Design considerations for a space-based transit search for Earth analogs

Abstract

An extrasolar planet discovered around one of the brightest stars in the sky would be a prime target for follow-on observing and characterization, especially if the planet were within the star's habitable-zone. The brightest stars are, however, often not considered to be practical targets for photometric planetary transit searches, despite the scientific rewards that finding a planet around these stars would provide. Exo-PlanetSat is a proposed satellite constellation that will survey the brightest G- and K-dwarfs across the entire sky and look for transits of Earth-sized planets in the habitable-zone. By using measurements of stellar orientations provided primarily by asteroseismic observations, only those stars whose axes lie near 900 to our line of sight will be observed. This increases the transit probability of a habitable-zone planet from 0.5% to 5%, lowering the number of stars that need to be examined by a factor of ten. This in turn enables the design of a practical survey of the brightest stars. A proposed concept of operations envisions thirty small spacecraft in low Earth-orbit, each individually targeted onto a bright star. Over the course of four years, these spacecraft will able to observe a sufficient number of solar-like stars to expect the detection of three habitable-zone Earths with high statistical confidence, assuming that every star has such a planet in orbit.