Exoplanet secondary atmosphere loss and revival

Abstract

The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus—high–molecular-weight (secondary) atmospheres—on rocky exoplanets. Many rocky exoplanets are born with thick (>10 kbar) H2-dominated atmospheres but subsequently lose their H2; this process has no known Solar System analog. We study the consequences of early loss of a thick H2 atmosphere for subsequent occurrence of a high–molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H2. Our results imply that most rocky exoplanets orbiting closer to their star than the habitable zone that were formed with thick H2-dominated atmospheres lack high–molecular-weight atmospheres today. During early magma ocean crystallization, high–molecular-weight species usually do not form long-lived high–molecular-weight atmospheres; instead, they are lost to space alongside H2. This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, water worlds). Our results imply that in order to find high–molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H2-poor, that have anomalously large radii, or that orbit less active stars.

A compositional link between rocky exoplanets and their host stars

“…The planet structure model we adopt does not include an atmospheric layer. However, the atmospheric build-up on planets that lost their H 2 -dominated envelopes is very limited, especially when planets are at close-in orbits (68) which is the case for all the planets in our sample. If the planets of our sample have Earth-like atmospheres, then neglecting them would have negligible impact on our results.…”

Oxygen controls on magmatism in rocky exoplanets

“…Equilibration between globally distributed magma and the overlying atmosphere, accompanied by significant outgassing from the convecting magma, can fundamentally alter the thickness and composition of the initial atmospheres that may have been inherited from the disk stage of a growing exoplanetary system ( 35 , 36 ). The extent of melting of a rocky exoplanet during its magma ocean stage is determined by a combination of a planet’s temperature profile and the solidus (temperature below which no liquid is present) and liquidus (temperature above which only liquid is present) of the rocks constituting the planet’s outer silicate reservoir ( 35 – 37 ).…”

“…Studies of the effect of f O 2 on magma oceans have, to date, been limited to quantifying the link between f O 2 variations and changes in the redox states of the major cation iron ( 13 , 26 , 46 ), or linked directly to models of gas speciation in atmospheres influenced by magma ocean outgassing ( 35 , 36 , 47 ). Variations in solidus and liquidus temperatures of cooling magma oceans have largely been assessed in terms of variations in volatile abundances ( 48 , 49 ).…”

TOI-1634 b: An Ultra-short-period Keystone Planet Sitting inside the M-dwarf Radius Valley

“…Given the unusual combination of TOI-1634 b's location in mass-radius space and its close orbital separation, TOI-1634 b occupies a unique region of the parameter space wherein it would be particularly interesting to distinguish between different atmospheric compositions. For example, a H 2 O-rich atmosphere would likely be indicative of a substantial initial water reservoir (Schaefer et al 2016;Kite & Barnett 2020) whereas a CO 2 -rich atmosphere may be produced by a runaway greenhouse if a significant portion of the planet's water inventory was photolyzed and lost to space. Given its USP, TOI-1634 b is an attractive candidate to distinguish between these atmosphere models via thermal emission observations as atmospheric signatures are likely to be more easily accessible than in transmission (Morley et al 2017b).…”