Bigger than Earth, but No Giant

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Science  18 Nov 2011:
Vol. 334, Issue 6058, pp. 878
DOI: 10.1126/science.334.6058.878-d

The microlensing method of detecting planets takes advantage of the fact that the light of a background star gets deflected by the gravitational field of a foreground star with which it is spatially aligned. If the foreground star hosts a planet, light will be deflected in a way that furthermore depends on the planet's mass and distance from its host star. Using this method to analyze observations from 13 different telescopes around the world and one in space, Muraki et al. detected a planet 10.4 times more massive than Earth. The planet orbits a star 0.56 times as massive as the Sun, at a distance comparable to Jupiter's distance from the Sun. This puts the planet's orbit beyond the snow line—the distance from a star beyond which ice can condensate—and thus in the sort of environs where giant planets such as Jupiter and Saturn form. However, the new planet looks more like a failed giant planet; one, in other words, that accreted enough solid material to form the core of a giant planet but never acquired a gaseous envelope because the protoplanetary disk lost its gas before the solid core was massive enough to efficiently attract hydrogen and helium.

Astrophys. J. 741, 22 (2011)

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