Wednesday, July 22, 2015

A Saturn-Mass Planet beyond the Snowline of an M Dwarf Star

Gravitational microlensing is a powerful technique for detecting exoplanets that orbit their host stars beyond the snowline (i.e. the distance from a star where temperatures are cool enough for water-ice and other volatiles to condense). Beyond the snowline, a protoplanetary disk around a star is expected to contain more material, allowing for the formation of more massive planets. When a foreground star crosses the line-of-sight to a background star, the gravity of the foreground star can act as a lens, magnifying the light from the background star. The change in the brightness of the background star with time is measured in the form of a light curve. If the background star hosts a planet, the planet’s own gravity can induce a “spike” in the light curve of the background star.

Using the technique of gravitational microlensing, Fukui et al. (2015) present the discovery of a Saturn-mass planet with ~0.34 times the mass of Jupiter orbiting an M dwarf star with ~0.39 times the Sun’s mass at a projected separation of either ~0.74 AU (close model) or ~4.3 AU (wide model). The planet is identified as OGLE-2012-BLG-0563Lb and it is the 5th sub-Jupiter-mass (i.e. between 0.2 to 1.0 times the mass of Jupiter) to be found around an M dwarf star through gravitational microlensing. Although it is clear that there is a population of sub-Jupiter-mass planets around M dwarf stars, there appears to be a paucity of Jupiter-mass planets (i.e. planets with ~1 to 2 times the mass of Jupiter) around the same type of star. This suggests that planet formation via the core-accretion mechanism rarely produces Jupiter-mass planets around M dwarf stars due to the lack of material in the protoplanetary disk.

Fukui et al. (2015), “OGLE-2012-BLG-0563Lb: a Saturn-mass Planet around an M Dwarf with the Mass Constrained by Subaru AO imaging”, arXiv:1506.08850 [astro-ph.EP