Microlensing and the Search for Extraterrestrial Life

Are microlensing searches likely to discover planets that harbor life? Given our present state of knowledge, this is a difficult question to answer. We therefore begin by asking a more narrowly focused question: are conditions on planets discovered via microlensing likely to be similar to those we experience on Earth? In this paper I link the microlensing observations to the well-known "Goldilocks problem" (conditions on the Earth-like planets need to be "just right") to find that Earth-like planets discovered via microlensing are likely to be orbiting stars more luminous than the Sun. This means that light from the planetary system's central star may contribute a significant fraction of the baseline flux relative to the star that is lensed. Such blending of light from the lens with light from the lensed source can, in principle, limit our ability to detect these events. This turns out not to be a significant problem, however. A second consequence of blending is the opportunity to determine the spectral type of the lensed star. This circumstance, plus the possibility that finite-source-size effects are important, implies that some meaningful follow-up observations are likely to be possible for a subset of Earth-like planets discovered via microlensing. In addition, calculations indicate that reasonable requirements on the planet's density and surface gravity imply that the mass of Earth-like planets is likely to be within a factor of ~15 of an Earth mass.