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One or more bound planets per Milky Way star from microlensing observations.
Cassan, A. 1,2,3,*; Kubas, D. 1,2,4; Beaulieu, J. P. 1,2,25; Dominik, M. 1,5; Horne, K. 1,5; Greenhill, J. 1,6; Wambsganss, J. 1,3; Menzies, J. 1,7; Williams, A. 1,8; Jorgensen, U. G. 1,9; Udalski, A. 10,11; Bennett, D. P. 1,12; Albrow, M. D. 1,13; Batista, V. 1,2; Brillant, S. 1,4; Caldwell, J. A. R. 1,14; Cole, A. 1,6; Coutures, Ch. 1,2; Cook, K. H. 1,15; Dieters, S. 1,6; Prester, Dominis D. 1,16; Donatowicz, J. 1,17; Fouque, P. 1,18; Hill, K. 1,6; Kains, N. 1,19; Kane, S. 1,20; Marquette, J. B. 1,2; Martin, R. 1,8; Pollard, K. R. 1,13; Sahu, K. C. 1,14; Vinter, C. 1,9; Warren, D. 1,6; Watson, B. 1,6; Zub, M. 1,3; Sumi, T. 21,22; Szymanski, M. K. 10,11; Kubiak, M. 10,11; Poleski, R. 10,11; Soszynski, I. 10,11; Ulaczyk, K. 10,11; Pietrzynski, G. 10,11,23; Wyrzykowski, L. 10,11,24
[Letter] Nature. 481(7380):167-169, January 12, 2012.

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: Most known extrasolar planets (exoplanets) have been discovered using the radial velocity 1,2 or transit 3 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 microlensing 6,7,8,9, 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 microlensing 10. These planets are at least as numerous as the stars in the Milky Way 10. 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 Symbol of stars host Jupiter-mass planets (0.3-10 MJ, where MJ = 318 M[circled plus] and M[circled plus] is Earth's mass). Cool Neptunes (10-30 M[circled plus]) and super-Earths (5-10 M[circled plus]) are even more common: their respective abundances per star are Symbol and Symbol. We conclude that stars are orbited by planets as a rule, rather than the exception.