Characterisation of the hydrospheres of TRAPPIST-1 planets

Abstract

Context. Planetary mass and radius data suggest that low-mass exoplanets show a wide variety of densities. This includes sub-Neptunes, whose low densities can be explained with the presence of a volatile-rich layer. Water is one of the most abundant volatiles, which can be in the form of different phases depending on the planetary surface conditions. To constrain their composition and interior structure, models must be developed that accurately calculate the properties of water at its different phases. Aims. We present an interior structure model that includes a multiphase water layer with steam, supercritical, and condensed phases. We derive the constraints for planetary compositional parameters and their uncertainties, focusing on the multi-planetary system TRAPPIST-1, which presents both warm and temperate planets. Methods. We use a 1D steam atmosphere in radiative-convective equilibrium with an interior whose water layer is in supercritical phase self-consistently. For temperate surface conditions, we implement liquid and ice Ih to ice VII phases in the hydrosphere. We adopt a Markov chain Monte Carlo inversion scheme to derive the probability distributions of core and water compositional parameters. Results. We refine the composition of all planets and derive atmospheric parameters for planets ‘b’ and ‘c’. The latter would be in a post-runaway greenhouse state and could be extended enough to be probed by space missions such as JWST. Planets ‘d’ to ‘h’ present condensed ice phases, with maximum water mass fractions below 20%. Conclusions. The derived amounts of water for TRAPPIST-1 planets show a general increase with semi-major axis, with the exception of planet d. This deviation from the trend could be due to formation mechanisms, such as migration and an enrichment of water in the region where planet d formed, or an extended CO2-rich atmosphere.

Rapid-then-slow migration reproduces mass distribution of TRAPPIST-1 system

“…2018; Acuña et al 2021). The estimates consistently indicate that all seven planets have rocky interiors with a small water mass fraction ( 10wt%).…”

“…The second column, a, indicates the semimajor axis. Data are obtained from Agol et al (2021).2018; Acuña et al 2021). The estimates consistently indicate that all seven planets have rocky interiors with a small water mass fraction ( 10wt%).…”

A model Earth-sized planet in the habitable zone of $α$ Centauri A/B

“…It is worth stressing, however, that "mantle water-storage capacity" neither implies presence of water in a particular abundance (cf. Dorn et al 2015;Unterborn et al 2018;Dencs & Regály 2019;Shah et al 2020;Acuna et al 2021), nor does it define the source of water (cf. Hallis et al 2015;Peslier et al 2017;Wu et al 2018;O'Brien et al 2018;Lichtenberg et al 2019).…”

TOI-220 b: a warm sub-Neptune discovered byTESS

“…TOI-220 b being strongly irradiated with an equilibrium temperature of 806 K, the model allows us to address how its composition could differ from that of a Neptune-like planet with a large gaseous envelope overlying a rocky core. We followed the method described in Lillo-Box et al (2020) and Acuña et al (2021) exploring the core mass fraction (CMF, the ratio of the mass of the core and the total planetary mass) and the water mass fraction (WMF, the ratio of the mass of the water layer -steam and supercriticaland the total mass) of the planet as free parameters in an MCMC Bayesian framework adapted from Dorn et al (2015). We use as input parameters the planetary mass and radius, and the Fe/Si and Mg/Si mole ratios.…”