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Don't panic: threat to Earth may be delayed 200 million years
By ROBERT MATTHEWS
Monday 7 January 2002
The end is not as nigh as we thought. Scientists have found a mistake in the standard account of the future fate of the solar system and now believe that the Earth will not be destroyed when the sun runs out of fuel.
For decades, astronomy textbooks have insisted that the Earth will be engulfed in an inferno billions of years from now as the sun burns up its nuclear fuel and swells to become a gigantic red star.
Surrounded by the searing gas of the sun's outer atmosphere, the Earth was expected to be dragged down to its doom deep within the sun. Now a team of astrophysicists at Sussex University in England has uncovered a significant flaw in the standard view of how the sun will evolve, with dramatic consequences for the fate of our planet.
According to the conventional wisdom from astronomers, the sun has been kept alight for the past 4.5b illion years by burning up hydrogen at the rate of several million tonnes every second. As this fuel runs out, the theory predicts that stars such as the sun will start to expand and cool into red giants.
Calculations based on this standard theory suggested that it would balloon out and engulf the Earth about 7.5 billion years from now. According to the British team, however, these calculations missed out a crucial effect: the loss of mass by the ageing sun as it expands and its gravity weakens.
Taking this effect into account, the team found that the Earth would manage to dodge a fiery fate, its orbit expanding away from the swelling sun.
According to Robert Smith, one of the team that made the discovery, the dying sun will make two attempts to destroy the Earth. In the first, about 7.7 billion years from now, it will expand to about 120 times its current size, engulfing the two innermost planets, Mercury and Venus.
The sun's weakened gravity will allow the Earth to escape a similar fate, however, with our planet settling down in an orbit about 25 per cent bigger than the one it now follows - well clear of the sun's outer atmosphere.
About 100 million years later the dying sun will have another go at the Earth, but will fail again, with our planet having moved out even further.
According to Dr Smith, the sun will then collapse into a harmless white dwarf star, about 16,000 kilometres across. "The Earth won't wander off into space," Dr Smith said. "But whether it will be anything like we see today seems pretty doubtful."
The team reports its findings in the current issue of the journal Astronomy and Geophysics. "They differ from the standard conclusion by taking account of mass loss and including the latest data based on studies of real stars," said Dr Smith. "To that extent, the textbooks will have to be rewritten."
He added that, although the Earth is safe from destruction, life here still faces some formidable challenges in the far future. The new calculations suggest that the surface of the Earth will become too hot to sustain human life for a few million years about 5.7 billion years from now.
This is about 200 million years later than previously thought - an extra period of grace that humans could use to develop technologies for living on a hotter Earth, such as building communities deep underground. Alternatively, the human race could move to another planet for a while.
"Unfortunately none of the surviving planets, such as Mars, are warm enough at the time we will need them - though we could think about altering conditions on them," said Dr Smith. "We might not have to leave the solar system."
The findings are likely to rekindle the age-old debate about the ultimate fate of humanity. Astronomer Patrick Moore said: "In the end, no one really knows what is going to happen. But my message would be 'don't panic'."