The Boundary between Gas-rich and Gas-poor Planets

Abstract

Sub-Saturns straddle the boundary between gas-rich Jupiters and gas-poor super-Earths/sub-Neptunes. Their large radii (4--8$R_{\oplus }$) suggest that their gas-to-core mass ratios range $\sim$0.1--1.0. With their envelopes as massive as their cores, sub-Saturns are just on the verge of runaway gas accretion; they are expected to be significantly less populous than gas giants. Yet, the observed occurrence rates of sub-Saturns and Jupiters are comparable within $\sim$100 days. We show that in these inner regions of planetary systems, the growth of sub-Saturns/Jupiters is ultimately limited by local and global hydrodynamic flows---runaway accretion terminates and the formation of gas giants is suppressed. We derive a simple analytic formula for the local hydrodynamic accretion rate---an expression that has been previously reported only as an empirical fit to numerical simulations. Evolving simultaneously the background disk gas and the gas accretion onto planetary cores, we find that both the ubiquity of super-Earths/sub-Neptunes and the rarity of gas-rich planets are best explained if an underlying core mass distribution is peaked at $\sim$4.3$M_{\oplus }$. Within a finite disk lifetime $\sim$10 Myrs, massive cores ($\gtrsim 10M_{\oplus }$) can become either gas-poor or gas-rich depending on when they assemble but smaller cores ($\lesssim 10M_{\oplus }$) can only become gas-poor. This wider range of possible outcomes afforded by more massive cores may explain why metal-rich stars harbor a more diverse set of planets.

ApJ in press. Figure 6 now compares model gas-to-core mass ratio vs. core mass to that inferred from observed exoplanets

References in corpus (19)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• On the Luminosity of Young Jupiters
• Separating gas-giant and ice-giant planets by halting pebble accretion
• Three regimes of extrasolar planets inferred from host star metallicities
• The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux
• A Three-Dimensional View of Turbulence: Constraints on Turbulent Motions in the HD 163296 Protoplanetary Disk using DCO${}^{+}$
• Water Vapour Absorption in the Clear Atmosphere of an exo-Neptune
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• A resonant chain of four transiting, sub-Neptune planets
• HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance
• Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid Recycling of Atmospheric Gas
• Kepler Multi-Planet Systems Exhibit Unexpected Intra-system Uniformity in Mass and Radius
• Reduced gas accretion on super-Earths and ice giants
• Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets
• Minimum Core Masses for Giant Planet Formation With Realistic Equations of State and Opacities
• Critical core mass for enriched envelopes: the role of H2O condensation
• Four Sub-Saturns with Dissimilar Densities: Windows into Planetary Cores and Envelopes
• A Systematic Study of the Final Masses of Gas Giant Planets
• Dusty outflows in planetary atmospheres: Understanding "super-puffs" and transmission spectra of sub-Neptunes

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• The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age
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• Mapping the exo-Neptunian landscape. A ridge between the desert and savanna
• Constraining the entropy of formation from young transiting planets
• Envelopes of embedded super-Earths II. Three-dimensional isothermal simulations
• Breaking the centrifugal barrier to giant planet contraction by magnetic disc braking
• Global 3D Radiation Hydrodynamic Simulations of Proto-Jupiter's Convective Envelope
• The Endgame of Gas Giant Formation: Accretion Luminosity and Contraction Post-Runaway
• CKS IX: Revisiting the Minimum-Mass Extrasolar Nebula with Precise Stellar Parameters
• Unified simulations of planetary formation and atmospheric evolution II: Rapid disk clearing by photoevaporation yields low-mass super-Earth atmospheres
• Planet Mass and Metallicity: The Exoplanets and Solar System Connection
• A Theoretical Framework for the Mass Distribution of Gas Giant Planets forming through the Core Accretion Paradigm
• Formation of Planetary Populations II: Effects of Initial Disk Size & Radial Dust Drift
• Architecture Classification for Extrasolar Planetary Systems
• Impact of M-dwarf Stellar Wind and Photoevaporation on the Atmospheric Evolution of Small Planets
• An Adolescent and Near-Resonant Planetary System Near the End of Photoevaporation
• A Multi-Planet System's Sole Super-Puff: Exploring Allowable Physical Parameters for the Cold Super-Puff HIP 41378 f