Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence

Abstract

In this paper we propose a new mechanism that could explain the survival of hydrogen atmospheres on some hot super-Earths. We argue that on close-orbiting tidally-locked super-Earths the tidal forces with the orbital and rotational centrifugal forces can partially confine the atmosphere on the nightside. Assuming a super terran body with an atmosphere dominated by volcanic species and a large hydrogen component, the heavier molecules can be shown to be confined within latitudes of 80 • whilst the volatile hydrogen is not. Because of this disparity the hydrogen has to slowly diffuse out into the dayside where XUV irradiation destroys it. For this mechanism to take effect it is necessary for the exoplanet to become tidally locked before losing the totality of its hydrogen envelop. Consequently, for super-Earths with this proposed configuration it is possible to solve the tidal-locking and mass-loss timescales in order to constrain their formation 'birth' masses. Our model predicts that 55 Cancri e formed with a day-length between approximately 17 − 18.5 hours and an initial mass less than ∼ 12M ⊕ hence allowing it to become tidally locked before the complete destruction of its atmosphere. For comparison, CoRoT-7b, an exoplanet with very similar properties to 55 Cancri e but lacking an atmosphere, formed with a day-length significantly different from ∼ 20.5 hours whilst also having an initial mass smaller than ∼ 9M ⊕ .

ARES.* V. No Evidence For Molecular Absorption in the HST WFC3 Spectrum of GJ 1132 b

“…The remnant core model (i.e., a planet that is highly compressed as a result of once hosting a large primordial atmosphere that was then subsequently lost) is from Mocquet et al (2014). It is important to note that the mass-radius models shown above are merely illustrative, as more complex setups are possible (e.g., Jespersen & Stevenson 2020;Modirrousta-Galian et al 2020a;Mousis et al 2020). The planets listed are Trappist-1 b, g (de Wit et al 2016;Grimm et al 2018) (Van Grootel et al 2014;Guo et al 2020), HD 3167 c (Christiansen et al 2017;Guilluy et al 2021;Mikal-Evans et al 2021), LHS 1140 b (Ment et al 2019), and K2-18b (Benneke et al 2019;Tsiaras et al 2019;Bézard et al 2020;Blain et al 2021).…”

“…The HST WFC3 transmission spectrum of the highly irradiated super-Earth 55 Cnc e shows evidence for hydrogen cyanide (HCN; Tsiaras et al 2016a). However, the exact nature of its atmosphere, and whether it exists, is still highly debated (Madhusudhan et al 2012;Dorn et al 2019;Jindal et al 2020;Modirrousta-Galian et al 2020a;Zhang et al 2021;Zilinskas et al 2021). 3.…”

GJ 357 b

“…It is common within the literature to perform backwards reconstructions of exoplanet atmospheres in order to determine their histories (e.g. Locci et al 2019;Modirrousta-Galian et al 2020b). This is important as it can provide us with useful information concerning the formation and evolution of exoplanets.…”

“…In order to account for this, we assumed a constant interior luminosity from 10 to 100 Myr. However, we did not incorporate planetary migration into our mass-loss model (see Modirrousta-Galian et al 2020b, for an example of evaporation-induced planetary migration). Therefore, by estimating how much hydrogen was lost from the moment the planet was born, we constrained the maximum allowed initial atmospheric reservoir.…”

Disentangling atmospheric compositions of K2-18 b with next generation facilities

“…Observations of 55-Cancri e, an extremely hot and irradiated low mass planet with a radius ≤ 2 R ⊕ , suggest the presence of a volatile-rich atmosphere [20,[45][46][47]. However, further observations are needed to constrain current models of the atmospheric composition and stability [46,[48][49][50]. Hubble observations of the TRAPPIST-1 planetary system [51,52] did not reveal the atmospheric composition of these rocky worlds [51][52][53].…”