05 February 2009 by David Shiga
HOW common are alien Earths - 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 Kepler space telescope, 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 "habitable zone" of their stars - the region where the temperature on the planet should be right for liquid water to exist at its surface.
"We all hope that there will be lots of such planets," says Kepler's chief scientist, William Borucki of NASA's Ames Research Center in Moffett Field, California. "If there are lots of Earths out there, there may be lots of life out there," he says - and perhaps even alien civilisations "just waiting for us to contact them".
If there are lots of Earths out there, there may be lots of life - perhaps even alien civilisations
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 "Europe's alien-watcher"), 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's boundary, so it is far from certain that they could sustain liquid water on their surfaces (New Scientist, 22 November 2008, p 36).
"The detection of a truly terrestrial-mass planet in a truly habitable orbit still remains very much a holy grail," says Gregory Laughlin of the University of California, Santa Cruz, who leads a ground-based hunt for transiting planets called Transitsearch.
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't 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 "planet transits". 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's. 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's 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. "We think it's harder to build a Jupiter than to build an Earth," 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 Terrestrial Planet Finder and Darwin 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 Jonathan Lunine of the University of Arizona, Tucson, who chaired a committee called the Exoplanet Task Force that produced a report last year on alien worlds. If Earths turn out be abundant, we should be able to perform this sort of close analysis using "the sorts of designs that people have talked about that are implementable in the next 15 years at costs that we can contemplate", 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. "It turned out the Earth was just one planet out of many," Lunine says. "Now we're also seeing that our solar system is not necessarily particularly special; we know of hundreds of planets around other stars."
But the big question remains: are planets the size and with the properties of the Earth common in the universe? "Kepler is the one that will tell us that," Lunine says.
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