2016ApJ...825..112S

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) x 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 {M}_{p}={21}{-7}^{+6}{M}_⊕ orbiting a low-mass M-dwarf with a mass of {M}_{h}={0.19}{-0.06}^{+0.05}{M}_☉ , (ii) a mini-Neptune with {M}_{p}={7.9}{-1.2}^{+1.8}{M}_⊕ orbiting a brown dwarf host with {M}_{h}={0.068}{-0.011}^{+0.019}{M}_☉ , and (iii) a super-Earth with {M}_{p}={3.2}{-0.3}^{+0.5}{M}_⊕ orbiting a low-mass brown dwarf host with {M}_{h}={0.025}{-0.004}^{+0.005}{M}_☉ , which is slightly favored. The 3D planet-host separations are {4.6}_{-1.2}^{+4.7} au, {2.1}_{-0.2}^{+1.0} au, and {0.94}_{-0.02}^{+0.67} au, 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.