Earth-like planet may be first of many

my edits in bold

• 28 January 2006
• Maggie McKee

PLANET hunters have detected what seems to be the smallest extrasolar planet so far, orbiting a red dwarf 22,000 light years away. Because red dwarfs are the most common type of star in the Milky Way, this might mean that Earth-like planets are abundant in our galaxy. In any case, it bodes well for "gravitational microlensing", the technique used to find this exoplanet.

Most planet-hunting techniques pick up massive planets in tight orbits around their host stars. About 170 exoplanets have been found around sun-like stars and until now the smallest had weighed in at 7 Earth masses. Now Jean-Philippe Beaulieu at the Institute of Astrophysics in Paris, France, and his team have found a planet that seems to be just 5.5 times the mass of Earth. To pick out the star they exploited the phenomenon called gravitational microlensing. When one star passes in front of another as seen from Earth, light from the background star is bent and magnified, or "lensed", by the gravity of the foreground star. If the star in the front is playing host to a planet, the planet's gravity can boost the light of the background star for a few hours.

The microlensing event that revealed the new planet was one of about 1000 picked up each year by OGLE, an international collaboration which monitors 170 million stars in the Milky Way's central bulge. Astronomers in Perth, Australia, who follow up some of these events for another consortium called PLANET, found the exoplanet's telltale signal on 9 August 2005.

But there is still some uncertainty as to the planet's mass - it could be anything from 2.8 to 11 Earth masses. That is because the microlensing measures only the ratio of the mass of the host star to the planet's mass, and researchers then have to use models to estimate the most likely mass of the star and the planet (Nature, vol 439, p 437).

The planet appears to orbit its star at about 2.6 times the distance of the Earth to the sun and is as frigid as Neptune and Pluto, with a surface temperature of about -220 °C. It is significant that the planet has been found around a red dwarf because such stars make up about 70 per cent of the stars in the Milky Way, so there could be many more such planets out there.

Other techniques have shown that Jupiter-sized planets are rare around red dwarfs. "This suggests that lower-mass planets are a lot more common than Jupiters around low-mass stars," says team member David Bennett at the University of Notre Dame in Indiana.

The find is also an important milestone for microlensing. "They've proven they can detect these low-mass [planets]," says Sara Seager of the Carnegie Institution of Washington in Washington DC. "It's just a matter of time before they get more."

From issue 2536 of New Scientist magazine, 28 January 2006, page 12