What can be learned from accurate radii and masses of terrestrial exoplanets, in spite of the limitations in observables compared to our solar system?

After many years of effort from the thousands of Kepler and CoRoT planet candidates with R<2R_E we now have a handful of objects with masses believed accurate to ±30% and this is starting to reveal tantalising correlations in the mass vs. radius relationship for small planets (Dressing et al 2015; Buchhave et al. 2016).

In all apart from one case, the masses have been determined from radial velocity instruments (the exception is Kepler-36b with a mass derived from a transit timing variation model, Carter et al. 2012). According to Dressing et al. (2015), there are other planets with less accurate masses that are not consistent with this correlation suggesting that other bulk compositions are likely. However, in general Kepler and CoRoT candidates are too faint to permit an accurate mass determination via RV observations. In many cases, even a rocky or icy nature cannot be distinguished owing to the size of the uncertainties. TESS will make little impact on the detection and precise characterisation of these small planets except for those around the coolest stars.

The mass vs. radius relationship for small planets with the best determined masses (30%) from Buchhave et al. (2016). Planets with less well determined masses suggest other compositions are likely. The grey region to the lower right indicates the region where planets would have an iron content exceeding the maximum value predicted from models of collisional stripping.

Our knowledge of bulk planet density (and therefore composition) is, first and foremost, dependent on the quality of the stellar mass and radius determinations that feed into the determinations of planetary mass and radius. One of the main goals of PLATO is therefore to provide highly precise and accurate measurements of the planet host stars’ characteristics, in particular their radii, masses and ages. Typical current uncertainties for radius and mass determinations of small planets are around ±6% and ±20%, respectively, leading to uncertainties of 30 to 50% in bulk density. The observational precision envisaged for PLATO will reduce the uncertainty in planetary bulk density to about 10%.

Current detection limits have prevented the discovery of more than a few rocky exoplanets, although low-mass planets around other stars are most likely abundant. PLATO will provide masses and radii of a large number of terrestrial planets from close-in orbits up to 1 AU distant from their host stars. Studying temperate planets at large orbital separations allows us to address the architecture of planetary systems and its connection to proto-planetary disk properties, and also to study the relationship between planet interiors and atmospheres in planets up to the HZ. These will be complemented by the detection of giant planets at larger orbital separations expected from the Gaia mission, expanding our characterisation of these systems. Constraining the mean composition and bulk interior structure of small planets, PLATO will enable us to answer the following questions:

• Are there other planetary systems that include a terrestrial planet like Earth?
• What is the typical bulk density distribution (and mass function) in planetary systems?
• How is the planet bulk density distribution correlated with stellar parameters (e.g. metallicity, mass, age, etc.)?

References

PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017