Figure 1: Artist’s impression of a pair of exoplanets.
Measuring a planet’s size and mass is essential in determining the planet’s density and hence its bulk composition. The masses of most of the small planets detected by NASA’s Kepler space telescope are beyond the sensitivity of radial velocity (RV) measurements. As a result, another technique known as transit timing variations (TTV) is required to determine the masses of these small planets. TTV involves precisely measuring the gravitational perturbation one planet has on another in the same planetary system with two or more planets. The gravitational perturbation shows up as a change in the transit timing of the planet (i.e. the planet transits its host star slightly earlier or slightly later).
From the analysis of TTV data, Jontof-Hutter et al. (2015) present mass measurements of 10 exoplanets that range from super-Earth-size to Neptune-size. All 10 planets are below 8 Mᴇ, whereby Mᴇ denotes the mass of Earth. Although their radii range from 1.31 (Kepler-105c) to 3.35 (Kepler-29b) times the radius of Earth, they span over an order of magnitude in density, indicating a huge compositional diversity. The densest of the 10 planets, Kepler-105c, has a density that is consistent with an Earth-like rocky composition. The other planets have lower densities that are consistent with mixtures of rock and ice, or rock and gas. None of the 10 planets are habitable as they receive between a few times to a few hundred times the amount of flux Earth gets from the Sun.
Figure 2: Parameters of the 10 planets. The mass, radius, and flux are in units of the mass and radius of Earth, and in units of the amount of flux Earth gets from the Sun. Jontof-Hutter et al. (2015)
Figure 3: Mass-radius diagram of super-Earth-mass exoplanets from 1 to 4 times the radius of Earth, compared to theoretical curves of pure water ice, silicate rock and iron. The 10 well-characterized exoplanets below 8 Mᴇ are new additions to the planetary mass-radius diagram, and they are represented by open circles and orange error bars. Despite a small range in mass, they span a wide range in density and hence bulk composition. Jontof-Hutter et al. (2015)
Jontof-Hutter et al. (2015), “Robust TTV Mass Measurements: Ten Kepler Exoplanets between 3 and 8 Earth Masses with Diverse Densities and Incident Fluxes”, arXiv:1512.02003 [astro-ph.EP]