The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?

TL;DR

This study uses dynamical analysis of the HD 40307 system to infer whether its super-Earths are rocky or gaseous, suggesting they are more likely similar to giant planets based on orbital stability and tidal effects.

Contribution

The paper introduces a method to constrain exoplanet physical properties using orbital dynamics and tidal effects, applied specifically to the HD 40307 system.

Findings

Planets are likely not terrestrial, resembling giant planets.

Orbital eccentricities must be at least 0.0001 for stability.

Inner planet's tidal heating could be significantly high.

Abstract

Three planets with minimum masses less than 10 Earth masses orbit the star HD 40307, suggesting these planets may be rocky. However, with only radial velocity data, it is impossible to determine if these planets are rocky or gaseous. Here we exploit various dynamical features of the system in order to assess the physical properties of the planets. Observations allow for circular orbits, but a numerical integration shows that the eccentricities must be at least 0.0001. Also, planets b and c are so close to the star that tidal effects are significant. If planet b has tidal parameters similar to the terrestrial planets in the Solar System and a remnant eccentricity larger than 0.001, then, going back in time, the system would have been unstable within the lifetime of the star (which we estimate to be 6.1 +/- 1.6 Gyr). Moreover, if the eccentricities are that large and the inner planet is rocky, then its tidal heating may be an order of magnitude greater than extremely volcanic Io, on a per unit surface area basis. If planet b is not terrestrial, e.g. Neptune-like, these physical constraints would not apply. This analysis suggests the planets are not terrestrial-like, and are more like our giant planets. In either case, we find that the planets probably formed at larger radii and migrated early-on (via disk interactions) into their current orbits. This study demonstrates how the orbital and dynamical properties of exoplanet systems may be used to constrain the planets' physical properties.