Exoplanetary Architectures

My group is also leading studies to explore the architectures of exoplanetary systems: the mass- and orbital semi-major axis distributions and occurrence rates of exoplanets. We use different methods to characterize the frequency of different planets: for large-separation giant planets utilize direct imaging and for close-in planets we use planetary transits. We compare  occurrence rates between different studies using statistical methods, including Monte Carlo simulations.

Kasper_Apai_PlanetDistribution

The low occurrence rate of giant planets around Sun-like stars

In Kasper, Apai et al. (2007) we carried out the first L’-band high-contrast imaging adaptive optics survey in search of giant exoplanets around the young stars in the Tuc-Hor and Beta Pictoris co-moving groups. We used the upper limits and compared these to the results of simulated observations of hypothetical planet populations, which were consistent with extrapolations of the short-period exoplanet populations as established by radial velocity surveys. The L’ imaging method and the Monte Carlo approach are now widely used.

More Planets around Lower-Mass Stars

In Mulders, Pascucci, Apai 2015a we modeled selection and detection biases for the Kepler mission to derive intrinsic planet-radius and semi-major axis distributions. This detailed statistical work led to the discovery of a stellar-mass dependent drop in Mulders_PlanetOccurrenceRatesexoplanet occurrence rates. Interestingly, the planet populations around stars with different masses show the same overall distribution, but – counterintuitively – less massive stars have more planets and these planets are shifted toward shorter periods.

More Planet Mass in Heavy Elements around Lower-Mass Stars

In a second study led by Gijs Mulders and Ilaria Pascucci we estimated the heavy element

The heavy element mass in planets surprisingly increases with decreasing stellar mass.

The heavy element mass in planets surprisingly increases with decreasing stellar mass.

budget that is stored in planets in the inner planetary systems as a function of stellar mass. The conversion of planet radius to planet mass was done through mass-radius relationships and we explored the possible range of such relationships. The surprising result of our study is that lower-mass stars not only have more planets (as we found in our earlier study), but there is more heavy element mass in those planets than in those around sun-like stars. This goes contrary to the expectation that low-mass stars have more planets because they fail to form larger ones. The trend we identify is also opposite to the trend found for dust mass in disks as a function of disk mass.