Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets

Abstract

The population of close-in super-Earths, with gas mass fractions of up to 10% represents a challenge for planet formation theory: how did they avoid runaway gas accretion and collapsing to hot Jupiters despite their core masses being in the critical range of $M_{\mathrm{c}}\simeq 10M_{\oplus }$? Previous three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of low-mass planets cannot be considered isolated from the protoplanetary disc, contrary to what is assumed in 1D-evolutionary calculations. This finding is referred to as the recycling hypothesis. In this Paper we investigate the recycling hypothesis for super-Earth planets, accounting for realistic 3D radiation hydrodynamics. Also, we conduct a direct comparison in terms of the evolution of the entropy between 1D and 3D geometries. We clearly see that 3D atmospheres maintain higher entropy: although gas in the atmosphere loses entropy through radiative cooling, the advection of high entropy gas from the disc into the Bondi/Hill sphere slows down Kelvin-Helmholtz contraction, potentially arresting envelope growth at a sub-critical gas mass fraction. Recycling, therefore, operates vigorously, in line with results by previous studies. However, we also identify an "inner core" -- in size $\approx$ 25% of the Bondi radius -- where streamlines are more circular and entropies are much lower than in the outer atmosphere. Future studies at higher resolutions are needed to assess whether this region can become hydrodynamically-isolated on long time-scales.

16 pages, 12 figures, accepted for publication at MNRAS

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• Hydrogen dominated atmospheres on terrestrial mass planets: evidence, origin and evolution
• A Tale of Planet Formation: From Dust to Planets
• The CARMENES search for exoplanets around M dwarfs -- LP 714-47b (TOI 442.01): Populating the Neptune desert
• Envelopes of embedded super-Earths II. Three-dimensional isothermal simulations
• Steady State by Recycling prevents Premature Collapse of Protoplanetary Atmospheres
• Setting the Stage: Planet formation and Volatile Delivery
• Gas flow around a planet embedded in a protoplanetary disc: the dependence on the planetary mass
• Makemake + Sedna: A Continuum Radiation Transport and Photoionization Framework for Astrophysical Newtonian Fluid Dynamics
• Global 3D Radiation Hydrodynamic Simulations of Proto-Jupiter's Convective Envelope
• ALMA observations require slower Core Accretion runaway growth
• How planets grow by pebble accretion IV: Envelope opacity trends from sedimenting dust and pebbles
• Influences of three-dimensional gas flow induced by protoplanets on pebble accretion --$\mathrm{I}$. shear regime
• Oscillatory migration of accreting protoplanets driven by a 3D distortion of the gas flow
• Close-in Super-Earths: The first and the last stages of planet formation in an MRI-accreting disc
• Unified simulations of planetary formation and atmospheric evolution II: Rapid disk clearing by photoevaporation yields low-mass super-Earth atmospheres
• TW Hya: an old protoplanetary disc revived by its planet
• The Growth of Protoplanets via the Accretion of Small Bodies in Disks Perturbed by the Planetary Gravity
• Influence of grain sizes and composition on the contraction rates of planetary envelopes and on planetary migration
• The Dynamic Proto-atmospheres around Low-Mass Planets with Eccentric Orbits