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 ($R_{\mathrm{p}}=1-4{\mathrm{R}}_{\oplus }$) have a bimodal radius distribution. This 'planet radius valley' separates compact, rocky super-Earths ($R_{\mathrm{p}}=1.0-1.8{\mathrm{R}}_{\oplus }$) from larger sub-Neptunes ($R_{\mathrm{p}}=1.8-3.5{\mathrm{R}}_{\oplus }$) 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 \textit{Gaia}. When dividing the planet sample into 'field' and 'overdensity' sub-samples, we find that planetary systems in the field exhibit a statistically significant ($p=5.5\times 10^{-3}$) 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.

8 pages, 3 figures, 1 table; ApJ Letters in press (accepted 2020 November 22; revised 2020 November 19; received 2020 October 22)

References in corpus (7)

• The NumPy array: a structure for efficient numerical computation
• The Gaia mission
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age
• On the ridges, undulations & streams in Gaia DR2: Linking the topography of phase-space to the orbital structure of an N-body bar
• The signatures of the parental cluster on field planetary systems

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• Exploring the link between star and planet formation with Ariel
• Constraining planetary mass-loss rates by simulating Parker wind profiles with Cloudy
• The influence of the environment on the spin evolution of low-mass stars. I. External photoevaporation of circumstellar disks
• Warm millimetre dust in protoplanetary discs near massive stars
• When the Peas Jump around the Pod: How Stellar Clustering Affects the Observed Correlations between Planet Properties in Multi-Planet Systems