Aten is the prototype of the Aten group of asteroids. These are Earth-approaching objects with orbits lying mostly within that of the Earth.

2161 Grissom is named in the memory of Apollo astronaut Virgil Ivan "Gus" Grissom who was killed in the Apollo 1 launchpad fire on 27 January 1967.

3200 Phaethon passes closer to the Sun than any other object in the solar system with the exception of a few comets. It was discovered by the IRAS satellite and is thought to be the parent body of the Geminid meteor shower.

The Astronomy Unit at Queen Mary, University of London, is proud to have four of its faculty members represented in the far reaches of the solar system. 2417 McVittie is named for the late George McVittie who was appointed as the first Professor of Mathematics at Queen Mary College in 1948. 3634 Iwan is named for Prof. Iwan P. Williams, 5598 Carlmurray is named for Prof. Carl D. Murray and 12395 Richnelson is named for Prof. Richard P. Nelson.

4179 Toutatis had a close encounter with the Earth in December 1996 and again in September 2004.

Star Trek creator Gene Roddenberry is immortalised in the heavens as 4659 Roddenberry. Actors George Takei (Lt. Sulu) and Nichelle Nichols (Lt. Uhura) from the series have been similarly honoured with 7307 Takei and 68410 Nichols.

After 200 years of being the biggest asteroid in the solar system, 1 Ceres was dethroned in 2001 by Kuiper Belt object 28978 Ixion. However, the following year, 50000 Quaoar claimed "largest asteroid" honours.

Lost just five days after its initial discovery in 1937 and only recovered in October 2003, 69230 Hermes turns out to be a binary object. Two asteroids for the price of one!

136199 Eris is the largest of the dwarf planets and the object which ultimately led to the demotion of Pluto! Informally known as "Xena" since its discovery in 2003, it is appropriately named for the goddess of discord who caused Trojan War. It has a satellite named Dysnomia.

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