杏吧原创

Update: NASA鈥檚 Kepler mission successfully blasted off on Friday, 6 March aboard a Delta II rocket from Florida鈥檚 Cape Canaveral Air Force Station at 2250 EST (0350 GMT on Saturday). About an hour later, the spacecraft separated completely from its launcher as planned, detaching from the rocket鈥檚 third stage.

HOW common are alien Earths 鈥� small, rocky planets orbiting at the right distance to be not so hot that water boils and not so cold that it stays frozen? Till now clues have been hard to come by, because surveys have not been sensitive enough to find many such planets.

That should soon change thanks to the , which NASA is expecting to launch on 5 March. Its unique positioning in the solar system and unprecedented sensitivity mean that for the first time we will be able to see Earth-size planets in the 鈥渉abitable zone鈥� of their stars 鈥� the region where the temperature on the planet should be right for liquid water to exist at its surface.

鈥淲e all hope that there will be lots of such planets,鈥� says Kepler鈥檚 chief scientist, of NASA鈥檚 Ames Research Center in Moffett Field, California. 鈥淚f there are lots of Earths out there, there may be lots of life out there,鈥� he says 鈥� and perhaps even alien civilisations 鈥渏ust waiting for us to contact them鈥�.

鈥淚f there are lots of Earths out there, there may be lots of life 鈥� perhaps even alien civilisations鈥�

We know that planets only slightly bigger than Earth do exist. One was found by ground-based telescopes using a technique called gravitational lensing, and another by the Corot satellite (see 鈥淓urope鈥檚 alien-watcher鈥�), a transit-hunting mission with more limited capabilities than Kepler, which launched in 2006. There are also two potentially rocky planets with just a few times the mass of Earth known to orbit the red dwarf star Gliese 581, one or the other of which could be just within the habitable zone. But they are, at best, near the zone鈥檚 boundary, so it is far from certain that they could sustain liquid water on their surfaces (New 杏吧原创, 22 November 2008, p 36).

鈥淭he detection of a truly terrestrial-mass planet in a truly habitable orbit still remains very much a holy grail,鈥� says of the University of California, Santa Cruz, who leads a ground-based hunt for transiting planets called .

The problem lies with the main method used so far to spot exoplanets. The vast majority were discovered by the so-called radial velocity technique, which searches the light spectra of stars for signs of periodic motion towards and away from us due to gravitational tugs from their planets. This method isn鈥檛 yet sensitive enough to detect planets as lightweight as Earth.

Kepler has been designed specifically to remedy that. It will use an increasingly fruitful way to find other worlds that looks for what are called 鈥減lanet transits鈥�. By monitoring more than 100,000 stars for periodic dips in brightness, it can spot when a planet passes in front of them.

Kepler will be in its own orbit around the sun, relatively far from Earth, so our planet will not interfere with observations. This will allow the mission to observe the same patch of sky uninterrupted for the entire mission of at least three-and-a-half years. By contrast, Corot is in orbit around Earth, where our planet blocks much of the sky. This, combined with the need to avoid stray light from the sun, limits Corot to viewing a patch of sky for only five months at a time.

By staring longer, Kepler can observe at least three transits of any planets it finds that are in one-year orbits, like Earth鈥檚. That is the minimum needed to confirm that the event recurs at a precise interval, and so rule out confounding factors such as fluctuations in the brightness of the star itself.

Kepler will also be much more sensitive than Corot. It will have an effective light-gathering aperture of 95 centimetres, compared with Corot鈥檚 27 centimetres, which should allow it to see planets as small as half the size of Earth, or about the width of Mars.

No one knows how many Earth-size planets Kepler will find, but one hopeful sign is that Jupiter-size planets turn out to be quite common. They have been found around roughly 15 per cent of sun-like stars. 鈥淲e think it鈥檚 harder to build a Jupiter than to build an Earth,鈥� Borucki says, since a Jupiter requires more material.

Whatever Kepler discovers, it will have set the scene for the next stage of planet-hunting. If Earth-like planets are common, then a few are likely to orbit stars near enough for follow-up missions to scrutinise them for evidence of oxygen and other clues that could hint at the presence of life.

NASA and ESA have been weighing up missions called and respectively that could launch before 2020, to try to observe the feeble light from extrasolar planets in the habitable zone of their stars. A major goal would be to measure the spectrum of the light passing though their atmospheres, looking for the signature of oxygen and other chemicals that would hint at the presence of life.

The success of these missions hinges on Kepler, says of the University of Arizona, Tucson, who chaired a committee called the that produced a on alien worlds. If Earths turn out be abundant, we should be able to perform this sort of close analysis using 鈥渢he sorts of designs that people have talked about that are implementable in the next 15 years at costs that we can contemplate鈥�, he says. But if Kepler finds that rocky worlds in the habitable zone are rare, then there may not be any sufficiently close to us to observe in detail. A mission capable of looking at more distant worlds might be too technically difficult and costly to be realised for a long time to come.

At least history gives us reason to hope that alien Earths will not be a rarity. Ever since Copernicus displaced our own planet from the centre of the universe, its place in the cosmos has been steadily shown to be less and less privileged. 鈥淚t turned out the Earth was just one planet out of many,鈥� Lunine says. 鈥淣ow we鈥檙e also seeing that our solar system is not necessarily particularly special; we know of hundreds of planets around other stars.鈥�

But the big question remains: are planets the size and with the properties of the Earth common in the universe? 鈥淜epler is the one that will tell us that,鈥� Lunine says.