Tidal evolution of exoplanetary systems hosting Potentially Habitable Exoplanets. The cases of LHS-1140 b-c and K2-18 b-c

TL;DR

This study models the tidal and secular evolution of two exoplanetary systems with potentially habitable planets, revealing orbital characteristics and planetary compositions consistent with recent observations and theoretical constraints.

Contribution

It introduces a combined secular and tidal evolution model for three-body systems with low-mass planets, applied to specific exoplanet systems to infer orbital and planetary properties.

Findings

LHS-1140 b and c are nearly circular in orbit.

K2-18 c is likely a Neptune-like gaseous planet.

K2-18 b's eccentricity constraints depend on its viscosity and system age.

Abstract

We present a model to study secularly and tidally evolving three-body systems composed by two low-mass planets orbiting a star, in the case where the bodies rotation axes are always perpendicular to the orbital plane. The tidal theory allows us to study the spin and orbit evolution of both stiff Earth-like planets and predominantly gaseous Neptune-like planets. The model is applied to study two recently-discovered exoplanetary systems containing potentially habitable exoplanets (PHE): LHS-1140 b-c and K2-18 b-c. For the former system, we show that both LHS-1140 b and c must be in nearly-circular orbits. For K2-18 b-c, the combined analysis of orbital evolution timescales with the current eccentricity estimation of K2-18 b allows us to conclude that the inner planet (K2-18 c) must be a Neptune-like gaseous body. Only this would allow for the eccentricity of K2-18 b to be in the range of values estimated in recent works ($e=0.20 \pm 0.08$), provided that the uniform viscosity coefficient of K2-18 b is greater than $2.4 \times 10^{19} \ \textrm{Pa s}$ (which is a value characteristic of stiff bodies) and supposing that such system has an age of some Gyr.