The earth

New Delhi, Feb. 24: The sun will expand and swallow the earth in about 7.59 billion years, but the planet’s oceans will boil and the continents will roast long before that fiery end, new calculations have predicted.

As the dying sun expands into a red giant, the earth will be engulfed, according to research by astronomers Robert Smith at the University of Sussex in the UK and Klaus-Peter Shroder at the University of Guanajuato, Mexico.

Previous theories have suggested that the sun will expand, but only toast the earth to a cinder. But the two astronomers have now shown that a burgeoning sun will lose mass in the form of a strong wind that will counter its gravity, allowing the earth to move away from the sun. Yet the planet will still find itself orbiting dense outer layers of the sun that will slow its escape.

“The drag caused by this low density gas is enough to cause the earth to drift inwards, and finally to be captured and vaporised by the sun,” said Smith, an emeritus reader at Sussex.

“Engulfment and loss of planet earth will take place just before the sun reaches the tip of the Red Giant Phase, 7.59 billion years from now,” the astronomers said in a report just published in the Monthly Notices of the Royal Astronomical Society. The sun would have gobbled up Mercury 3.8 million years and Venus 1 million years before claiming the earth.

But the calculations also suggest that the earth will become uninhabitable in about a billion years. A 10 per cent increase in the sun’s luminosity in about a billion years will edge the earth out of the habitable zone.

In about a billion years, the habitable zone — the region near a star where a planet’s surface can support liquid water — would have moved outward and extended to the orbit of Mars, the astronomers said.

The increased solar luminosity would cause excess ocean evaporation, triggering a runaway evaporation that would cause the oceans to boil dry and all water vapour to escape into space, they said.

This would be followed by a dry greenhouse phase when temperatures of continents would rise faster and faster, the astronomers said.

But the astronomers have also suggested that doomsday may be avoidable. A research team at the University of California, Santa Cruz, had seven years ago proposed a technique to harness gravitational effects of a passing asteroid to nudge the earth’s orbit gradually away from the expanding sun.