Astronomers dismiss Nasa's claim to have found a new planet

A new planet has been discovered ­ the first to be found since Pluto in 1930 ­ Nasa, the US space agency, said last night.

A new planet has been discovered ­ the first to be found since Pluto in 1930 ­ Nasa, the US space agency, said last night.

There is just one problem: most astronomers do not think it is a planet, and they are sure that schoolchildren will continue to learn that there are nine, not 10, planets in the solar system.

The new discovery, named Sedna ­ after the Inuit goddess who created the sea creatures of the Arctic ­lies about 170 billion kilometres away from the Sun, roughly 90 times further away than the Earth. But its eccentric orbit means that it can moveup to 10 times further away. That takes it far into the Kuiper Belt, a collection of cosmic debris in the outer reaches of the solar system, filled with rocks and ice which never coalesced when the planets formed about six billion years ago.

Sedna is estimated to be up to 1,100 miles (1,770 kilometres) in diameter, or about three-quarters the size of Pluto, based on the light reflected from its surface detected by telescopes on Earth.

Mike Brown, an astronomer at the California Institute of Technology who led the Nasa-funded team that found Sedna, said the Sun would appear so small from its surface that "you could completely block it out with the head of a pin".

Disputes about whether Sedna is a planet began well before the formal announcement of its discovery last night.

Professor Iwan Williams, the president of the International Astronomical Union (IAU), which has the final say on the classification of celestial objects, said: "If it's smaller than Pluto then it won't be classed as a planet. The fact is that, if Pluto was discovered today, it wouldn't be called a planet. It's too small, and it's not that different from any number of celestial bodies of that kind of size."

Sir Patrick Moore, the astronomer, was even more dismissive. He said: "It's not a planet; it's a Kuiper Belt object. Nor is Pluto a planet. It's a planetary body."

The difficulties in classifying such objects ­ and the growing number that are being found with increasingly powerful telescopes ­ means the IAU is considering the creation of a committee to produce a formal definition of a planet.