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Wednesday, 9 January, 2002, 12:06 GMT
Earth granted reprieve
Space, BBC
Theory predicts that the Sun will expand
By BBC News Online's Helen Briggs

The Earth has been granted a reprieve.

Astronomers believe the planet may now escape being swallowed up when the Sun dies in about 7.5 billion years' time.

The new calculations actually extend the length of time the Earth will be habitable by 200 million years.

We no longer think that the Earth will be swallowed up by the Sun but it will be frazzled to a cinder

Dr Robert Smith, Sussex University
But, in the end, the surface of the planet will simply become too hot for life to survive. Earth-dwellers will have to find alternative homes in space, say astrophysicists in the UK.

Dr Robert Smith, Reader in Astronomy at the University of Sussex, said: "We had better get used to the idea that we shall need to build our own survival capsules - the planets are simply too far apart for planet-hopping to be a viable solution.

"Perhaps this is the ultimate justification for developing an International Space Station."

Fuel crisis

Solar evolution theory predicts that our star will eventually run out of fuel.

As it does so, it will expand to an enormous size, becoming what is known as a red giant.

What is a red giant?
Stars convert hydrogen to helium
As time goes by, the hydrogen is depleted and the star starts to collapse
The pressure and temperature inside the star rises and it starts to burn helium
The dying star will appear red in colour, and larger in size, hence the term red giant
It will then swallow the closest planets, including Mercury and Venus. Until now, astronomers had always thought that the Earth would be engulfed too.

But the Sussex team thinks the figures are wrong. According to new calculations, the orbit of the Earth will increase slightly beyond the outer atmosphere of the red giant, as its gravitational pull weakens.

If this is the case, the Earth will escape destruction - although its surface will be charred.

"Previous calculations suggested that the Earth will be vaporised by being swallowed up by the Sun," Dr Smith told BBC News Online.

"Our calculations show that the Earth will survive as a body but it will still be lifeless because it will get so hot that nothing will be able to survive on its surface."

Text book dilemma

The new figures are based on theoretical calculations checked against data from real stars.

The story for life on Earth is long over by the time the Sun becomes a red giant

Professor James Kasting, Pennsylvania State University
They predict that it will be 5.7 billion years before the planet becomes too hot to sustain life.

Dr Smith told BBC News Online: "One effect of the calculations is that we may have 200 million years longer than previous people have thought."

Other scientists are less optimistic. Professor James Kasting, a geoscientist at Pennsylvania State University, US, believes water on Earth will boil away in about one billion years' time, spelling doom for the planet.

He told BBC News Online: "The story for life on Earth is long over by the time the Sun becomes a red giant.

"The question of what happens 6-7 billion years from now is interesting from an academic point of view but that's not when life will end."

The new calculations raise another concern, albeit more mundane.

"The text books will have to be slightly changed because we no longer think that the Earth will be swallowed up by the Sun but it will be frazzled to a cinder," said Dr Smith.

See also:

21 Feb 00 | Washington 2000
Date set for desert Earth
05 Feb 01 | Sci/Tech
Planet Earth on the move
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