On 29 July 2005 came announcement of the discovery of a trans-Neptunian object larger than Pluto. The issue of whether the astronomical community will officially designate it a planet has yet to be determined--but provisionally, based on size, I'll refer to it here as the tenth planet in our solar system.
On 28 July 2005, J. L. Ortiz et al. announced discovery of an object nearly as large as Pluto, now designated 2003 EL61. Apparently the object had been previously observed by a team led by Brown, who was withholding announcement of this and two other large trans-Neptunian objects (TNOs) until a conference in September. Discovery credit in astronomy is based on who announces first, which was the Ortiz team. Reportedly someone accessed one or more computers of the Brown team, prompting them to announce discovery of the other two large TNOs on 29 July. One of these, now designated 2003 UB313, is clearly larger than Pluto based on its observed brightness. The other, designated 2005 FY9, is over half the diameter of Pluto. Additionally, the Brown team reported having observed a satellite of 2003 EL61, constraining mass of the system to about 30% that of Pluto. A second satellite of 2003 EL61 was discovered a short time later.
The new planet 2003 UB313 (until it is permanently named), appears to be between Pluto and the Earth's Moon in size. It is probably mostly ice (water ice and frozen methane). It takes about 560 years to orbit the Sun once in its very eccentric orbit. At its closest to the Sun, it is just inside Pluto's average distance from the Sun; at its furthest, it is three times as far from the Sun as Neptune. Its orbit is inclined 44° to the ecliptic, the plane of the Earth's orbit. All the planets out to Neptune orbit close to the ecliptic, and the high inclination of 2003 UB313 is part of why it had not been discovered sooner.
On 1 October 2005, M. Brown and his team announced discovery of a satellite to 2003 UB313. The satellite is estimated to be about 350 km across.
To add to these finds, two more satellites of Pluto were discovered, both estimated at on the order of 100 km across and both orbiting outside the orbit of Charon.
The first measurement of the diameter of 2003 UB313 was reported in February 2006. The team of Bertoldi, Altenhoff, Weiss, Menten, and Thum made thermal infrared measurements from the IRAM telescope in Spain and found a diameter of 3000 km plus or minus 400 km. In April the team of Brown, Schaller, Roe, Rabiowitz, and Trujillo reported results from direct imagery of 2003 UB313, indicating a diameter of 2400 km plus or minus 100 km.
The table below lists information on Neptune, Pluto, the newly discovered "planet" temporarily designated 2003 UB313, and the other largest known TNOs.
| object | diameter (km) | perihelion distance (AU) | aphelion distance (AU) | semimajor axis (AU) | eccentricity | discovered | satellites |
|---|---|---|---|---|---|---|---|
| Neptune | 50,400 | 29.8 | 30.4 | 30.1 | 0.01 | 1846 | Proteus (D=420 km, a=117,600 km); Triton (D=2,710 km, a=354,800 km); Nereid (D=340 km, a=5,513,000 km); 10 others known |
| (90482) Orcus | 1,530 | 30.8 | 48.1 | 39.4 | 0.22 | 2004 | |
| Pluto | 2,340 | 29.7 | 49.3 | 39.5 | 0.25 | 1930 | Charon (D=1212 km, a=19,570 km); S/2005 P2 (D=90?, a=48,700 km); S/2005 P1 (D=110?, a=64,780 km) |
| (20000) Varuna | 870 | 40.8 | 45.2 | 43.0 | 0.05 | 2000 | |
| 2003 EL61 | 1,380 | 35.2 | 51.5 | 43.3 | 0.19 | 2003* | S/2005 (2003 EL61) 2 (D=170 km?, a=39,300 km); S/2005 (2003 EL61) 1 (D=300 km?, a=49,500 km) |
| (50000) Quaoar | 1,260 | 42.0 | 45.0 | 43.5 | 0.04 | 2002 | |
| 2005 FY9 | 1,500 | 38.7 | 52.6 | 45.6 | 0.15 | 2005 | |
| (84522) 2002 TC302 | 1,200 | 39.0 | 71.2 | 55.1 | 0.29 | 2002 | |
| 2003 UB313 | 2,700 | 37.8 | 97.6 | 67.7 | 0.44 | 2003* | S/2005 (2003 UB313) 1 (D=350 km?, a=36,000 km?) |
| (90377) Sedna | 1,700 | 76.1 | 913 | 495 | 0.85 | 2003 |
* As their designations indicate, 2003 UB313 and 2003 EL61 were discovered in images taken in 2003; they were not comfirmed/announced until 2005.
See this page for more information on 2003 UB313 and its satellite.
See this page for more information on 2003 EL61 and its satellites.
More data on the largest objects beyond Neptune:
| object | absolute magnitude H | diameter (km) | albedo | density (g/cm3) | B-V color | surface constituents |
|---|---|---|---|---|---|---|
| 2003 UB313 | -1.16 | 2700± 400 | 0.73±0.15 | -- | 0.71 | CH4 |
| Pluto | -1.0 | 2360± 70 | 0.60 | 1.92±0.12 | 0.87 | CH4, N2, CO |
| 2005 FY9 | -0.25 | 1100-2200 | 0.2-0.8 | -- | 0.9 | CH4 |
| 2003 EL61 | 0.44 | 1380± 60 | 0.66±0.06 | 2.97±0.37 | 0.63 | H2O, ? |
| Charon | 0.9 | 1212± 3 | 0.37±0.02 | 1.63±0.07 | 0.71 | H2O, NH3-H2O |
| (90377) Sedna | 1.56 | 450-1800 | 0.05-0.9 | -- | 1.23 | CH4, N2 |
| (90482) Orcus | 2.27 | 600-1550 | 0.04-0.3 | -- | 0.68 | H2O, ? |
| (50000) Quaoar | 2.72 | 1260± 190 | 0.10±0.03 | -- | 0.94 | H2O, NH3-H2O |
Other links (external):
© 2005, 2006 by Wm. Robert Johnston.
Last modified 14 April 2006.
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