Published online 15 March 2004 | Nature | doi:10.1038/news040315-1


Astronomers spy new 'planet'

Sedna spotted far beyond Pluto.

The Spitzer telescope has spied Sedna.The Spitzer telescope has spied Sedna.© NASA

Mapsof our Solar System may have to be redrawn to include a tenth planet.

The object has been named Sedna, after the Inuit goddess of the ocean. It is the largest body orbiting the Sun to be discovered since Pluto was spotted in 1930.

Sedna is similar in size to Pluto, measuring about 2000 kilometres across. It has a highly elliptical orbit: at its furthest point, the 'planet' is around 130 billion kilometres from the Sun. At its closest point, Sedna is roughly 13 billion kilometres away, three times further from the Sun than Pluto.

Mike Brown, astronomer at the California Institute of Technology, Pasadena, led the team that first saw Sedna with the 48-inch telescope at Mount Palomar Observatory, California, in November 2003. The Spitzer and Hubble space telescopes later confirmed the find. NASA is due to announce full details at 1 pm EST today.

Sedna lives in the Kuiper belt, a region of space beyond Pluto filled with at least 70,000 icy rocks. The new find is the biggest Kuiper-belt object ever seen and has a relatively stable orbit, making it a strong contender for the title of 'tenth planet'.

Status anxiety

This discovery will reignite the debate about whether Pluto can really be called a planet, says Robin Catchpole, astronomer at London's Royal Observatory, Greenwich. "What constitutes a planet is the sort of question I set for my summer students," he laughs, "but it is all semantics, really. These objects are just part of the solar system."

Some astronomers say that a planet should be roughly spherical and have a nearly circular orbit around the Sun. But this would exclude Pluto and Sedna, which have very elliptical orbits.

Astronomers have identified other Kuiper-belt objects of similar size in recent years, such as Quaoar and Varuna. But Sedna is a more interesting planetary candidate because it is larger, further away and has a more regular orbit, says Catchpole.

Kuiper-belt objects are hard to see because so little sunlight reaches them at their enormous distances from the Sun. They are mostly asteroids, bits of rubble left over from the formation of the solar system more than four billion years ago. Catchpole thinks there are probably many more objects like Sedna just waiting to be discovered in the Kuiper belt.

It is now up to the International Astronomical Union to decide whether Sedna will officially be called a planet.