When a foreground star happens to cross the line-of-sight to a distant background star, the gravitational field of the foreground star can act as a lens, magnifying light from the background star. The brightening of the background star is recorded in the form of a light curve as the foreground star crosses its line-of-sight. If the foreground star hosts a planet, the gravitational field of the planet can induce a "blip" in the light curve. This phenomenon is known as gravitational microlensing and it is one of the methods used to detect planets around other stars.
Figure 1: Artist's impression of a Saturn-mass planet.
Using this technique, Hirao et al. (2016) present the discovery of a Saturn-mass planet around a red dwarf star with ~30 percent the mass of the Sun. The planet is identified as OGLE2012-BLG-0724Lb. By analysing the gravitational microlensing light curve, the planet is estimated to have ~0.47 times the mass of Jupiter and its estimated projected separation from its host star is ~1.6 AU.
The discovery of this Saturn-mass planet around a red dwarf star adds to the population of known sub-Jupiters (i.e. planets with 0.2 to 1.0 times the mass of Jupiter) around red dwarf stars. On the contrary, there appears to be a lack of Jupiter-mass planets (i.e. planets with 1 to 2 times the mass of Jupiter) around red dwarf stars. This supports the core accretion model of planet formation which predicts that Jupiter-mass planets, and planets that are even more massive, are unlikely to form around red dwarf stars.
Figure 2: The top panel shows the gravitational microlensing light curve. The middle panel shows a close up of the "blip" in the gravitational microlensing light curve indicating the presence of OGLE2012-BLG-0724Lb. The bottom panel shows the residual from the best fit model. Hirao et al. (2016)
Hirao et al. (2016), "OGLE-2012-BLG-0724Lb: A Saturn-mass Planet around an M-dwarf", arXiv:1604.05463 [astro-ph.EP]