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Yet Another Trans-Neptunian Planet Found


The discovery has been announced of yet another tiny planet beyond Neptune, second only to Pluto in size. The object is designated 2003 VB12 and is presently 89 times Earth's distance from the sun. It was noticed as a very faint, very slow-moving 'star' on pictures taken with the 1.2-metre Schmidt camera at Mt Palomar, California in November 2003. It was then found on photos back to 2001 and has been tracked at several observatories during 2004. It is extremely faint, magnitude 21, one- millionth of the brightness of the faintest star the eye can see.

The orbit of 2003 VB12 is very elliptical. At its closest in 2076 it will be 11 billion km from the sun, 75 times Earth's distance or twice Pluto's average distance. Its greatest distance is 1000 times our distance, or 150 billion km from the sun. It takes approximately 12 000 years to complete one orbit.

The size of 2003 VB12 can be estimated only from its brightness; it is much too small and distant to see any planetary disk. The size estimate depends on assumptions about the reflectivity or whiteness of the object. A small light-coloured planet will appear as bright as a large dark planet. Early indications are that 2003 VB12 is around 1500 km in diameter but with a large uncertainty. The Spitzer Space Telescope, working in the infra-red, was unable to detect the asteroid, showing it can't be more than 1700 km in diameter. To get this in perspective, Pluto, with a diameter of 2400 km, is 1/500th of Earth's mass, or one- fifth a heavy as our moon.

Technically, 2003 VB12 has an absolute magnitude (H) of 1.7. The next largest trans-Neptunian, 2004 DW, found in February, has an H of 2.2. Asteroid (50 000) Quaoar, the previous second-largest trans-Neptunian, is 2.6. Pluto's absolute magnitude is -1.0, but it is probably much whiter than the other trans-Neptunian asteroids because its frost layer is renewed at every perihelion.

All three 'second biggest' planets, Quaoar, 2004 DW and 2003 VB12 have been discovered by the same team led by Dr Mike Brown of the California Institute of Technology, along with Drs. Chad Trujillo of the Gemini Observatory, Hawaii, and David Rabinowitz of Yale University. They have given the unofficial name "Sedna" for the Inuit goddess of the ocean, to 2003 VB12. There is a fashion for referring to the next biggest planet after Pluto as 'the 10th planet'. This is misleading since Pluto, the so-called Ninth Planet, is simply the largest, so far, and first discovered, of the cloud of small icy asteroids beyond Neptune. All evidence shows that there are no big planets -- even as small as earth size! -- beyond Neptune.

- The 2003 VB12 information above is from International Astronomical Union Circular No. 8304; Minor Planet Electronic Circular 2004-E45; and NASA News Release 2004-085 forwarded by Dr Grant Christie.

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