Wednesday, May 18, 2011

Free floating planets might outnumber the stars in the Galaxy

A recent gravitational microlensing survey indicates that there may be twice as many free floating planets in our Galaxy than stars. Gravitational microlensing is the brightening that occurs when a dim but massive object passes along the line of sight to a more distant brighter object. According to Einstein's theory of relativity, mass bends spacetime, so the path followed by a light ray is deflected (ie lensed) if passing near enough to a star or a planet. So an astronomer observing a lensed star will see it brighten for a month or two if a very dim star (such as a white dwarf or neutron star) passes near the line of sight (LOS). This also occurs if a planet passes near the LOS, but the lower mass planet has a smaller gravitational influence, so the lensing event is briefer, only a few days.

This is illustrated in the above figure, which shows an otherwise steady star brightening by 40% during three days. These planetary microlensing events are quite rare, so astronomers must continuously monitor millions of stars just to detect 10 such microlensing events in one year. From the observed frequency of these microlensing events, it can be shown that most of the lensing objects are free-floating Jupiter-mass planets that are not bound to any star. But this unusual finding is consistent with some models of planet formation, which predict that when multiple planets form around a star, the planets' gravitational interactions can eject one or more planets from the system. Those ejected bodies are free-floating planets, and their fate is to roam the Galaxy unseen, except in these microlensing surveys. These results were obtained by astrophysicists T. Sumi and K. Kamiya (Osaka Japan) and others, with further details reported in their preprint.

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