Nature. 2011 May 19;473(7347):349-52. doi: 10.1038/nature10092.
Since 1995, more than 500 exoplanets have been detected using different techniques, of which 12 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions, 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 (1.8(+1.7)(-0.8)) 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 imaging 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.
自 1995 年以来，已经使用不同的技术检测到了 500 多个系外行星，其中 12 个是通过引力微引力透镜技术检测到的。其中大多数与它们的宿主恒星有引力束缚。在年轻的恒星形成区域有一些自由漂浮的行星质量物体的证据，但这些物体仅限于质量为 3 到 15 木星质量的大质量物体，并且在光度质量估计及其丰度方面存在很大的不确定性。在这里，我们报告了在银河系核区进行了两年的引力微引力透镜调查观测，发现了一类无束缚或远距离木星质量物体的存在，其数量几乎是主序星的两倍（1.8(+1.7)(-0.8)）。这些行星质量物体没有宿主恒星，可以通过引力微引力透镜在大约十个天文单位内检测到。然而，与直接成像的约束条件进行比较表明，这些行星质量物体中的大多数都没有与任何宿主恒星绑定。在大约一个木星质量处质量函数的突然变化，支持了这样一种观点，即它们的形成过程与恒星和褐矮星的形成过程不同。它们可能是在原行星盘中形成的，随后散射到无束缚或非常遥远的轨道中。
