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One or more bound planets per Milky Way star from microlensing observations


Most known extrasolar planets (exoplanets) have been discovered using the radial velocity1,2 or transit3 methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing6,7,8,9, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing10. These planets are at least as numerous as the stars in the Milky Way10. Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 au (Sun–Earth distance) from their stars. We find that of stars host Jupiter-mass planets (0.3–10 MJ, where MJ = 318 M and M is Earth’s mass). Cool Neptunes (10–30 M) and super-Earths (5–10 M) are even more common: their respective abundances per star are and . We conclude that stars are orbited by planets as a rule, rather than the exception.

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Figure 1: Survey-sensitivity diagram.
Figure 2: Cool-planet mass function.


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Support for the PLANET project was provided by the HOLMES grant from the French Agence Nationale de la Recherche (ANR), the French National Centre for Scientific Research (CNRS), NASA, the US National Science Foundation, the Lawrence Livermore National Laboratory/National Nuclear Security Administration/Department of Energy, the French National Programme of Planetology, the Program of International Cooperation in Science France–Australia, D. Warren, the German Research Foundation, the Instrument Center for Danish Astronomy and the Danish Natural Science Research Council. The OGLE collaboration is grateful for funding from the European Research Council Advanced Grants Program. K.Ho. acknowledges support from the Qatar National Research Fund. M.D. is a Royal Society University Research Fellow.

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Authors and Affiliations



A.Ca. led the analysis and conducted the modelling and statistical analyses. A.Ca. and D.K. selected light curves from 2002–07 PLANET/OGLE microlensing seasons, analysed the data and wrote the Letter and Supplement. D.K. computed the magnification maps used for the detection-efficiency calculations. J.-P.B. and Ch.C. wrote the software for online data reduction at the telescopes. J.-P.B. led the PLANET collaboration, with M.D., J.G., J.M. and A.W.; P.F. and M.D.A. contributed to online and offline data reduction. M.D. contributed to the conversion of the detection efficiencies to physical parameter space and developed the PLANET real-time display system with A.W., M.D.A. and Ch.C.; K.Ho. and A.Ca. developed and tested the Bayesian formulation for fitting the two-parameter power-law mass function. J.G. edited the manuscript, conducted the main data cleaning and managed telescope operations at Mount Canopus (1 m) in Hobart. J.W. wrote the original magnification maps software, discussed the main implications and edited the manuscript. J.M., A.W. and U.G.J. respectively managed telescope operations in South Africa (South African Astronomical Observatory 1 m), Australia (Perth 0.61 m) and La Silla (Danish 1.54 m). A.U. led the OGLE campaign and provided the final OGLE photometry. D.P.B, V.B., S.B., J.A.R.C., A.Co., K.H.C., S.D., D.D.P., J.D., P.F., K.Hi., N.K., S.K., J.-B.M., R.M., K.R.P., K.C.S., C.V., D.W., B.W. and M.Z. were involved in the PLANET observing strategy and/or PLANET data acquisition, reduction, real-time analysis and/or commented on the manuscript. T.S. commented on the manuscript. M.K.S., M.K., R.P., I.S., K.U., G.P. and Ł.W. contributed to OGLE data.

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Correspondence to A. Cassan or A. Cole.

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The authors declare no competing financial interests.

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The file contains Supplementary Text and Data, Supplementary Figures 1-5 with legends, Supplementary Table 1 and additional references. (PDF 692 kb)

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Cassan, A., Kubas, D., Beaulieu, JP. et al. One or more bound planets per Milky Way star from microlensing observations. Nature 481, 167–169 (2012).

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