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Unbound or distant planetary mass population detected by gravitational microlensing


Since 1995, more than 500 exoplanets have been detected using different techniques1,2, of which 12 were detected with gravitational microlensing3,4. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions5,6,7,8, but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice () as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge. These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However, a comparison with constraints from direct imaging9 suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about one Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.

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Figure 1: Light curves of event MOA-ip-3 and event MOA-ip-10.
Figure 2: Observed and theoretical distributions of the event timescale, tE.
Figure 3: Likelihood contours for the planetary-mass function parameters.


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The MOA collaboration thanks the JSPS and MEXT of Japan, and the Marsden Fund of New Zealand, for support. D.P.B. acknowledges support by the NSF and NASA. The OGLE collaboration is grateful for funding from the European Research Council Advanced Grants Program.

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T.S. and K.K. conducted data reduction and statistical analysis. A.U. produced OGLE-III light curves. I.A.B. generated the extended MOA-II light curves. D.P.B. conducted the detailed analysis of binary events. T.S. wrote the manuscript. D.P.B. and I.A.B. edited the manuscript. All other authors contributed to the observation and maintenance of observational facilities, discussed the results and commented on the manuscript.

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Correspondence to T. Sumi.

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The Microlensing Observations in Astrophysics (MOA) Collaboration., The Optical Gravitational Lensing Experiment (OGLE) Collaboration. Unbound or distant planetary mass population detected by gravitational microlensing. Nature 473, 349–352 (2011).

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