Thermal phase curves of non-transiting terrestrial exoplanets 2. Characterizing airless planets

TL;DR

This study demonstrates that multi-wavelength infrared phase curves can effectively determine the radius, albedo, and orbital inclination of non-transiting, airless terrestrial exoplanets, aiding their characterization with upcoming telescopes.

Contribution

It introduces a method to retrieve key physical parameters of non-transiting rocky exoplanets using thermal phase curves, without atmospheric assumptions.

Findings

Airless planets can be distinguished from those with atmospheres.

Radius, albedo, and inclination can be retrieved from multiband observations.

Applicable to nearby terrestrial exoplanets detected by radial velocity.

Abstract

Context. The photometric signal we receive from a star hosting a planet is modulated by the variation of the planet signal with its orbital phase. Such phase variations are observed for transiting hot Jupiters with current instrumentation, and have also been measured for one transiting terrestrial planet (Kepler 10 b) and one non-transiting gas giant (Ups A b). Future telescopes (JWST and EChO) will have the capability to measure thermal phase curves of exoplanets including hot rocky planets in transiting and non-transiting configurations, and at different wavelengths. Short-period planets with a mass below 10 R_EARTH are indeed frequent and nearby targets (within 10 pc) are already known and more are to be found. Aims. To test the possibility to use multi-wavelengths infrared phase curves to constrain the radius, the albedo and the orbital inclination of a non-transiting planet with no atmosphere and on a 1:1 spin orbit resonance. Methods. We model the thermal emission of a synchronous rocky planet with no atmosphere and its apparent variation with the orbital phase for a given orbital inclination. We assume that the planet is detected by radial velocity so its orbital period and minimum mass are known. We simulate observed noisy phase curves and then apply a procedure to retrieve the radius and albedo of the planet and the inclination of the orbit. Results. Airless planets can be distinguished from planets having a dense atmosphere and their radius, albedo and inclination (and therefore true mass) can be retrieved from multiband observations with MIRI-JWST and EChO in the 5-15 {\mu}m range. [...] As inclination above 60{\deg} represents half of randomly oriented orbits, the growing population of short-period terrestrial-sized planets detected by radial velocity surveys and transits should offer several nearby promising targets for this method [...]. See full abstract in the paper.