Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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.

Your institute does not have access to this article

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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


  1. Mayor, M. & Queloz, D. A. Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995)

    ADS  CAS  Article  Google Scholar 

  2. Marcy, G. W. & Butler, R. P. A planetary companion to 70 Virginis. Astrophys. J. 464, L147–L151 (1996)

    ADS  Article  Google Scholar 

  3. Charbonneau, D., Brown, T. M., Latham, D. W. & Mayor, M. Detection of planetary transits across a Sun-like star. Astrophys. J. 529, L45–L48 (2000)

    ADS  CAS  Article  Google Scholar 

  4. Howard, A. et al. Planet occurrence within 0.25 AU of Solar-type stars from Kepler. Astrophys. J. (submitted); preprint at (2011)

  5. Mayor, M. et al. The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets. Astron. Astrophys. (submitted); preprint at (2011)

  6. Mao, S. & Paczynski, B. Gravitational microlensing by double stars and planetary systems. Astrophys. J. 374, L37–L40 (1991)

    ADS  Article  Google Scholar 

  7. Gould, A. & Loeb, A. Discovering planetary systems through gravitational microlenses. Astrophys. J. 396, 104–114 (1992)

    ADS  Article  Google Scholar 

  8. Bennett, D. P. & Rhie, S. H. Detecting Earth-mass planets with gravitational microlensing. Astrophys. J. 472, 660–664 (1996)

    ADS  Article  Google Scholar 

  9. Wambsganss, J. Discovering Galactic planets by gravitational microlensing: magnification patterns and light curves. Mon. Not. R. Astron. Soc. 284, 172–188 (1997)

    ADS  Article  Google Scholar 

  10. Sumi, T. et al. Unbound or distant planetary mass population detected by gravitational microlensing. Nature 473, 349–352 (2011)

    ADS  CAS  Article  Google Scholar 

  11. Udalski, A. The Optical Gravitational Lensing Experiment. Real time data analysis systems in the OGLE-III survey. Acta Astronaut. 53, 291–305 (2003)

    Google Scholar 

  12. Bond, I. A. et al. Real-time difference imaging analysis of MOA Galactic bulge observations during 2000. Mon. Not. R. Astron. Soc. 327, 868–880 (2001)

    ADS  Article  Google Scholar 

  13. Albrow, M. et al. The 1995 pilot campaign of PLANET: searching for microlensing anomalies through precise, rapid, round-the-clock monitoring. Astrophys. J. 509, 687–702 (1998)

    ADS  Article  Google Scholar 

  14. Gould, A. et al. Microlens OGLE-2005-BLG-169 implies that cool Neptune-like planets are common. Astrophys. J. 644, L37–L40 (2006)

    ADS  Article  Google Scholar 

  15. Gaudi, B. S. et al. Discovery of a Jupiter/Saturn analog with gravitational microlensing. Science 319, 927–930 (2008)

    ADS  CAS  Article  Google Scholar 

  16. Udalski, A. et al. A Jovian-mass planet in microlensing event OGLE-2005-BLG-071. Astrophys. J. 628, L109–L112 (2005)

    ADS  Article  Google Scholar 

  17. Dong, S. et al. OGLE-2005-BLG-071Lb, the most massive M dwarf planetary companion? Astrophys. J. 695, 970–987 (2009)

    ADS  Article  Google Scholar 

  18. Gould, A. et al. Frequency of solar-like systems and of ice and gas giants beyond the snow line from high-magnification microlensing events in 2005–2008. Astrophys. J. 720, 1073–1089 (2010)

    ADS  Article  Google Scholar 

  19. Beaulieu, J.-P. et al. Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing. Nature 439, 437–440 (2006)

    ADS  CAS  Article  Google Scholar 

  20. Kubas, D. et al. Limits on additional planetary companions to OGLE 2005-BLG-390L. Astron. Astrophys. 483, 317–324 (2008)

    ADS  Article  Google Scholar 

  21. Gaudi, B. S. et al. Microlensing constraints on the frequency of Jupiter-mass companions: analysis of 5 years of PLANET photometry. Astrophys. J. 566, 463–499 (2002)

    ADS  Article  Google Scholar 

  22. Einstein, A. Lens-like action of a star by the deviation of light in the gravitational field. Science 84, 506–507 (1936)

    ADS  CAS  Article  Google Scholar 

  23. Dominik, M. Stochastic distributions of lens and source properties for observed galactic microlensing events. Mon. Not. R. Astron. Soc. 367, 669–692 (2006)

    ADS  Article  Google Scholar 

  24. Cassan, A. An alternative parameterisation for binary-lens caustic-crossing events. Astron. Astrophys. 491, 587–595 (2008)

    ADS  Article  Google Scholar 

  25. Sumi, T. et al. A cold Neptune-mass planet OGLE-2007-BLG-368Lb: cold Neptunes are common. Astrophys. J. 710, 1641–1653 (2010)

    ADS  Article  Google Scholar 

  26. Howard, A. W. et al. The occurrence and mass distribution of close-in super-Earths, Neptunes, and Jupiters. Science 330, 653–655 (2010)

    ADS  CAS  Article  Google Scholar 

  27. Cumming, A. et al. The Keck Planet Search: Detectability and the minimum mass and orbital period distribution of extrasolar planets. Publ. Astron. Soc. Pacif. 120, 531–554 (2008)

    ADS  Article  Google Scholar 

  28. Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62–85 (1996)

    ADS  Article  Google Scholar 

  29. Tsapras, Y. et al. Microlensing limits on numbers and orbits of extrasolar planets from the 1998–2000 OGLE events. Mon. Not. R. Astron. Soc. 343, 1131–1144 (2003)

    ADS  Article  Google Scholar 

  30. Snodgrass, C. et al. The abundance of Galactic planets from OGLE-III 2002 microlensing data. Mon. Not. R. Astron. Soc. 351, 967–975 (2004)

    ADS  Article  Google Scholar 

Download references


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.

Author information

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.

Corresponding authors

Correspondence to A. Cassan or A. Cole.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

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

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing