The First Neptune Analog or Super-Earth with Neptune-like Orbit: MOA-2013-BLG-605Lb

Abstract

We present the discovery of the first Neptune analog exoplanet or super-Earth with 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 $9\sim 14$ times the expected position of the snow-line, $a_{\mathrm{snow}}$, which is similar to Neptune's separation of $11a_{\mathrm{snow}}$ from the Sun. The planet/host-star mass ratio is $q=(3.6\pm 0.7)\times 10^{-4}$ and the projected separation normalized by the Einstein radius is $s=2.39\pm 0.05$. 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 $M_{\mathrm{p}}=21_{-7}^{+6}{M}_{Earth}$ orbiting a low-mass M-dwarf with a mass of $M_{\mathrm{h}}=0.19_{-0.06}^{+0.05}{M}_{\odot }$, (ii) a mini-Neptune with $M_{\mathrm{p}}=7.9_{-1.2}^{+1.8}{M}_{Earth}$ orbiting a brown dwarf host with $M_{\mathrm{h}}=0.068_{-0.011}^{+0.019}{M}_{\odot }$ and (iii) a super-Earth with $M_{\mathrm{p}}=3.2_{-0.3}^{+0.5}{M}_{Earth}$ orbiting a low-mass brown dwarf host with $M_{\mathrm{h}}=0.025_{-0.004}^{+0.005}{M}_{\odot }$ which is slightly favored. The 3-D 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 $a_{\mathrm{snow}}$ for these models, respectively. The Keck AO observation confirm that the lens is faint. This discovery suggests that low-mass planets with Neptune-like orbit are common. So processes similar to the one that formed Neptune in our own Solar System or cold super-Earth may be common in other solar systems.

54 pages, 10 figures, 13 tables, Accepted for publication in the ApJ