Bridging the Planet Radius Valley: Stellar Clustering as a Key Driver for Turning Sub-Neptunes into Super-Earths

Abstract

Extrasolar planets with sizes between that of the Earth and Neptune (R p = 1−4 R ⊕ ) have a bimodal radius distribution. This 'planet radius valley' separates compact, rocky super-Earths (R p = 1.0−1.8 R ⊕ ) from larger sub-Neptunes (R p = 1.8−3.5 R ⊕ ) hosting a gaseous hydrogen-helium envelope around their rocky core. Various hypotheses for this radius valley have been put forward, which all rely on physics internal to the planetary system: photoevaporation by the host star, long-term mass loss driven by the cooling planetary core, or the transition between two fundamentally different planet formation modes as gas is lost from the protoplanetary disc.Here we report the discovery that the planet radius distribution exhibits a strong dependence on ambient stellar clustering, characterised by measuring the position-velocity phase space density with Gaia. When dividing the planet sample into 'field' and 'overdensity' sub-samples, we find that planetary systems in the field exhibit a statistically significant (p = 5.5 × 10 −3 ) dearth of planets below the radius valley compared to systems in phase space overdensities. This implies that the large-scale stellar environment of a planetary system is a key factor setting the planet radius distribution. We discuss how models for the radius valley might be revised following our findings and conclude that a multi-scale, multi-physics scenario is needed, connecting planet formation and evolution, star and stellar cluster formation, and galaxy evolution.

Investigating the architecture and internal structure of the TOI-561 system planets with CHEOPS, HARPS-N and TESS

“…According to our analysis, performed as described in Mustill et al (2021), TOI-561 is located in a low-density region of the 6-dimensional Galactic phase space (see Winter et al 2020, Mustill et al 2021, and Kruijssen et al 2021 for definition and discussion), which is not surprising given that TOI-561 is a thick disk star (Mustill et al 2021). Kruijssen et al (2020) showed that stars in low-density regions seem to host no super-Earths, but only sub-Neptunes, i.e. planets having a significant H/He envelope and therefore located above the radius gap.…”

The New Generation Planetary Population Synthesis (NGPPS)

“…One example is the bimodal distribution of planet radii in the observed exoplanet sample (Fulton et al 2017;Fulton & Petigura 2018;Hsu et al 2018;Van Eylen et al 2018 Mordasini 2020), which was theoretically predicted to be caused by photoevaporation of planetary envelopes by high-energy radiation from their host star (Jin et al 2014;Owen & Wu 2013;Lopez & Fortney 2013). Other mechanisms have been proposed to produce this "radius valley" at roughly 2 R ⊕ as well, including atmospheric loss due to internal heat from cooling planetary cores (Ginzburg et al 2016(Ginzburg et al , 2018Gupta & Schlichting 2019), impacts of planetesimals (Wyatt et al 2019) or other protoplanets (Liu et al 2015), different internal compositions of planets residing above or below the valley (Zeng et al 2019;Venturini et al 2020), and atmospheric stripping by external radiation sources in stellar cluster environments (Kruijssen et al 2020). In the Generation III Bern Model, photoevaporation by the host star and collisional stripping are taken into account.…”

Exploring the link between star and planet formation with Ariel

“…Major efforts are currently being undertaken to link the properties of planetary systems to their large-scale stellar environment, with promising results (e.g. [112][113][114][115]). These studies show that planetary system architectures and planetary properties (e.g.…”