Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations

Abstract

Small planets (∼1–3.9 ) constitute more than half of the inventory of the 4000-plus exoplanets discovered so far. Smaller planets are sufficiently dense to be rocky, but those with radii larger than ∼1.6 are thought to display in many cases hydrogen/helium gaseous envelopes up to ∼30% of the planetary mass. These low-mass planets are highly irradiated and the question of their origin, evolution, and possible links remains open. Here we show that close-in ocean planets affected by the greenhouse effect display hydrospheres in supercritical state, which generate inflated atmospheres without invoking the presence of large hydrogen/helium gaseous envelopes. We present a new set of mass–radius relationships for ocean planets with different compositions and different equilibrium temperatures, which are found to be well adapted to low-density sub-Neptune planets. Our model suggests that super-Earths and water-rich sub-Neptunes could belong to the same family of planets, i.e., hydrogen/helium-free planets, with differences between their interiors simply resulting from the variation in the water content.

Characterisation of the hydrospheres of TRAPPIST-1 planets

“…In Sect. 2.1 we describe the updates to the supercritical water layer with respect to the model depicted in Mousis et al (2020), while in Sect. 2.2 we present the implementation of the hydrosphere in ice phases.…”

“…If the planet is close enough to its host star, the upper layer of the hydrosphere corresponds to a hot steam atmosphere, whose temperature at the base is determined by the radiativeconvective balance calculated by the atmosphere model (Marcq 2012;Marcq et al 2017). When the pressure and temperature at the surface, which is defined as the base of the hydrosphere layer, are above the critical point of water, we include a supercritical water layer extending from the base of the hydrosphere to a height corresponding to the phase change to steam (Mousis et al 2020). We updated the EOS for this layer to the EOS introduced by Mazevet et al (2019), which is a fit to the experimental data provided by the International Association for the Properties of Water and Steam (IAPWS; Wagner & Pruß 2002) for the supercritical regime, and quantum molecular dynamics (QMD) simulations data for plasma and superionic water (French et al 2009).…”

“…In this study, we develop a planet interior model suitable for the different conditions at which water can be found in low-mass planets. Our implementation includes a supercritical water layer, introduced in Mousis et al (2020), coupled with a 1D radiative-convective atmosphere model (Marcq 2012;Marcq et al 2017;Pluriel et al 2019) to calculate the total radius of the highly irradiated planets with water self-consistently. Furthermore, for temperate planets, we have updated the interior model presented in Brugger et al (2016Brugger et al ( , 2017 to include ice phases Ih, II, III, V, and VI.…”

Planetary system LHS 1140 revisited with ESPRESSO and TESS

“…This is valid for LHS 1140 b, but we note that LHS 1140 c is closer to the star, and the WMF estimated in this study therefore only represents an upper limit to its water content (which assumes a layer of liquid water, which is not viable in this case). Furthermore, for LHS 1140 c, we extended the interior structure model to include the modeling of water in supercritical phase, whose implementation is described in Mousis et al (2020). We considered an atmosphere with a composition of 97% water and 3% carbon dioxide, and a surface pressure at its bottom of 300 bar.…”

Transmission Spectroscopy for the Warm Sub-Neptune HD 3167c: Evidence for Molecular Absorption and a Possible High-metallicity Atmosphere

“…Under this scenario, the sub-Neptunes are planets drawn from the same initial population, only they managed to retain their H/He envelopes. An alternative suggestion is that many of the sub-Neptunes could instead be composed of rock and water in comparable proportions, with little or no H/He (Zeng et al 2019;Mousis et al 2020). Such worlds would need to form beyond the snow line, where water ice is abundant, and subsequently migrate to their present close-in orbits.…”