ACCORDING to The Hitch-Hiker’s Guide to the Galaxy, it is the answer to life, the Universe and everything. Now scientists have turned to Douglas Adams’s magic number — 42 — to help them to answer that question for real.

Not satisfied with the spectacular views afforded by the Very Large Telescope (VLT), which is kept at Paranal Observatory in Chile, astronomers plan to build a bigger version, the Extremely Large Telescope, with a mirror measuring 42m in diameter.

The project, to be undertaken by the European Southern Observatory (ESO), will take about ten years and cost between €500 million and €1 billion (£700 million).

It will transform our knowledge and understanding of the cosmos. The larger the mirrors in a telescope, the more light it captures and therefore the more powerful it is.

Originally, the scientists had hoped to build an Overwhelmingly Large Telescope, with a 100m mirror, but this idea was abandoned as over-ambitious. The 42m size was deemed more feasible.

The coincidence between the mirror’s diameter and Douglas Adams’s answer to life, the Universe and everything has delighted astronomers working on the project, as the ELT is designed to shed light on exactly the same questions.

Andreas Kaufer, director of the Paranal Observatory, home to the VLT, said: “That is one of the big questions: where does the Universe come from, and where is it going to? The other big one, of course, is: where does life come from?” The VLT, which came into service in 2001, has already taken science closer to the answers. It consists of four 8.2m telescopes. They are used separately or together to simulate a 200m telescope, albeit with less sensitivity than a real mirror of that size.

In 2004, the VLT captured the first images of a planet beyond our solar system, a gas giant about five times the mass of Jupiter named 2M1207b.

Research into planets orbiting other stars is critical to discovering the prospects for extraterrestrial life.

The ELT’s much bigger mirror will allow it to see farther than the VLT, and in greater detail. It should be able to pick up much smaller planets closer in size to Earth; those found to date have been gas giants that could not harbour life.

Scientists estimate that a 42m telescope would be powerful enough to search about 100 star systems for Earth-like planets. It should even be capable of examining their light for signatures of life, such as the presence of liquid water and methane.

The ELT would also be able to resolve stars in nearby galaxies, and to look in detail at galaxies more than 13 billion light years away, which were formed after the Big Bang about 13.7 billion years ago.

“In astronomy, looking far means looking back in time,” said Dr Kaufer. “You can look right at the beginning of the Universe, while the first stars and galaxies form. The gain will be to get decent data that will allow us to analyse what we can see, to see how they formed and evolved.”

British scientists have had access to the VLT since Britain joined the ESO in 2002. The ELT project is likely to consider six sites. Chile is the front-runner, thanks to its exceptional conditions for astronomy.

The Paranal Observatory, which sits on the top of Cerro Paranal, a 2,500m Andean mountain in the Atacama Desert, which is the driest on Earth with an average of just 3mm of rainfall each year. This provides almost perfect viewing conditions on 90 per cent of nights. Its remoteness offers guaranteed dark skies.

Other potential sites are in South Africa, Morocco and Tibet. There is even a possibility of building it in Antarctica, where some regions are even drier than the Atacama.

At present, the world’s largest telescopes are the twin Keck telescopes in Hawaii, with their 10m segmented mirrors. The four VLT telescopes have the largest single mirrors.

• A giant infra-red camera is to be shipped from Oxfordshire to Chile next month to map the southern night sky. The camera, which barely fits into the cargo bay of a Boeing 747, will be fitted to the Visible and Infrared Survey Telescope for Astronomy (Vista).

  The £36 million telescope was conceived as a tool for British astronomers, but was handed to the European Southern Observatory in 2002 as part of Britain’s £72 million fee for joining the international partnership. Its mirror was also built at the Rutherford Appleton Laboratory in Didcot, Oxfordshire.

  Vista’s wide-angle view will capture and map millions of stars at once. Although other telescopes such as the VLT are more powerful, it offers the best combination of power and angle of any telescope in the world. It will be able to detect relatively cool objects, such as failed stars or brown dwarfs, and to see through clouds of dust. It will also be able to see very distant and old objects, which are visible only in the infra-red part of the spectrum.

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