THE striking success of a new planet-hunting technique that has found a chilly 鈥渟uper-Earth鈥� nearly 9000 light years away is not only suggesting that such planets are common, it is even encouraging astronomers to look for planets beyond our own galaxy.
Around 170 extrasolar planets have been discovered so far, most of them gas giants like Jupiter circling nearby stars. The majority have come to light because their immense gravity makes their parent stars wobble, a motion that has revealed planets up to a few hundred light years away.
Last year, observations of a phenomenon called microlensing found a planet nearly 22,000 light years away. Microlensing occurs when one star passes directly in front of another, as seen from Earth, and the gravity of the foreground star briefly lenses the light of the background star, producing a characteristic brightening. If the foreground star has a planet, that can create a telltale blip in the magnified light.
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The latest planet was also found using microlensing. Three international teams 鈥� called OGLE, MicroFUN and RoboNet 鈥� together analysed a bright microlensing event in the central bulge of the Milky Way using five telescopes in the US, Chile and New Zealand. They found that the lensing star is about half as massive as the sun and has a planet of 13 Earth masses in tow. This Neptune-mass planet lies in the outer reaches of its planetary system, about 2.7 times the Earth-sun distance from its star, and the astronomers estimate that its temperature is a frigid -200 掳C.
The pattern of microlensing showed that the new planet has no giant Jupiter-like siblings. 鈥淚t鈥檚 the first time that we can rule out gas giants,鈥� says Andrew Gould of Ohio State University in Columbus, team leader for MicroFUN. 鈥淐learly, there鈥檚 just a Neptune dominating the system by itself.鈥� This suggests that the planetary system did not contain enough gas to build gas giants, in which case the new planet is probably a 鈥渘aked super-Earth鈥� of rock and ice without a thick atmosphere. Gould has submitted the findings to Astrophysical Journal Letters.
This is the second such Neptune-mass planet discovered in the outer reaches of a planetary system from about half a dozen candidate microlensing events studied so far. 鈥淭he fact that they鈥檙e picking up so many Neptune-mass planets does suggest that they really are abundant,鈥� says Andrew Collier Cameron, a planet hunter at the University of St Andrews, UK.
鈥淭he discovery of a second chilly 鈥榮uper-Earth鈥� is encouraging astronomers to look for planets beyond our galaxy鈥�
Collier Cameron says that microlensing is showing great promise for finding distant planets throughout our galaxy. What鈥檚 more, next year Gould hopes to begin an ambitious microlensing search for planets in the giant Andromeda galaxy, 2.5 million light years away.
