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Detecting habitable new worlds
The next blue planet
Aug 13th 2009 | RIO DE JANEIRO
From The Economist print edition
The race is on to discover a second Earth
IN 1995, when Michel Mayor of the University of Geneva detected the first exoplanet (a planet that orbits
a star other than the sun) he started a race that has gained pace ever since. Some 360 such planets have
now been detected, but none is exactly equivalent to the Earth.
The closest so far is Gliese 581c, which was discovered in 2007 by Dr Mayor’s colleague, Stéphane Udry.
It is both rocky and orbits its parent star at a distance where liquid water could reasonably be expected to
exist. However, since its parent star is a red dwarf—a far smaller and fainter object than the sun—that
orbit is, in fact, much smaller that the Earth’s around the sun. That, in turn, suggests Gliese 581c is likely
to be tidally locked to its orbital period, so that one side of the planet always faces the star and the other
never does. Having half a planet in permanent daylight and the other half in permanent darkness does not
sound like a good recipe for life.
As astronomers heard this week at the International Astronomical Union meeting in Rio, two new
missions—a French one launched in December 2006 and an American one launched on March 6th—are in
the process of trying to add to the list. Dr Mayor told the meeting that the French mission, CoRoT, has
now found 80 exoplanets. It does so by watching for small diminutions in the amount of light from a star
as the planet in question passes in front of it, a phenomenon known technically as a transit. The details of
all but seven of these transiting planets are still unpublished, but Dr Mayor gave the meeting a preview.
The planets discovered so far by CoRoT typically have a mass that is less than 30 times that of Earth,
making them likely to have a solid, rocky surface. But they also orbit their stars rapidly, typically taking
two or three months, rather than a year, to do so. For those who hanker after extraterrestrial life that is a
pity. Such rapid orbits mean the planets in question are close to their parent stars, and thus likely to be
tidally locked.
Other news from CoRoT is better, though. Some 80% of the planets Dr Mayor has found have siblings.
The existence of so many neighbours suggests that planetary systems tend to be stable, and stability is
good for the evolution of life. Dr Mayor described a system he has seen that has five rocky planets in it.
They have masses of 11, 14, 26, 27 and 76 times that of the Earth. He concluded his talk by saying, “I am
really confident that we have an Earth-like planet coming in the next two years.”
He and his team may, however, be pipped at the post. On August 6th America’s space agency, NASA,
announced that its Kepler planet-detector (named after the man who worked out the laws of planetary
motion, as this article explains) is also behaving well. A paper published in Science by William Borucki of
the NASA Ames Research Centre based in Moffett Field, California, and his colleagues showed that Kepler,
which also uses the transit-detection technique, has confirmed the existence of a Jupiter-like planet
discovered in 2007 and provided more precise details of that planet’s mass and orbital period. And
Kepler’s instruments are more sensitive than CoRoT’s, so it should be capable of finding Earth-sized
planets more easily than its French cousin.
Yet such space probes are not the only way of searching for other Earths. As part of his efforts to find new
worlds, Dr Mayor is using the HARPS spectrograph, which is based at the European Southern Observatory
in La Silla, Chile. He and his colleagues are training HARPS on ten nearby, bright and quiet stars three
times a night, for 15 minutes at a time, for 50 nights a year, for at least two years, in the hope of spotting
a nearby Earth-sized planet. The device works by detecting the tiny wobble given to a parent star when a
planet passes it by. The spectrograph has already found 16 planets.
Meanwhile, David Bennett of the University of Notre Dame in Indiana wants to use a technique called
gravitational microlensing to spot planets that might be missed by other methods. He told the conference
that his approach would pick up not only small rocky planets orbiting at great distances from their parent
stars, but also planets that had been ejected from their orbits. The idea would be to stare at a distant star
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