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Abstract
The MEarth Project is a ground-based photometric survey to find planets transiting the closest and smallest main-sequence stars. In its first four years, MEarth discovered one transiting exoplanet, the 2.7 Earth radius planet GJ1214b. Here, we answer an outstanding question: in light of the bounty of small planets transiting small stars uncovered by the Kepler mission, should MEarth have found more than just one planet so far? We estimate MEarth's ensemble sensitivity to exoplanets by performing end-to-end simulations of 1.25 million observations of 988 nearby mid-to-late M dwarfs, gathered by MEarth between October 2008 and June 2012. For 2-4 Earth radius planets, we compare this sensitivity to results from Kepler and find that MEarth should have found planets at a rate of 0.05 - 0.36 planets/year in its first four years. As part of this analysis, we provide new analytic fits to the Kepler early M dwarf planet occurrence distribution. When extrapolating between Kepler's early M dwarfs and MEarth's mid-to-late M dwarfs, we find that assuming the planet occurrence distribution stays fixed with respect to planetary equilibrium temperature provides a good match to our detection of a planet with GJ1214b's observed properties. For larger planets, we find that the warm (600-700K), Neptune-sized (4 Earth radius) exoplanets that transit early M dwarfs like Gl436 and GJ3470 occur at a rate of <0.15/star (at 95% confidence) around MEarth's later M dwarf targets. We describe a strategy with which MEarth can increase its expected planet yield by 2.5X without new telescopes, by shifting its sensitivity toward the smaller and cooler exoplanets that Kepler has demonstrated to be abundant.
Highlights
IntroductionEmpirical measurements of the masses, radii, and atmospheres of such planets can serve as crucial inputs to developing this understanding
An overarching goal of exoplanetary science is to understand the physical processes that shape exoplanets smaller than Neptune
For planets smaller than 2 R⊕, the sensitivity increased relative to the naive estimate, thanks to some stars having radii smaller than originally assumed, so that MEarth’s exposure times designed to detect 2 R⊕ planets could pick up even smaller planets
Summary
Empirical measurements of the masses, radii, and atmospheres of such planets can serve as crucial inputs to developing this understanding These measurements are most feasible for transiting planetary systems that exhibit favorable planet-to-star mass, radius, and temperature ratios (for amplifying signal strengths) and that are bright (for suppressing photon noise). Planets can satisfy these criteria by transiting nearby small stars: the local M dwarfs. This finding that small planets are so abundant around Kepler’s distant M dwarfs bodes well for our prospects of finding such planets transiting nearby M dwarfs, the close exemplars for which detailed characterization studies will be most rewarding
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