Updated June 01, 2013

Dwarf planets

"A planet is a celestial body which is on orbit around the Sun, which possesses a sufficient mass so that its gravity takes it on the strengths of cohesion of the solid body and maintains it in hydrostatic equilibrium (spherical shape), and which eliminated any body moving on a close orbit ".
This definition was approved on August 24th, 2006, during the 26th General assembly of the UAI (International Astronomical Union) by a vote by a show of hands about 400 scientists and astronomers after ten days of discussions. In addition, the UAI created a new class of objects: the dwarf planets. A dwarf planet, since the new definition of August, 2006, is a celestial body on orbit around the Sun:
- witch has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near-spherical) shape,
- witch has not cleared the neighborhood around its orbit.
- witch is not a satellite
According to this last one, three bodies reach the status of dwarf planet: Pluto, Eris, and Ceres. Other bodies should soon join this nomenclature.

Image: Representation of the largest trans-Neptunian objects known from the Earth. The four biggest: Eris, Pluto, Makemake and Haumea were granted the status of dwarf planet.

Dwarf planets	Orbital period (years)	Diameter (km)

Eris	557	2326 ± 12
Pluto	247.74	2370 ± 10
Haumea	283.28	1240 ± 60
Makemake	309.90	1478 ± 17
Sedna	≈ 11400	995 ± 80
Ceres	4.60	974,6 ± 3.2
Orcus	245.18	917 ± 25
Quaoar	285.97	1070 ± 38
Charon	248.90	1207 ± 1.5
2002 TC302	408.03	≈ 1200
Varuna	283.20	≈ 936
2002 UX25	277	≈ 910
2002 TX300	284.69	≈ 900
Ixion	248.37	≈ 759
2002 AW197	321	700 ± 50

Pluto

Pluto Discovered in 1930 by the American astronomer Clyde Tombaugh, Pluto is out of the ordinary. Of an equatorial diameter lower than 2 500 km, it represents only tenth two of the Earth. Not satisfied to be, so far, the smallest member of the solar system, it also possesses an eccentric and very oblique orbit with regard to the plan of the ecliptic. was discovered in 1930 during the search for a celestial body allowing to explain the orbitales disturbances of Uranus and Neptune, hypothesis proposed by Percival Lowell as being the Planet X. Pluto possesses a satellite, Charon, discovered by James Christy in 1978, is a big satellite with regard to the planet mother (diameter 1270 km).
Charon has a mass about ten times lower than Pluto and the report of diameters is from 1 to 2.

Pluto	characteristics

Diameter	2 370 km ± 10 km
Mass	1.314×10²² kg
Gravity	0.625 m/s2
Average orbital speed	4.74 km/s
Escape velocity	1.229 km/s
Aphelion	7 375 927 031 km or 49,31 AU
Perihelion	4 436 824 613 km or 29.66 AU
Eccentricity	0.250024871
Orbital period	247.74 years
Inclination	17.089°
Discovered date	February 18, 1930
Discovered by	Clyde Tombaugh
Temperature	≈48 K
Number of satellites	5

Eris

Eris The object 2003UB313, is officially appointed by the international astronomical Union in August, 2006. The name chooses is that of the Greek goddess Eris. In the Greek mythology, Eris is the goddess of the Discord. According to Hésiode, she is a girl of Nyx (the Night) and a mother of the Pain, the Famine and the other plagues was photographed for the first time during made observations in October 21st, 2003 with the telescope Oschin of 1,22 meters of the Mountain Palomar, in California, by the team of Caltech was photographed for the first time during observations made which had already discovered several big beyond Neptune objects as (50000) Quaoar and (90377) Sedna.
Thanks to the spatial telescope Hubble and to that of the W.M. Keck Observatory the measure of the mass of Eris, the biggest member of the class of dwarfish planets in our Solar system, was able to be realized. Eris is 1,27 times as massive as Pluto, formerly the biggest member of the Belt of Kuiper of the ice-cold objects revolving beyond the orbit of Neptune.

Eris

diameter	2 400 km ± 100 km
surface
mass	1,668×10²² kg
gravity
inclination of the axis
albedo	0,6
liberation speed
absolute magnitude	-1,2
rotation period
surface temperature	≈ 30 K

Ceres

Ceres Ceres is the first bare asteroid. We recognize him the definition of dwarfish planet today, since the new definition of the international astronomical Union of August, 2006. With about a diameter 950 km, Ceres is also the biggest member of the belt of asteroids situated between the orbits of Mars and Jupiter was accidentally discovered.
Piazzi tried to observe a star listed by Francis Wollaston under the name of Mayer 87 because she was not in the position given in the zodiacal catalog of Mayer (it turned out afterward that it was in fact about Lacaille 87).
On the place, he observed an object moving on the sky, that he believed at first to be a comet.

Image: Ceres view from the Hubble Space Telescope (ACS). The contrasts were raised to reveal the details of the surface. Credit NASA, ESA

Ceres

diameter	959,2×932,6 km
surface
mass	9,445×10²⁰ kg
gravity	0,26 m/s²
inclination of the axis
albedo	0,113
liberation speed	0,51 km/s
absolute magnitude	3,34
rotation period	0,3781 d
surface temperature	≈167 K

Haumea

Haumea is one of the largest members of a family called TNO (Trans-Neptunian Object).
This object called 2003 EL61 first because his first appearance on a photograph from 2003, was officially named by the International Astronomical Union: Haumea.
Haumea is an object transneptunian (OTN) of oblong shape, the largest dimension is between 1 960 and 2500 km. He was ranked Dwarf planet September 17, 2008. The discovery of Haumea is controversial and contested by two teams.
Haumea has two satellites Hi'iaka and Namaka, rotates on itself under 4 h, has an ellipsoidal shape and a high albedo due to its composition of water ice.

Haumea

Diameter	1 960 à 2 500 km
Surface temperature	32±3 K

Image: image artist Haumea and its satellites Hi'iaka and Namaka.
crédit: A. Feild (Space Telescope Science Institute)

Makemake

Makemake, after the name of the creator god of humanity in the mythology of Rapa Nui Easter Island, is a trans-Neptunian object (OTN), of a dimension between 1300 and 1900 km. Makemake discovered in 2005 by Michael E. Brown, Chadwick Trujillo and David Rabinowitz of the Caltech almost simultaneously than Eris, is a dwarf planet. This celestial body is one of the largest known objects in the outer solar system.
Makemake is a dwarf planet reddish, probably covered with frozen methane (CH4). The team, led by José Luis Ortiz (Instituto de Astrofísica de Andalucía, CSIC, Spain), combined observations from multiple telescopes using several different observatories in South America to watch Makemake when passed a distant star in April 2011. Publication November 22, 2012 in the journal Nature: "When Makemake passes the star and obscures its light, the star disappears and reappears in a very abrupt, instead of fading and gradually turn back. This means that the small dwarf planet has no significant atmosphere.".
This member of the Kuiper belt, slightly smaller than Pluto, orbiting the Sun in an orbit slightly more distant. It was registered under number 2005 FY9 soon after its discovery by the team of Mike Brown, before getting its present name July 13, 2008. No image of the surface of Makemake has yet been taken, but its surface is covered with methane, ethane, and possibly nitrogen.

Its average temperature is very low (≈ 30 K, ≈ -243.2 ° C). Makemake not seem to have the moon, like other major dwarf planets Eris has one, Haumea has two and Pluto five.

Makemake	characteristics

Dimensions	1 400 à 1 500 km
discovered	31 mars 2005
Surface temperature	≈35 K
Semi-major axis	6.846 x 10⁹ km
Perihelion	5.807 x 10⁹ km
Aphelion	7.885 x 10⁹ km
Eccentricity	0.15
Period of revolution	3308 years
Average orbital speed	4.389 km/s
Inclination	29°
Ascending node	79.42°
Argument of perihelion	245.85°

Makemake, as seen by the Hubble telescope

Image: Makemake as seen by the Hubble Space .

Sedna

Sedna Sedna (Goddess Inuit of the ice-cold oceans of the north pole), is the biggest beyond Neptune object after Pluto and Eris. Situated in an almost empty space, Sedna is more red and more brilliant than any object of the solar system.
The scientists have not determined the reason of these unique characteristics yet. He could possess the small moon. In April, 2005 a more precise measure of the celestial body allowed to determine a complete rotation speed about 10 hours.
At the time of its discovery, Sedna was the biggest object discovered in the solar system since the discovery of Pluto. Bigger objects (dwarf planets) were since discovered, as Eris.

Sedna

diameter	1180×1800 km
surface
mass	1.7-6.1×10²¹ kg
gravity
inclination of the axis	11.934°
albedo	0.2
liberation speed	0,62-0,95 km/s
absolute magnitude	1.6
rotation period	0.42 d
surface temperature	≈33 K

Quaoar

Quaoar is an object transneptunian (OTN) within the Kuiper belt and has a size greater than half of that of Pluto.
The Kuiper belt consists of objects made of rock and ice orbiting the Sun and located beyond Neptune.
The discovery in June 2002, made by Michael Brown and Chadwick Trujillo of Caltech (Pasadena, USA), tends to confirm the hypothesis that the Kuiper Belt is home dimension objects comparable to Pluto and beyond.
This strengthens the position of classifying Pluto astronomers, not among the planets but among the objects of the Kuiper belt.
The new object, referred to as Quaoar has a diameter of 1250 km.

It follows a circular path about 6.3 billion kilometers from the Sun and 1.8 billion kilometers from Neptune. Pluto follows an elliptical path causing it to the inside of the orbit of Neptune and beyond that of Quaoar. Composed half ice and half rock, Quaoar is black, "dirty ice," said Brown.
It was made black by ultraviolet light which has gradually transformed its organic compounds, and since the beginning of the solar system.
The objects of the Kuiper belt is the remains of the time of the formation of planets.