Fact mirrors fiction in the search for life on other planets

In The Hitchhiker's Guide to the Galaxy it was the answer to life, the universe and everything - and now 42 has become the number that will help scientists to find a sister planet to Earth.

For astronomers are planning to build a huge optical telescope to scan for Earth-like planets, and have worked out it needs a mirror diameter of 42 metres.

It could be the first scientific instrument to capture an image of a habitable rocky planet like Earth with water and possibly life. And they hope to build the Extremely Large Telescope (ELT) - as the project has been imaginatively named - over the next 10 to 15 years, deploying a budget of €500m (£340m).

The ELT is the brainchild of the club of European astronomers who have already built the world's biggest optical telescope - this time called the Very Large Telescope (VLT) - on top of the Cerro Paranal mountain in the Atacama desert in the foothills of the Chilean Andes.

This instrument was the first optical telescope to capture the faint light from distant "exoplanets" orbiting far-away stars. To date, it has confirmed sightings of three exoplanets but all of them are huge objects many times the size of Jupiter and orbiting so near to their own suns that they are highly unlikely to harbour life.

What astronomers at the European Southern Observatory are dreaming of is an even bigger telescope that could pick out the faint specks of light reflecting from a small, rocky planet that is in a close, but not too close, orbit around a bright star similar to the Sun.

"The real question is: can we find planets in what is called the habitable zone?" said Andreas Kaufer, director of the Paranal Observatory, which operates the VLT for the European Southern Observatory.

"You have to be at the right distance from a star for it not to be too hot or too cold - if you believe that life has to have something to do with liquid water," Dr Kaufer said.

The VLT has four separate telescopes, each of which is equipped with 8.2-metre mirrors weighing 24 tons each. With clever electronic trickery these telescopes can be made to act in unison, giving astronomers an instrument with an effective mirror size exceeding 100 metres.

However, although this method of operating the VLT is good at resolving two points of light hundreds of millions of miles away, it is not so good at collecting the few photons of light necessary to detect some of the faintest objects, such as Earth-sized planets.

For that, a telescope with a real mirror some 100 metres in diameter is needed. It would be technically difficult to built a single mirror this size, and so the astronomers are planning to build one which is composed of many smaller segments which will be joined together to act in unison.

But building such a telescope would have cost more than £1.5bn - which is way beyond the budget of the European Southern Observatory - so astronomers came up with something more modest, which is why they have decided on a telescope with a mirror diameter of 42 metres.

"To go from a 10-metre telescope to a 100-metre telescope would have been a huge step. It would have been the same kind of step taken by Galileo when he went from the 6mm eye to the 6cm first telescope. That was a revolution," Dr Kaufer said. "That was the pushing argument for building a 100-metre telescope, but I think we have to be realistic. If we manage to increase it by a factor of four, it would be great," he said.

To find an Earth-like planet a telescope would have to be able to detect the light of an object that is about 10 billion times fainter than its parent star. The further away such a star is from Earth, the more difficult the task becomes.

A 100-metre telescope would in principle be able to see an Earth-like planet orbiting a Sun-like star out to a distance of about 100 light years.

Astronomers have estimated that there would be about 1,000 Sun-like stars in this region of space that could be surveyed for Earth-like planets.

However, with a 42-metre telescope the number of candidate stars would drop to less than 200.

Dr Kaufer said that, although a telescope with a 42-metre mirror would not be as good as one with a 100-metre mirror, he feels that 42 is the number that could still lead us to seeing the first Earth-like planet. "It opens up the possibility of seeing more stars and fainter stars in other galaxies. We can go much further out. Seeing faint objects is important but seeing further out is also important," Dr Kaufer said.

"But one definite driver for the big telescopes was to image terrestrial planets around other stars," he said.