Astronomers have found an extrasolar planet that may be just 5.5 times as massive as the Earth - that would make it the smallest exoplanet ever detected around a normal star. The find suggests Earth-like planets are abundant in the galaxy and validates a technique that should be able to find them.
Most planet-hunting techniques are best at detecting massive planets in tight orbits around their host stars - but such planets are poor candidates for fostering life. About 170 exoplanets have been found around normal stars, but until now the smallest has weighed in at 7 Earth masses and was found by analysing the gravitational wobble it induced in its host star.
Now, a team led by Jean-Philippe Beaulieu at the Astrophysical Institute of Paris, France, has identified a planet that may be just 5.5 Earth masses. It appears to orbit a small star, called a red dwarf, which lies about 22,000 light years from the Sun.
It was found using a technique called gravitational microlensing, in which one star passes in front of another as seen from Earth. Light from the background star is gravitationally bent and magnified for a period of days to weeks during the event. But if the nearer star hosts a planet, the planet's gravity can give an added boost to the background star's light for a few hours.
The events occur randomly and do not repeat themselves, making finding them very labour-intensive. This was one of about 1000 microlensing events identified annually by an international collaboration called OGLE (Optical Gravitational Lensing Experiment) which monitors 170 million stars in the Milky Way's central "bulge". Astronomers in Perth, Australia, who follow up some of these events as part of another consortium called PLANET (Probing Lensing Anomalies NETwork) found the planet's telltale signal on 9 August 2005.
But there is some uncertainty in the planet's mass - it could be as small as 2.8 Earth masses, or as large as 11.0. That is because the method measures only the ratio of the masses of the planet and its star - researchers then use galactic models to estimate the star's most likely mass.
Nonetheless, researchers say the find is important as it allows them to test theories of planet formation around red dwarfs, which make up about 85% of the stars in the galaxy. Other techniques reveal Jupiter-sized planets are rare around the stars - only a few red dwarfs have been found to harbour them - but those techniques are simply not sensitive enough to detect smaller bodies.
"This suggests lower-mass planets are a lot more common than Jupiters around low-mass stars, which are the most common stars in the galaxy," says team member David Bennett at the University of Notre Dame in Indiana, US.
Small and puny
Scott Gaudi of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, offers a possible explanation. The stars are so much smaller than the Sun - ranging from 10% to 50% of the Sun's mass - that objects orbit them much more slowly and therefore take longer to collide, coalesce and grow into planets.
"Everything happens much slower around red dwarfs," he told New Scientist. Planets simply do not have the opportunity to grow as large as Jupiter - which is 318 times as massive as Earth - because the star's radiation blows away their dusty, gassy building materials before they can grow. "You end up with small, puny things and not a lot of giant things," says Gaudi.
This planet, dubbed OGLE-2005-BLG-390Lb, appears to orbit its star at a distance about 2.6 times greater than between the Sun and the Earth. But the red dwarf shines so weakly compared to the Sun that the planet's surface is thought to hover at about -220°C, as frigid as Neptune and Pluto.
That is too cold to harbour life as we know it. But Gaudi says the new find suggests most of the red dwarfs in the galaxy have such chilly "super Earths". "This hints that there are probably planets even closer, in the habitable zone," he says.
Matter of time
Astronomers say the find is also an important milestone for microlensing. Two other planets had been found with the technique since 2004, but each was several times the mass of Jupiter. "They've proven they can detect these low-mass things," says Sara Seager of the Carnegie Institution of Washington in Washington DC, US. "It's just a matter of time before they get more."
"There are really no other methods at present that can detect planets of this mass and at this separation," agrees Andrew Gould of Ohio State University in Columbus, US. He argues that microlensing searches should be intensified by upgrading cameras to cover a wider view of the sky and dedicating more time - and more telescopes - to monitor the galactic bulge.
"For a long time, we've worked on detecting planets with whatever was at hand, making use of existing small telescopes or even amateur telescopes," Gould told New Scientist. "It's time to move on to the next stage."
Journal reference: Nature (vol 439, p 437)
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