Exoplanet Demographics

Exoplanets are a diverse population. The more than 5000 known exoplanets different greatly in their masses, sizes, orbital periods, and even the type of stars they orbit. There is, however, also order in this chaos, with certain type of planets and planetary systems occurring more frequently than others. By correcting for the biases inherent to different exoplanet detection methods, the demographics of planets orbiting other stars can be constructed.

In this review I will paint a picture of exoplanet demographics consisting of two main populations: The first is a population of (ice) giant planets that extends over orders of magnitude in star-planet separation. These planets are detected by radial velocity, micro-lensing, direct imaging, and transit surveys. Their planet host stars tend to be more massive and more metal rich. The second is a population of planetary systems containing super-earths and sub-Neptunes detected by transit and radial velocity surveys. Our observational view of these systems is limited to within 1 astronomical unit from their host stars. Still, they orbit an estimated 50% of sun-like stars, and are also frequently found around stars that are in low in mass or metal poor.

What lies beyond the detection limits of current exoplanets surveys remains to be discovered: I will conclude with what to expect from missions that will increase the number of known exoplanets by thousands more in the coming years: the Transiting Exoplanet Survey Satellite, the Gaia astrometry mission, and the Roman microlensing survey.