Using the technique of gravitational microlensing, Sumi et al. (2015) present the discovery of the coldest low-mass planet ever found. This planet is identified as MOA-2013-BLG-605Lb. Analysis of the gravitational microlensing light curve reveals three physical solutions. At present, it is impossible to distinguish which physical solution is the more likely one. Regardless, MOA-2013-BLG-605Lb has a mass similar to that of Neptune or a super-Earth. The planet orbits its host star/brown dwarf at a distance that is roughly 9 to 14 times the expected position of the snowline around its host star/brown dwarf. The snowline is basically the distance from a young star or brown dwarf where the temperature becomes cool enough for water to condense into solid ice grains. For the Sun, the snowline is located at a distance of 2.7 AU.
Figure 1: Artist’s impression of a cold Neptune-mass planet.
Figure 2: Gravitational microlensing light curve indicating the presence of MOA-2013-BLG-605Lb. The middle and bottom panels show the detail of the planetary signal and the residuals from the best fit model. Sumi et al. (2015)
The three physical solutions to the gravitational microlensing lightcurve depend on the distance of MOA-2013-BLG-605Lb from Earth. For the small parallax model, the distance is 11.7 [-2.6, +2.0] thousand light years; for the medium parallax model, the distance is 5.9 [-0.7, +1.3] thousand light years; and for the large parallax model, the distance is 2.8 [-0.3, +0.4] thousand light years.
In the small parallax model, MOA-2013-BLG-605Lb has 21 [-7, +6] times the mass of Earth and it orbits a low-mass red dwarf star with 0.19 [-0.06, +0.05] times the mass of the Sun at a projected separation of 4.2 [-0.9, +0.7] AU, which is 8.9 [-1.4, +10.5] times the expected position of the snowline around its host star. Basically for this model, MOA-2013-BLG-605Lb is a Neptune-mass planet at a wide separation from a low-mass red dwarf star.
In the medium parallax model, MOA-2013-BLG-605Lb has 7.9 [-1.2, +1.8] times the mass of Earth and it orbits a high-mass brown dwarf with 0.068 [-0.011, +0.019] times the mass of the Sun at a projected separation of 2.1 [-0.2, +0.4] AU, which is 12 [-1, +7] times the expected position of the snowline around its host brown dwarf. Basically for this model, MOA-2013-BLG-605Lb is a mini-Neptune in a distant orbit around a high-mass brown dwarf.
In the large parallax model, MOA-2013-BLG-605Lb has 3.2 [-0.3, +0.5] times the mass of Earth and it orbits a low-mass brown dwarf with 0.025 [-0.004, +0.005] times the mass of the Sun at a projected separation of 0.94 [-0.09, +0.12] AU, which is 14 [-1, +11] times the expected position of the snowline around its host brown dwarf. Basically for this model, MOA-2013-BLG-605Lb is a super-Earth in a distant orbit around a low-mass brown dwarf.
Figure 3: Artist’s impression of a cold Neptune-mass planet.
For the small, medium and large parallax models, the estimated temperature on MOA-2013-BLG-605Lb is ∼26 K, ∼13 K and ∼7 K, respectively. This makes MOA-2013-BLG-605Lb the coldest low-mass planet found to date and it could be the first known example of a Neptune-like planet in terms of mass and temperature. Future observations can resolve which of the three models are more likely. For example, direct imaging may be able to detect the system if it is a low-mass red dwarf star since a brown dwarf will be too dim to be detectable.
Reference:
Sumi et al. (2015), “The First Cold Neptune Analog Exoplanet: MOA-2013-BLG-605Lb”, arXiv:1512.00134 [astro-ph.EP]