Role of Planetary Obliquity in Regulating Atmospheric Escape: G-dwarf versus M-dwarf Earth-like Exoplanets

Abstract

We present a three-species (H + , O + and e − ) multi-fluid magnetohydrodynamic (MHD) model, endowed with the requisite upper atmospheric chemistry, that is capable of accurately quantifying the magnitude of oxygen ion losses from "Earth-like" exoplanets in habitable zones, whose magnetic and rotational axes are roughly coincidental with one another. We apply this model to investigate the role of planetary obliquity in regulating atmospheric losses from a magnetic perspective. For Earth-like exoplanets orbiting solar-type stars, we demonstrate that the dependence of the total atmospheric ion loss rate on the planetary (magnetic) obliquity is relatively weak; the escape rates are found to vary between 2.19 × 10 26 s −1 to 2.37 × 10 26 s −1 . In contrast, the obliquity can influence the atmospheric escape rate (∼ 10 28 s −1 ) by more than a factor of 2 (or 200%) in the case of Earth-like exoplanets orbiting late-type M-dwarfs. Thus, our simulations indicate that planetary obliquity may play a weak-to-moderate role insofar as the retention of an atmosphere (necessary for surface habitability) is concerned.

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“… 2017 ; Dong et al. 2017 , 2018 , 2019 ; Fraschetti et al. 2019 ) for planets orbiting such a low mass star.…”

“…In addition, atmospheric stripping by the strong stellar winds of TRAPPIST-1 is thought to be efficient (Garraffo et al 2017;Dong et al 2017Dong et al , 2018Dong et al , 2019Fraschetti et al 2019) for planets orbiting such a low mass star.…”

“…This number could be lowered by up to two orders of magnitude when considering the planet has a strong magnetic field (Dong et al 2017). Dong et al (2019) showed with the case of TRAPPIST-1g that the rate of atmospheric escape due to stellar wind can actually increase by a factor of ∼100 for O 2 -dominated atmospheres, since O 2 -dominated exospheres are not expected to cool as efficiently as CO 2dominated ones. Although to the best of our knowledge no numerical experiments have yet been conducted to study the stellar wind-driven atmospheric escape rate for N 2 -dominated atmospheres around low mass stars, we argue that quantitatively similar results are expected.…”

Global Ten‐Moment Multifluid Simulations of the Solar Wind Interaction with Mercury: From the Planetary Conducting Core to the Dynamic Magnetosphere

“…In recent years, our understanding of terrestrial bodies has been significantly advanced by increasingly sophisticated numerical models. A large number of global models based on either fluid or hybrid (kinetic ion particles and massless electron fluid) approach have been developed for both magnetized planets such as Mercury (e.g., Exner et al, 2018;Jia et al, 2015;Kabin et al, 2008;Kidder et al, 2008;Müller et al, 2012;Richer et al, 2012;Trávníček et al, 2010) and unmagnetized planets such as Mars (Dong et al, 2014(Dong et al, , 2015(Dong et al, , 2018a(Dong et al, , 2018bLedvina et al, 2017;Ma et al, 2014;Modolo et al, 2016) as well as exoplanets (Johansson et al, 2011;Dong et al, 2017aDong et al, , 2017bDong et al, , 2018cDong et al, , 2019. However, none of these global models can accurately treat collisionless magnetic reconnection due to their lack of detailed electron physics.…”

QUEST: A New Frontiers Uranus orbiter mission concept study

“…Unraveling these processes, particularly in contrast to the magnetic interactions ongoing at the other giant planets, will provide further evidence to support current models of magnetospheric dynamics. Additionally, the study of the solar wind-Uranus interaction may also facilitate our understanding of the stellar wind-planet interaction of exoplanets with high magnetic obliquities [39,40] thereby expanding our magnetospheric comparative planetology capabilities beyond our own solar system. Additionally, Voyager 2 arrived when Uranus was near northern summer solstice [44] and the proposed arrival date of 2045 for the QUEST mission would be near the autumnal equinox.…”