MOA-2011-BLG-262: Free-floating planet with sub-Earth-mass moon?

A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge
http://arxiv.org/abs/1312.3951

Quote :

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 data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. Nevertheless, these results indicate the potential of microlensing to detect exomoons, albeit ones that are different from the giant planet moons in our solar system. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M pi_rel, where M is the lens system mass and pi_rel is the lens-source relative parallax. If the lens system is nearby (large pi_rel), then M 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, mu_rel = 19.6 +- 1.6 mas/yr, 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, the alternate solution has a lower proper motion, which 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_host = 0.12 (+0.19 -0.06) M_solar and m_comp = 18 (+28 -100 M_earth, at a projected separation of a_perp ~ 0.84 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 low mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.

As they note, the slow solution which gives a Neptune around a low-mass star represents a type of system that is well established to exist, while gas giants with ~0.5 Earth mass moons are not and moreover have not been seen in previous microlensing events with planetary-mass primaries.

Still, very intriguing and good to verify that moons can be detected via microlensing observations.

Still far As I know,ever by microlensing is very hard detected Low Mass object below the Earth mass,So isn't a definitive prof that exomoons of earth mass and below to 0.5 Earth mass are rare or not

New Scientist article: First exomoon glimpsed – 1800 light years from Earth

I suppose the Na'vi were inevitable.

New Method to Measure Proper Motions of Microlensed Sources: Application to Candidate Free-Floating-Planet Event MOA-2011-BLG-262
http://arxiv.org/abs/1312.7297

Quote :

We develop a new method to measure source proper motions in microlensing events, which can partially overcome problems due to blending. It takes advantage of the fact that the source position is known precisely from the microlensing event itself. We apply this method to the event MOA-2011-BLG-262, which has a short timescale t_E=3.8 day, a companion mass ratio q=0.0047 and a very high or high lens-source relative proper motion mu_rel=20 mas/yr or 12 mas/yr (for two possible models). These three characteristics imply that the lens could be a brown dwarf or a massive planet with a roughly Earth-mass "moon". The probability of such an interpretation would be greatly increased if it could be shown that the high lens-source relative proper motion was primarily due to the lens rather than the source. Based on the long-term monitoring data of the Galactic bulge from the Optical Gravitational Lensing Experiment (OGLE), we measure the source proper motion that is small, mu_s = (-2.3, -0.9) +- (2.8,2.6) mas/yr in a (North, East) Galactic coordinate frame. These values are then important input into a Bayesian analysis of the event presented in a companion paper by Bennett et al.

Quote :

The stellar lens with the Jupiter-mass planetary companion located in the Galactic bulge is, a priori, much more likely explanation for the MOA-2011-BLG-262 microlensing event, than the close-by Jupiter-mass planet with the Earth-mass moon in the Galactic disk

The key part is immediately after the sentence you quoted... in full:

Quote :

The stellar lens with the Jupiter-mass planetary companion located in the Galactic bulge is, a priori, much more likely explanation for the MOA-2011-BLG-262 microlensing event, than the close-by Jupiter-mass planet with the Earth-mass moon in the Galactic disk. By reducing the likelihood of the lens being in the bulge, our measurement brings those two explanations on par.

This actually strengthens the exomoon interpretation, though only in as much as it gives it similar likelihood to the star+jovian interpretation.

Indeed EPE favours "low-mass star+Neptune-mass planet" solution:

http://exoplanet.eu/catalog/moa-2011-blg-262l_b/

Now with an artist rendering.
http://www.jpl.nasa.gov/news/news.php?release=2014-109&utm_source=iContact&utm_medium=email&utm_campaign=NASAJPL&utm_content=universe20140410