Figure 1: Artist’s impression of a Neptune-mass planet.
When a foreground star crosses the line-of-sight to a background star, the gravitational field of the foreground star can act as a lens and magnify light from the background star. This phenomenon is known as gravitational microlensing and the presence of a planet around the foreground star can induce perturbations in the resulting gravitational microlensing light curve. The core-accretion theory of planet formation predicts that gas giant planets and Neptune-mass planets form beyond the “snow-line” of their host stars due to the much greater abundance of solid material. In the hunt for planets, gravitational microlensing has proven to be sensitive to planets located beyond the “snow-line” of their host stars.
Skowron et al. (2015) present the discovery of a Neptune-mass planet identified as MOA 2011-BLG-028Lb. The planet is located ~20,000 light years away, near the center of the galaxy, and it was detected during a gravitational microlensing event that lasted from December 2010 until September 2011. The presence of MOA 2011-BLG-028Lb was inferred from a perturbation in the gravitational microlensing light curve that was observed from 12 to 14 May 2011. MOA 2011-BLG-028Lb is estimated to have ~30 times the mass of Earth and it orbits a host star with ~0.75 times the mass of the Sun at a projected separation of between 3.1 to 5.2 AU. This places MOA 2011-BLG-028Lb beyond the “snow-line” of its host star, which is at ~2.0 AU.
Figure 2: Gravitational microlensing light curve indicating the presence of MOA 2011-BLG-028Lb. Skowron et al. (2015)
Skowron et al. (2015), “MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge”, arXiv:1512.03422 [astro-ph.EP]