THE FIRST NEPTUNE ANALOG OR SUPER-EARTH WITH A NEPTUNE-LIKE ORBIT: MOA-2013-BLG-605LB

Abstract

We present the discovery of the first Neptune analog exoplanet or super-Earth with a Neptune-like orbit, MOA-2013-BLG-605Lb. This planet has a mass similar to that of Neptune or a super-Earth and it orbits at 9 ∼ 14 times the expected position of the snow line, a snow, which is similar to Neptune's separation of 11 a snow from the Sun. The planet/host-star mass ratio is q = (3.6 ± 0.7) × 10-4 and the projected separation normalized by the Einstein radius is s = 2.39 ± 0.05. There are three degenerate physical solutions and two of these are due to a new type of degeneracy in the microlensing parallax parameters, which we designate "the wide degeneracy." The three models have (i) a Neptune-mass planet with a mass of Mp = 21-7+6M⊕ orbiting a low-mass M-dwarf with a mass of Mh = 0.19-0.06+0.05M⊙, (ii) a mini-Neptune with Mp = 7.9-1.2+1.8M⊕ orbiting a brown dwarf host with Mh = 0.068-0.011+0.019M⊙, and (iii) a super-Earth with Mp = 3.2-0.3+0.5MM⊕ orbiting a low-mass brown dwarf host with Mh = 0.025-0.0004+0.005M⊙, which is slightly favored. The 3D planet-host separations are 4.6-1.2+4.7au, 2.1-0.2+1.0au, and 0.94-0.02+0.67au, which are 8.9-1.4+10.5, 12-1+7, or 14-1+11 times larger than a snow for these models, respectively. Keck adaptive optics observations confirm that the lens is faint. This discovery suggests that low-mass planets with Neptune-like orbits are common. Therefore processes similar to the one that formed Neptune in our own solar system or cold super-Earths may be common in other solar systems.