# MOA-2011-BLG-262Lb: A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge

### Citation

Bennett, DP and Batista, V and Bond, IA and Bennett, CS and Suzuki, D and Beaulieu, JP and Udalski, A and Donatowicz, J and Bozza, V and Abe, F and Botzler, CS and Freeman, M and Fukunaga, D and Fukui, A and Itow, Y and Koshimoto, N and Ling, CH and Masuda, K and Matsubara, Y and Muraki, Y and Namba, S and Ohnishi, K and Rattenbury, NJ and Saito, T and Sullivan, DJ and Sumi, T and Sweatman, WL and Tristram, PJ and Tsurumi, N and Wada, K and Yock, PCM and Albrow, MD and Bachelet, E and Brillant, S and Caldwell, JAR and Cassan, A and Cole, AA and Corrales, E and Coutures, C and Dieters, SF and Dominis Prester, D and Fouque, P and Greenhill, J and Horne, K and Koo, JR and Kubas, D and Marquette, JB and Martin, R and Menzies, JW and Sahu, KC and Wambsganss, J and Williams, A and Zub, M and Choi, JY and DePoy, DL and Dong, S and Gaudi, BS and Gould, A and Han, C and Henderson, CB and McGregor, D and Lee, CU and Pogge, RW and Shin, IG and Yee, JC and Szymanski, MK and Skowron, J and Poleski, R and Kozlowski, S and Wyrzykowski, L and Kubiak, M and Pietrukowicz, P and Pietrzynski, G and Soszynski, I and Ulaczyk, K and Tsapras, Y and Street, RA and Dominik, M and Bramich, DM and Browne, P and Hundertmark, M and Kains, N and Snodgrass, C and Steele, IA and Dekany, I and Gonzalez, OA and Heyrovsky, D and Kandori, R and Kerins, E and Lucas, PW and Minniti, D and Nagayama, T and Rejkuba, M and Robin, AC and Saito, R, MOA-2011-BLG-262Lb: A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge, The Astrophysical Journal, 785 Article 155. ISSN 0067-0049 (2013) [Refereed Article]

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We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product ML πrel where ML is the lens system mass and πrel is the lens-source relative parallax. If the lens system is nearby (large πrel), then ML is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, μrel = 19.6 ± 1.6 mas yr–1, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, there is an alternate solution that has a lower proper motion and fits the data almost as well. This solution is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of $M_{\rm host} = 0.12^{+0.19}_{ -0.06}\,M_\odot$ and $m_{\rm comp} = 18^{+28}_{ -10}\,{M_\oplus }$, at a projected separation of $a_\perp = 0.84^{+0.25}_{ -0.14}$ AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.