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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.

Furthermore, on 11 June 2008, the International Astronomical Union defined a plutoid as a celestial body that

is in orbit around the Sun at a semi-major axis greater than that of the planet Neptune,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and
has not cleared the neighbourhood around its orbit.

Satellites of plutoids are not plutoids themselves.

Ceres, Pluto, Haumea, Eris and Makemake are now classified as dwarf planets and all but Ceres are also classified as plutoids.

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

Number	Name	Discovery		Semi-Major Axis (AU)	Period (years)	Eccentricity	Inclination
1	Ceres *	1801	Piazzi	2.766	4.60	0.0798	10.56°
134340	Pluto *	1930	Tombaugh	39.45	248	0.250	17.09°
136108	Haumea *	2003	Brown, Trujillo, Rabinowitz	43.35	285	0.189	28.19°
136199	Eris *	2003	Brown, Trujillo, Rabinowitz	67.73	557	0.440	44.16°
136472	Makemake *	2005	Brown, Trujillo, Rabinowitz	53.07	310	0.159	28.96°

2	Pallas	1802	Olbers	2.772	4.61	0.231	34.84°
3	Juno	1804	Harding	2.668	4.36	0.258	12.97°
4	Vesta	1807	Olbers	2.362	3.63	0.0894	7.134°
5	Astraea	1845	Hencke	2.574	4.13	0.193	5.369°
243	Ida	1884	Palisa	2.861	4.84	0.0459	1.138°
253	Mathilde	1885	Palisa	2.646	4.31	0.266	6.738°
298	Baptistina	1890	Charlois	2.264	3.41	0.0952	6.284°
433	Eros	1898	Witt	1.458	1.76	0.223	10.83°
588	Achilles	1906	Wolf	5.196	11.8	0.147	10.32°
624	Hektor	1907	Kopff	5.229	12.0	0.0232	18.19°
719	Albert	1911	Palisa	2.629	4.26	0.552	11.55°
944	Hidalgo	1920	Baade	5.753	13.8	0.660	42.51°
951	Gaspra	1916	Neujmin	2.209	3.28	0.174	4.102°
1221	Amor	1932	Delporte	1.920	2.66	0.435	11.88°
1566	Icarus	1949	Baade	1.078	1.12	0.827	22.85°
1862	Apollo	1932	Reinmuth	1.471	1.78	0.560	6.355°
2060	Chiron	1977	Kowal	13.70	50.7	0.381	6.935°
2062	Aten	1976	Helin	0.9667	0.950	0.183	18.93°
2161	Grissom	1963	Goethe Link Observatory	2.750	4.56	0.161	7.301°
2417	McVittie	1964	Goethe Link Observatory	3.183	5.68	0.221	3.108°
3200	Phaethon	1983	IRAS	1.271	1.43	0.890	22.18°
3634	Iwan	1980	Lagerkvist	2.246	3.37	0.0910	4.305°
4179	Toutatis	1989	Pollas	2.531	4.03	0.631	0.4459°
4659	Roddenberry	1981	Bus	2.371	3.65	0.225	2.471°
7307	Takei	1994	Shimizu, Urata	2.741	4.54	0.0836	6.989°
10455	Donnison	1978	Lagerkvist	2.361	3.63	0.2212	6.117°
12395	Richnelson	1995	Asher	3.234	5.82	0.0455	21.17°
28978	Ixion	2001	Buie	39.69	250	0.242	19.58°
50000	Quaoar	2002	Trujillo, Brown	43.61	288	0.0370	7.985°
68410	Nichols	2001	White, Collins	2.679	4.38	0.0780	4.862°
69230	Hermes	1937	Reinmuth	1.655	2.13	0.624	6.067°

Notes

Ceres is the largest of all the objects in the asteroid belt and is classified as a dwarf planet. Like 134340 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.

Pluto was demoted from planet to dwarf planet status on 24 August 2006 and given the number 134340 a month later. 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).

Like Pluto, Haumea has satellites. They are named Hi'iaka and Namaka, both discovered in 2005 by Brown, Bouchez and the Keck Observatory Adaptive team. Haumea is the Hawaiian goddess of childbirth and fertility, and Hi'iaka and Namaka are two of her children. The minor planet Haumea is accompanied not only by her two satellites but lots of tiny fragments which resulted from an ancient collision.

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.

Like other classical Kuiper belt objects, Makemake has the name of a creator deity, in this case, the creator of humanity according to the myths of Easter Islanders. It is pronounced ma kay ma kay.

Pallas has an unusually high orbital inclination.

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, Astraea was discovered.

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 Mathilde on its way to a year 2000 Valentine's Day encounter with 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 Eros.

Numerical modelling suggests that members of the Baptistina asteroid group, named for 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 Achilles but the largest is Hektor.

The last of the "lost" named and numbered asteroids, 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.

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 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 Gaspra, a member of the Flora group of asteroids.

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

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

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.

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.

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

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.

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 five of its faculty members represented in the far reaches of the solar system. McVittie is named for the late George McVittie who was appointed as the first Professor of Mathematics at Queen Mary College in 1948. Iwan is named for Prof. Iwan P. Williams, Carlmurray is named for Prof. Carl D. Murray, Donnison is named for Dr. Richard Donnison and Richnelson is named for Prof. Richard P. Nelson.

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 Roddenberry. Actors George Takei (Lt. Sulu) and Nichelle Nichols (Lt. Uhura) from the series have been similarly honoured with Takei and Nichols.

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

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

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