Astrological Notes: Are we alone in the Universe?

AS WE enter the new millennium, astronomers stand on the brink of answering the age-old question of whether there are planets like the Earth, and life-forms like us, elsewhere in the Universe. The caveats are important, because although discoveries of other planets orbiting other stars have made headlines in recent years, those discoveries all relate to planets far bigger than Earth, more like the giant Jupiter, which has 300 times as much mass as the Earth. But soon, it seems, we will be able to detect Earth-like planets.

The recent discoveries push existing technology to the limit. Improvements in telescope design make it just possible to detect the "wobble" in the motion of some stars caused by the presence of a massive planet in orbit around the star and tugging on it gravitationally. In fact, the planet doesn't orbit the star - they both orbit the common centre of gravity. As a star is much more massive than a planet, this centre around which the star "orbits" is actually inside the star - so all we see is the barely detectable wobble.

Detecting Earth-like planets needs technology that is literally out of this world. A proposed telescope designed to look for evidence of life in the Universe, and dubbed "Darwin", could be put into orbit by the middle of the next century. Designs exist for a telescope which would obtain images of Earth-like planets up to a hundred light years away, and even provide information about life on those planets.

Such planets are too small to produce a detectable wobble in the parent stars, and have to be imaged directly. The light they reflect from the parent star makes them only one-billionth as bright as the star - but in the extreme infra-red, light with wavelengths much longer than those our eyes can see, they are a million times brighter, one-thousandth as bright as the star. With several infra-red telescopes joined together to make one huge instrument, Darwin would be able to detect any Earth- like planets in orbit around the 118 nearest stars. And 44 of those stars are sufficiently like our Sun that such planets might well be there.

But the cleverest feature of Darwin is that it will be able to monitor the chemical composition of the atmospheres of those planets. Every chemical substance emits and absorbs light at different wavelengths, producing a characteristic "fingerprint" in the spectrum of light we see, or detect with our telescopes. Darwin will be able to see the fingerprints of three key compounds - carbon dioxide, water and ozone.

Of the three more or less Earth-like planets in our Solar System, Venus and Mars both have carbon dioxide atmospheres, typical of a dead and lifeless world (which makes the debate about whather there is life on Mars today rather pointless). But the atmosphere of Earth contains both water and, far more significantly, ozone. Ozone is a form of oxygen, and oxygen is produced by the activity of life. Oxygen is highly reactive, as we see every time we strike a match, and cannot survive without being replenished. If all life on Earth ceased tomorrow, all the oxygen would soon react with other substances and be locked up in stable compounds such as carbon dioxide. There would be no more ozone.

There are just 39 stars close enough to us that Darwin would be able to detect ozone on an Earth-like planet in orbit around one of them. But it only takes one observation. If Darwin finds ozone on just a single planet, we will know that we are not alone - and people alive on Earth today may still be alive to witness the discovery.

Dr Simon Goodwin and Dr John Gribbin work in the Astronomy group at Sussex University. They are the authors of `Empire of the Sun' (Constable, pounds 16.99)