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Microlensing Finds a Rocky Planet

Gravitational microlensing, promises to find planets down to 10 Earth masses, much farther out from their parent stars. Using this technique, a team of astronomers has just announced the detection of a rocky planet in this range.
According to the Extrasolar Planet Encyclopaedia, astronomers have discovered 13 planets using gravitational microlensing. The newly announced one, MOA-2009-BLG-266Lb, is estimated to be just over 10 times the mass of Earth and orbits at a distance of 3.2 AUs around a parent star with roughly half the mass of the Sun.
The new finding is important because it is one of the first planets in this mass range that lies beyond the "snow line", the distance during formation of a planetary system beyond which ice can form from water, ammonia, and methane. This presence of icy grains is expected to assist in the formation of planets since it creates additional, solid material to form the planetary core.

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Title: OGLE-2008-BLG-513Lb: The Orbital Solution for a Microlensing Planet
Authors: J.C. Yee, A. Udalski, Subo Dong, J. Greenhill, Y. Tsapras, I.A. Bond, A. Gould, S. Kozlowski, P. Fouque, M.D. Albrow, C. Han, L.A.G. Monard, J. McCormick, A. Williams, N. Kains, J. An, M. Dominik, et al

The dominant features of the microlensing event OGLE-2008-BLG-513 arise from a 2-body lens with a mass ratio q=0.027±0.001. The light curve cannot be adequately described by a static, 2-body lens model, which forces us to consider the orbital motion of the lens system. Including orbital motion improves the fit by Delta chiČ>1000. We model the orbital motion as a Keplerian orbit, and with the additional information from microlens parallax, we are able to place constraints on all eight parameters of the orbit. If our model is correct, this gives us the most complete orbital information of any microlensing planet. We find that the host star is 0.18<M_L/M_Sun<0.34, and the planet is 5.0<m_p/M_Jup<9.6 with semimajor axis a ~3.0 AU. Although this model fully explains the shape of the light curve, it has the unusual requirement that if the lens system is bound, the source is at a distance of 1.2<D_S/kpc<1.8 rather than the typical distance of 8 kpc. This event is also unusual because the position of the source in the VIH colour-colour diagram, which is determined independent of the light curve model, is strange compared to the stars in the field and cannot be easily explained. A nearby source may help explain why the colours of the source are so peculiar, although this explanation is not completely satisfactory. This peculiarity provides us with a means to confirm or refute our microlensing model by obtaining a spectrum of the source, which we predict will show a metal-poor, late-K/early-M dwarf. If our model is confirmed by such a spectrum, this massive planet close to its M dwarf host likely presents a challenge to both core accretion and gravitational instability planet formation theories.

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