2. Planets like Earth
If there is life out there, how do we know what to look for? Is it a given that living things must crawl over the surface of their home planets? Not according to Martin Rees, who asks us to drop our parochial notions and imagine underground biospheres or living beings that float in the fogs of dense atmospheres.

This is all very well but we only actually know of life in one place. If we are to search for other life in any meaningful way, life on Earth is our only reference point. Rees takes the trouble to point this out: ‘It makes sense to look at planets which we feel might have resembled the young Earth, because there, in principle, the same processes could have happened, which we believe happened here on Earth.’

One of the undeniable specialities of Earth that makes it a good home for life is that it has liquid water on its surface. Life is inconceivable without it. The chemical reactions that are believed to have originally sparked the evolution of life couldn’t have happened without liquid water. And no lifeform that we know of can live and reproduce if devoid of water.

But a planet can only maintain liquid water if it is just the right distance from its heater – its sun. Too close and water boils off, as it did on Venus. Too far away and it’s all locked up in ice, as it is on Mars.

There are thought to be one million million planets that possess the necessary criteria for having liquid water. Endless potential for life ...

< PREVIOUS PAGE | 1 | 2 | 3 | 4 | 5 | 6 | NEXT PAGE >