Using asteroseismology to understand exoplanet orbits
Vincent Van Eylen (Leiden University), Simon Albrecht (Aarhus University)

Eccentricity is a fundamental orbital parameter which holds information about planet formation and evolution as well as habitability. Surprisingly, many massive gas giant planets travel on highly elliptical orbits, in contrast to the orbits of solar system planets which are nearly circular. So far, the orbital shape of smaller, more terrestrial, exoplanets remained largely elusive, because the stellar radial velocity caused by these small planets is extremely challenging to measure. I sidestepped this problem by using photometry from the Kepler satellite and utilizing a method relying on Kepler's second law, which relates the duration of a planetary transit to its orbital eccentricity, if the stellar density is known. Here, asteroseismology is key to delivering this stellar parameter with the required precision and accuracy. This approach enabled me to measure the eccentricity of planets even smaller than Earth, much smaller than what was previously possible. I present eccentricity measurements for 74 planets in multi-planet systems, and 50 systems with a single transiting planet. The multi-planet systems are nearly circular, in full agreement with solar system eccentricities, but in contrast to the eccentricity distributions previously derived for exoplanets from radial velocity studies. The systems with a single transiting planet have significantly higher eccentricities. I link these findings to planet formation and evolution theory and argue that the eccentricity of systems with a single transiting planet may be related to the presence of non-transiting planets on an inclined orbit. I also compare the eccentricities with stellar parameters derived from asteroseismology for this "gold standard" sample.