The Impact of Solar-like Variability on the Detectability of Transiting Terrestrial Planets

Transit photometry is a promising method for discovering extrasolar planets as small as Earth from space-based photometers, and several near-term photometric missions are on the drawing board. In particular, NASA's recently selected Kepler mission is devoted primarily to detecting extrasolar planets. The success of these efforts depends in part on the ability to detect transit signatures against the inherent photometric variability of the target stars. While other noise sources such as shot noise and CCD noise are under the control of the instrument designers, this one is not. The photometric variability of solar-like stars presents a fundamental lower limit to the minimum detectable planet radius for a given star and number of observed transits. In this paper we examine the capability of such missions using bolometric data for the only star for which sufficient photometric precision exists to address this question: the Sun. The results indicate that solar-like variability does not prevent the detection of Earth-sized planets even for stars rotating significantly faster than the Sun. Four transits are detectable for m_v=12 stars with rotation periods as short as ~21 days, while six transits allow detection for stellar rotation periods as short as ~16 days. Indeed, the limits posed by solar-like variability allow for the detection of planets significantly smaller than Earth orbiting Sun-like stars. Planets as small as 0.6 Earth radii exhibiting at least six transits can be detected orbiting bright (m_v=10) solar analogs.