Sir Patrick Moore has poured cold water on scientists' claims that they have discovered a new planet.

Nasa astronomers say they have found what could be the solar system's tenth planet, approximately ten billion miles from Earth.

What Nasa has named Sedna, after an Inuit goddess of the ocean, is believed to measure at least 1,250 miles across.

The icy rock may even be larger than the farthest known planet, Pluto, which has a diameter of 1,406 miles.

However, Selsey-based astronomer Sir Patrick, former presenter of BBC series The Sky At Night, is sceptical about using the term "planet".

He believes there could be similar discoveries even farther out towards the fringes of the solar system.

He told The Argus: "It's wrong to call it a planet. I think it's too small, in the wrong orbit and in the wrong place.

"The discovery is very interesting but it will not alter any theories we have, I'm sure.

"Sedna is smaller than a moon.

"Pluto was only classed as a planet because they originally thought it was larger than Earth, which was a mistake."

Sedna is in a region of space called the Kuiper Belt where more than 400 astral bodies have been found.

It is an estimated 10.56 billion miles from the sun.

Pluto is 3.67 billion miles from the sun and the earth 93 million miles.

Sir Patrick, 81, said: "Sedna is well into the Kuiper Belt but there could be a whole swarm of things around much farther out."

It was spotted using Nasa's Spitzer space telescope during a three-year survey headed by Dr Michael Brown of the California Institute of Technology.

Sedna is similar in size to Jupiter's ice moon Europa, discovered by Galileo and Marius in 1610.

Sir Patrick has not spotted Sedna himself but does at least approve of the name.

He said: "It's so difficult to see. This is something that is so far away.

"The name is not quite official. That has to be decided by the International Astronomical Union, of which I used to be a member. But it is a very good name."

Dr John Mason, trustee of the South Downs Planetarium Trust in Chichester, was unsure whether Sedna would be classed as a planet.

He said: "We know there is a swarm of icy bodies beyond Pluto.

"We have discovered many other icy bodies but the difference with Sedna is it happens to be big.

"It's rivalling Pluto in size and may well be bigger.

"I suspect not and I expect, if anything, it will be put in the group of Kuiper Belt objects.

"Pluto is one of these Kuiper Belt objects but it has always been the biggest and the only one called a planet.

"If Sedna is bigger than Pluto - and at the moment it seems comparable in size - it could be called a planet too.

"It's always possible Pluto will be downgraded or relegated to being another Kuiper Belt object.

"It's because of the historical importance of Pluto, as the first Kuiper Belt object found, that it has been known as a planet."

"This kind of discovery always stimulates interest in astronomy.

"I will be certainly mentioning it when I talk at the planetarium but, to know exactly what the significance is in general terms, we have to wait until we are certain of the object's diameter. At the moment, we are not certain."

Dr Robert Smith, head of physics and astronomy at Sussex University, said: "Most astronomers feel, unless it was something comparable to Neptune in size, they would not want to start introducing more planets.

"There is a vast range of objects in the Kuiper Belt and it's entirely possible there are actually more of the larger range of objects."