Microlensing and the Search for Extraterrestrial Life

Are microlensing searches likely to discover planets that harbor life? Givenour present state of knowledge, this is a difficult question to answer. Wetherefore begin by asking a more narrowly focused question: are conditions onplanets discovered via microlensing likely to be similar to those we experienceon Earth? In this paper I link the microlensing observations to the well-known"Goldilocks Problem" (conditions on the Earth-like planets need to be "justright"), to find that Earth-like planets discovered via microlensing are likelyto be orbiting stars more luminous than the sun. This means that light from theplanetary system's central star may contribute a significant fraction of thebaseline flux relative to the star that is lensed. Such blending of light fromthe lens with light from the lensed source can, in principle, limit our abilityto detect these events. This turns out not to be a significant problem,however. A second consequence of blending is the opportunity to determine thespectral type of the lensed spectral type of the lensed star. Thiscircumstance, plus the possibility that finite-source-size effects areimportant, implies that some meaningful follow-up observations are likely to bepossible for a subset Earth-like planets discovered via microlensing. Inaddition, calculations indicate that reasonable requirements on the planet'sdensity and surface gravity imply that the mass of Earth-like planets is likelyto be within a factor of $\sim 15$ of an Earth mass.Comment: 15 pages, 2 figures. To be published in the Astrophysical Journa