The Bimodal Distribution in Exoplanet Radii: Considering Varying Core Compositions and H₂ Envelope’s Sizes

Abstract

Several models have been introduced in order to explain the radius distribution in exoplanet radii observed by Fulton et al. (2017) with one peak at ∼ 1.3R ⊕ the other at ∼ 2.4R ⊕ and the minimum at ∼ 1.75R ⊕ . In this paper we focus on the hypothesis that the exoplanet size distribution is caused by stellar XUV-induced atmospheric loss. We evolve 10 6 synthetic exoplanets by exposing them to XUV irradiation from synthetic ZAMS stars. For each planet we set a different interior composition which ranged from 100 wt% Fe (very dense) through 100 wt% MgSiO 3 (average density) and to 100 wt% H 2 O ice (low density) with varying hydrogen envelop sizes which varied from 0 wt% (a negligible envelop) to 100 wt% (a negligible core). Our simulations were able to replicate the bimodal distribution in exoplanet radii. We argue that in order to reproduce the distribution by Fulton et al. (2017) it is mandatory for there to be a paucity of exoplanets with masses above ∼ 8M ⊕ . Furthermore, our bestfit result predicts an initial flat distribution in exoplanet occurrence for M P 8M ⊕ with a strong deficiency for planets with 3M ⊕ . Our results are consistent with the ∼ 1.3R ⊕ radius peak mostly encompassing denuded exoplanets whilst the ∼ 2.4R ⊕ radius peak mainly comprising exoplanets with large hydrogen envelops.

A HARPS-N mass for the elusive Kepler-37d: a case study in disentangling stellar activity and planetary signals

“…We note that Kepler-37d lies within the so-called small planet radius gap or 'Fulton gap' (Fulton et al 2017). According to recent XUV irradiation modelling by Modirrousta-Galian, Locci & Micela (2020), which reproduces the bimodal exoplanet radius distribution observed by Fulton, roughly equal proportions of planets with Kepler-37d's 1.9 R ⊕ radius are expected to have atmospheres versus no atmospheres.…”

GJ 357 b

“…This translates into higher masses and densities for GJ 357 b, which are also listed in Table 2 and shown in Figs. 2 and 4 2019) results are considered, the composition of the planet would be consistent with a telluric planet that has a core 60% (Zeng et al 2016) of the total mass or with the icy remnant core of a planet which once hosted a large primordial atmosphere (Mocquet et al 2014;Modirrousta-Galian et al 2020a). However, as explained previously, the uncertainties in the measured values allow for a more typical rocky composition.…”

“…For instance, the bimodal distribution of exoplanet radii is believed by many to be caused by photoevaporation, which has been scrutinised by performing backwards reconstructions of exoplanet populations (e.g. Owen & Wu 2013Modirrousta-Galian et al 2020a). Furthermore, the remnant cores of planets, which once hosted large primordial atmospheres, are of great interest to the exoplanetary community as they provide us with an inference for the interior structure of bodies like Jupiter and Saturn (e.g.…”

“…a core that is 60 wt.% of the total mass). However, an alternative explanation for this high density is that GJ 357 b is a compressed, icy remnant core of a planet that originally had a large hydrogen envelope (Mocquet et al 2014;Modirrousta-Galian et al 2020a). New observational data further support this possibility as shown by the "T2" trend in Swain et al (2019).…”

Exploring Super-Earth Surfaces: Albedo of Near-Airless Magma Ocean Planets and Topography

“…If this planet were then to migrate very close to its host star, XUV-irradiation may fully erode away its atmosphere leaving behind a denuded core (e.g. Lecavelier Des Etangs 2007; Ehrenreich and Désert 2011;Lammer et al 2013;Owen and Wu 2013;Jin et al 2014;Owen and Wu 2017;Jin and Mordasini 2018;Kubyshkina et al 2018;Locci et al 2019;Modirrousta-Galian et al 2020a). Due to iron being relatively heavy, it may be resilient to hydrodynamic escape (e.g.…”