Gravitational microlensing has led to the detection of planets with masses ranging from more than Jupiter to a few times the mass of Earth. It involves measuring the magnification of light from a distant background star due to the lensing effect by the gravitational field of a foreground star. During a microlensing event, a lightcurve of the background star is produced as the foreground star crosses in front of it. The presence of planets around the foreground star can produce sharp deviations in the otherwise smooth and symmetric lightcurve of the background star.
Figure 1: Artist’s impression of a habitable Earth-like moon around a gas giant planet.
Figure 2: An illustration of a microlensing event of a distant background star by a foreground star with and without a planet.
Using the Keck telescope near the summit of Mauna Kea in Hawaii, a team of astronomers observed a microlensing event and reported the discovery of a gas giant planet that is probably within the habitable zone of its parent star. This planet is identified as MOA-2011-BLG-293Lb. It has a mass of 4.8 ± 0.3 Jupiter mass and orbits its parent star at a distance of 1.1 ± 0.1 AU, where one AU is the average Earth-Sun separation distance. The planet’s parent star is a Sun-like star with a mass of 0.86 ± 0.06 solar mass. MOA-2011-BLG-293Lb and its parent star are located at an estimated distance of 25,000 light years away, near the centre of the Milky Way galaxy.
MOA-2011-BLG-293Lb is interesting because a hypothetical terrestrial-size moon in orbit around it can support Earth-like conditions and may be potentially habitable. Since MOA-2011-BLG-293Lb circles its parent star near the outer edge of the habitable zone, a terrestrial-size moon around this giant planet would require some sort of greenhouse warming effect to keep its surface warm enough to sustain liquid water. MOA-2011-BLG-293Lb is also one of the furthest planets discovered to date.
V. Batista et al. (14 October 2013), “MOA-2011-BLG-293Lb: First Microlensing Planet possibly in the Habitable Zone”, arXiv:1310.3706 [astro-ph.EP]