UNDER THE MICROSCOPE:Can Earth survive the last sundown? Some suggest it can
IN ABOUT FIVE billion years from now our sun will slowly start to die. It has long been predicted that when this happens the sun will swell into a giant star called a red giant, which will envelop and vaporise the Earth. Of course, the human race will not survive this event unless we have by then long since succeeded in leaving Earth to live in a more hospitable region of space. Recent observations suggest it is possible that the physical Earth may survive the red giant phase of the sun, and even, under some theoretically conceivable, but highly improbable, circumstances, remain habitable by humans for several billion more years, but nobody is suggesting that Earth will remain habitable by humans beyond that.
The sun is a nuclear fusion reactor that fuses hydrogen into helium, releasing enormous amounts of energy in the process. As it uses up the hydrogen, it gets hotter and brighter and burns the remaining fuel faster. The sun grows 10 per cent brighter every 1.1 billion years. In one billion years from now, the sun's heat will vaporise all liquid water on Earth's surface and all life on land will die. Mars will also heat up, frozen carbon dioxide and water will vaporise into the atmosphere creating a greenhouse effect, heating the planet to a point similar to Earth today and possibly creating a future temporary home for life.
About 5.5 billion years from now all the hydrogen in the core of the sun will have fused into helium and the core, no longer supported against gravitational collapse, will begin to contract. This contraction will heat a shell around the core until hydrogen begins to fuse within it. This will greatly expand the outer layers of the sun, causing the star to become a red giant.
In 7.5 billion years from now, the sun will be 256 times its current size. A strong stellar wind will carry away one-third of the mass of the sun. The sun will expand and contract a number of times in its red giant phase before settling down into a small white dwarf star. This will have a mass of about 54 per cent of the present mass of the sun, but will only be about the size of the Earth. It will shine brightly at first but will gradually grow ever dimmer.
As the sun swells up, it will engulf Mercury and Venus, but the fate of the Earth is less clear. The reduction of the mass of the sun will weaken its gravitational pull on the earth and the Earth's orbit will move outward from the sun.
However, recent calculations suggest that some of the Earth's orbit will still traverse the outer layers of the swollen sun and this will gradually "reel in" the Earth until it is vaporised by the sun. Other scientists argue that the loss of mass from the sun will so weaken its gravitational pull over time that the earth will escape vaporisation.
One fantastic possibility for ensuring that the Earth is saved has been made by Dan Korycansky of the University of California. He proposes that the Earth could be pulled further away from the sun using the gravitational force of a large asteroid - "it can be done by harnessing the gravitational effects of a large asteroid that passes by the Earth. The Earth's orbit is nudged gradually outwards away from the encroaching sun. By doing this incrementally every 6,000 years or so, it might be possible for life on Earth to survive for another five billion years and escape the wrath of the sun's first red giant phase".
Astronomers have now announced the discovery of a planet that seems to have survived the puffing up red giant phase of its home star. There is therefore reason to hope that Earth too could survive the dying swelling sun. The planet orbits a faint star in the constellation Pegasus, about 4,500 light years from Earth. Before the star became a red giant and lost half its mass, the planet orbited it at about 90 million miles. The loss of gravitational pull from the reduced mass red giant star pushed the planet into an orbit nearly twice as far away.
Of course there is no hope in the longer term that life can continue to inhabit the Earth - what we are discussing is whether or not the physical planet Earth can survive. If it does survive and if humanity successfully finds another home elsewhere in space, the charred remains of Earth will become a famous place of pilgrimage for humans, the birthplace of the human race, perhaps the only intelligent life in the universe.
• Dr Reville is associate professor of biochemistry and public awareness of science officer at UCC - www.understandingscience.ucc.ie