THE DISTRIBUTION OF TRANSIT DURATIONS FOR KEPLER PLANET CANDIDATES AND IMPLICATIONS FOR THEIR ORBITAL ECCENTRICITIES

Abstract

Doppler planet searches have discovered that giant planets follow orbits with a wide range of orbital eccentricities, revolutionizing theories of planet formation. The discovery of hundreds of exoplanet candidates by NASA's Kepler mission enables astronomers to characterize the eccentricity distribution of small exoplanets. Measuring the eccentricity of individual planets is only practical in favorable cases that are amenable to complementary techniques (e.g., radial velocities, transit timing variations, occultation photometry). Yet even in the absence of individual eccentricities, it is possible to study the distribution of eccentricities based on the distribution of transit durations (relative -2 -to the maximum transit duration for a circular orbit). We analyze the transit duration distribution of Kepler planet candidates. We find that for host stars with T eff > 5100K we cannot invert this to infer the eccentricity distribution at this time due to uncertainties and possible systematics in the host star densities. With this limitation in mind, we compare the observed transit duration distribution with models to rule out extreme distributions. If we assume a Rayleigh eccentricity distribution for Kepler planet candidates, then we find best-fits with a mean eccentricity of 0.1-0.25 for host stars with T eff ≤ 5100K. We compare the transit duration distribution for different subsets of Kepler planet candidates and discuss tentative trends with planetary radius and multiplicity. High-precision spectroscopic follow-up observations for a large sample of host stars will be required to confirm which trends are real and which are the results of systematic errors in stellar radii. Finally, we identify planet candidates that must be eccentric or have a significantly underestimated stellar radius.

SECURE MASS MEASUREMENTS FROM TRANSIT TIMING: 10 KEPLER EXOPLANETS BETWEEN 3 AND 8 M ⊕ WITH DIVERSE DENSITIES AND INCIDENT FLUXES

“…This corresponds to a Rayleigh distribution with scale width 0.1 as the prior for scalar eccentricity. This is consistent with the distribution of eccentricities among Keplerʼs exoplanets found by Moorhead et al (2011) using an independent analysis of measured transit durations. This wide distribution is also consistent with the known eccentricities of giant RV planets Plavchan et al 2014).…”

LACK OF INFLATED RADII FOR KEPLER GIANT PLANET CANDIDATES RECEIVING MODEST STELLAR IRRADIATION

“…Instead of unrealistically assuming circular orbits, we assigned each candidate an eccentricity value drawn from the distribution presented in Wang & Ford (2011), as well as a random value for the argument of periastron. This approach is reasonable since no significant trend seems to exist between eccentricity and orbital period for Kepler candidates (Moorhead et al 2011).…”

The Solar System as an Exoplanetary System

“…For the RV planets, the best fit eccentricity is biased upwards from the true value leading to a reduced number of systems with a low eccentricity (e.g. Shen & Turner 2008;Hogg, Myers & Bovy 2010;Zakamska, Pan & Ford 2011;Moorhead et al 2011). However, the detection efficiency decreases only mildly with increasing eccentricity because despite being more difficult to detect, they have a larger RV amplitude for a fixed planet mass and semi-major axis (Shen & Turner 2008).…”