Secure Identification of Free‐floating Planets

Abstract

Among the methods proposed to detect extrasolar planets, microlensing is the only technique that can detect free-floating planets. Free-floating planets are detected through the channel of short-duration isolated lensing events. However, if a seemingly isolated planetary event is detected, it is difficult to firmly conclude that the event is caused by a free-floating planet, because a wide-separation planet can also produce an isolated event. There were several methods proposed to break the degeneracy between the isolated planetary events produced by the free-floating and wide-separation planets, but they are incomplete. In this paper, we show that free-floating planets can be securely identified by conducting astrometric follow-up observations of isolated events to be detected in future photometric lensing surveys by using high-precision interferometers to be operated contemporarily with the photometric surveys. The method is based on the fact that the astrometric lensing effect covers a much longer range of the lens-source separation than the photometric effect. We demonstrate that several astrometric follow-up observations of isolated planetary events associated with source stars brighter than V ~ 19 using the Space Interferometry Mission with an exposure time of 10 minutes for each observation will make it possible to measure the centroid shift induced by primaries with projected separations up to ~100 AU. Therefore, the proposed method is far more complete than previously proposed methods, which are flawed by limited applicability only to planets with projected separations 20 AU or planets accompanied by bright primaries.