Pluto鈥檚 title as the outermost planet could be in jeopardy, with the discovery of a large object orbiting the Sun far further out than any other.
The object, dubbed Sedna for the Inuit goddess of the sea, lies about 10 billion miles from the Sun, nearly twice as far as Pluto. Its estimated 2000-kilometre diameter is about 90 per cent the size of Pluto鈥檚, making Sedna the largest Solar System object discovered since Pluto itself in 1930.
鈥淪edna is very big, and much farther out than previous discoveries,鈥� discoverer Michael Brown of the California Institute of Technology told The Australian. 鈥淚鈥檓 pretty sure there are other large bodies there too.鈥� Brown鈥檚 team is expected to reveal further details of the discovery later on Monday.
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Sedna lies in the Kuiper Belt, a band of ice and rock left over from the birth of the Solar System that stretches outwards from Neptune. About 800 Kuiper-Belt Objects (KBOs) have been discovered since 1992, some with predicted orbits that would take them as far as 150 billion kilometres from the Sun.
Size matters
Sedna is 鈥渁n awesome thing to find, but it is not a surprise鈥�, writes David Jewitt, an astronomer at the University of Hawaii and co-discoverer of the first KBO, on his website. Astronomers 鈥渉ave expected for years to find objects of Pluto鈥檚 size and perhaps a little larger.鈥�
And size matters. 鈥淥n a practical level, [large KBOs] are bright enough to permit meaningful physical studies鈥� that will shed light on the origin of these icy relics, Jewitt notes. KBOs are so distant and their ice-rock compositions so unknown that even measurements of their diameters are 鈥渓ittle more than guesses鈥�, he adds.
But Sedna鈥檚 size is so great that it holds special scientific promise. 鈥淚t might be large enough to retain a tenuous atmosphere, as does Pluto, at least at certain parts of its orbit鈥�, says Jewitt.
Sedna was reportedly discovered in November 2003 using a wide-field telescope on Palomar Mountain in Southern California, with NASA鈥檚 Spitzer Space Telescope and the Tenagra Observatory in Arizona confirming the observation.
The new object is said to orbit the Sun every 10,500 years and may be trailed by a tiny moon. Sedna鈥檚 temperature is a chilly -200掳C 鈥� the coldest known object in the Solar System.
Planet or not
Sedna鈥檚 size is likely to revive arguments about what constitutes a planet. Many astronomers believe that Pluto was designated a planet only because it was found six decades before the first KBO.
鈥淚f you think Pluto is a planet, you should probably label Sedna that way, too, because it鈥檚 almost as big,鈥� says Jewitt. 鈥淕et ready for a 20-planet Solar System!鈥�
Since 2001, a team led by Brown, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University has surveyed about 15 per cent of the sky from Palomar Mountain and found over 30 KBOs.
In February, the team discovered an object about 70 per cent as wide as Pluto and in 2002 it found an object half Pluto鈥檚 diameter, called Quaoar. The team estimates it will find as many as 10 new, large KBOs in the next few years, including a few objects larger than Pluto.
