Eye on the cosmos

A new telescope - Darwin - positioned far beyond the Earth, could transform our view of the universe. And, says Ian Brown, it may provide our best chance yet of finding extra-terrestrial life

Imagine peering into the night sky and pointing to a cluster of stars in the knowledge that among them, invisible to the naked eye, is a planet much like Earth, with an atmosphere, running water and life. Within the next dozen or so years we might be able to do just that. A revolutionary new space-telescope called Darwin, stationed 1.5 million kilometres from Earth, could by 2014 be scanning the star fields, and pinpointing alien worlds that are not just capable of supporting life, but already home to it.

The mission would be the most ambitious science project proposed to date by the European Space Agency (ESA). Its findings could change our perceptions of our place in the universe.

Alan Penny of the Rutherford Appleton Laboratory in Oxfordshire says that Darwin could push astronomy far beyond the limits of telescopes on Earth. A member of ESA's Darwin science team, he has worked on designing the mission's parameters, including how it might confirm the existence of "pre-biotic" worlds that are ripe for life.

Nothing quite like Darwin has ever been launched into space. It will comprise a flotilla of six telescopes, each 1.5m in diameter, 100 metres apart, 1.5 million kilometres beyond the Moon. By keeping its back to the Earth, Darwin will observe the entire celestial sphere during a year.

"Darwin won't be in a near-Earth orbit like the Hubble Space Telescope," explains Penny. "It will be quite far out. Once you've put something out in space it will tend to just hang there, although that's not to downplay the technological challenges ahead." Tiny thrusters on each telescope will help to keep the instruments in tight formation.

Looking for planets around even nearby stars is like trying to discern the light from a candle next to a lighthouse – from 1,000km away. At optical wavelengths, stars would outshine an Earth-sized planet by a billion to one. Darwin will also look in the infrared wavelengths where the contrast is a million to one.

Organic activity on Earth produces gases that absorb certain wavelengths of infrared light. Spectral analysis will split the light from an extra-solar planet into its constituent wavelengths and reveal what gases are in its atmosphere. Free oxygen would heavily point to life.

Does Penny think we'll find evidence of extra-terrestrial life? "Haven't the faintest," he says. "No one really knows how life starts, so we don't know yet if it's common or rare. The theory of planet formation is still pretty uncertain too, although we do think small, rocky planets should be a by-product when a star gives birth to a solar system. Let's say we are fairly hopeful of finding these planets, but just don't know about finding life on them. But one cause for optimism might be that life seemed to have started very early on Earth. So that seems to indicate that life does start easily given the right conditions."

The Darwin mission must take place in space for a number of crucial reasons. On Earth, our atmosphere diffuses starlight and blocks mid-infrared wavelengths. Planets are most easily detected using infrared wavelengths because they are warm – observing from Earth is like peering through a fog. At room temperature, the telescopes would themselves emit infrared. Space is so cold that it all but stops the telescope radiating its own infrared signal.

And why Darwin's elaborate flotilla concept? Because to make out planets orbiting nearby stars, we'll need a telescope that is in effect 30m in diameter – way beyond our current technology. Darwin will in effect "cheat" by using a technique known as interferometry. Pioneered by Cambridge astronomers in the 1950s, the technique uses smaller telescopes to mimic a "virtual", much larger telescope. Darwin's six telescopes will combine their signals to give a particularly high-resolution image.

But Darwin must pull off a double bluff. To see a planet, it must also cut out that blinding light from the parent star. An interferometer can also do this, by slightly delaying some of the signals from some of the telescopes. By adjusting this delay, the alien sun is nullified, allowing any faint, nearby planet to stand out. This instrument is a "nulling interferometer".

The ESA will first test the concept on the ground. The Ground-based European Nulling Interferometer Experiment (Genie) will perform the biggest experiment in nulling interferometry ever using the European Southern Observatory's Very Large Telescope. Expected to be online by 2006, Genie will guide ESA engineers on how to build the hub-spacecraft of the Darwin flotilla. "If you see the way of getting to Darwin as a number of technological milestones, this is one of the most important," says Malcolm Fridlund, a Darwin project scientist.

Genie will also reducea target list of stars for Darwin to study. Already, there is a draft list of around 400 of the nearest and most Sun-like stars, including Sirius and Tau Ceti.

"We have calculated that with Darwin we could see an 'Earth' if it were 10 light-years away with a few hours of observation," says Fridlund. "With the Very Large Telescope, it would be impossible because of the atmosphere. So we have to go into space."

The ESA also plans to launch the Smart-2 mission in 2006, two free-flying spacecraft that will demonstrate the formation flying essential to Darwin. Meanwhile, Nasa is busy with its own version of Darwin, the Terrestrial Planet-Finder. Speculation is mounting that the Americans and Europeans will ultimately collaborate on one joint nullifying interferometer space-telescope, which they will launch and operate together.

But before then, several precursor missions are scheduled. The French-led Corot, a 30cm-diameter space-telescope, will be the first to search specifically for rocky Earth-like planets around other stars. It will peer unblinkingly through the universe for tiny changes in the brightness of nearby stars as any orbiting planets pass in front of them. The launch is scheduled in 2005.

Eddington, planned for launch in 2007, will be larger than Corot. It will search more than 500,000 stars and will be able to detect habitable planets half the size of Earth.

All these will be stepping-stones to Darwin. "Perhaps we are alone; perhaps we're not," says Alan Penny. Darwin, he believes, could open a window on to a whole new world – or an infinity of loneliness.