The effect of late giant collisions on the atmospheres of protoplanets and the formation of cold sub-Saturns

Abstract

We investigate the origins of cold sub-Saturns (CSS), an exoplanetary population inferred from microlensing surveys. If confirmed, these planets would rebut a theorised gap in planets' mass distribution between those of Neptune and Jupiter caused by the rapid runaway accretion of super-critical cores. In an attempt to resolve this theoretical-observational disparity, we examine the outcomes of giant collisions between sub-critical protoplanets. Due to the secular interaction among protoplanets, these events may occur in rapidly depleting discs. We show that impactors ~ 5% the mass of near-runaway envelopes around massive cores can efficiently remove these envelopes entirely via a thermally-driven super-Eddington wind emanating from the core itself, in contrast with the stellar Parker winds usually considered. After a brief cooling phase, the merged cores resume accretion. But, the evolution timescale of transitional discs is too brief for the cores to acquire sufficiently massive envelopes to undergo runaway accretion despite their large combined masses. Consequently, these events lead to the emergence of CSS without their transformation into gas giants. We show that these results are robust for a wide range of disc densities, grain opacities and silicate abundance in the envelope. Our fiducial case reproduces CSS with heavy (>= 30 M_Earth) cores and less massive (a few M_Earth) sub-critical envelopes. We also investigate the other limiting cases, where continuous mergers of comparable-mass cores yield CSS with wider ranges of core-to-envelope mass ratios and envelope opacities. Our results indicate that it is possible for CSS and Uranus and Neptune to emerge within the framework of well studied processes and they may be more common than previously postulated.

Accepted for publication in MNRAS, 17 pages, 9 figures

References in corpus (31)

• Separating gas-giant and ice-giant planets by halting pebble accretion
• Observations of Protoplanetary Disk Structures
• The TESS Objects of Interest Catalog from the TESS Prime Mission
• The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Grain Retention and Formation of Planetesimals near the Snow Line in MRI-driven Turbulent Protoplanetary Disks
• Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts
• Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid Recycling of Atmospheric Gas
• Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems
• The newborn planet population emerging from ring-like structures in discs
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars
• Minimum Core Masses for Giant Planet Formation With Realistic Equations of State and Opacities
• The Formation of Jupiter's Diluted Core by a Giant Impact
• Assembling the Building Blocks of Giant Planets around Intermediate Mass Stars
• Microlensing Results Challenge the Core Accretion Runaway Growth Scenario for Gas Giants
• Planetesimal Accretion onto Growing Proto-Gas-Giant Planets
• Dynamical Shakeup of Planetary Systems II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping
• The Formation of Uranus & Neptune: Challenges and Implications For Intermediate-Mass Exoplanets
• The Imprint of the Protoplanetary Disk in the Accretion of Super-Earth Envelopes
• Heavy-metal Jupiters by major mergers: metallicity vs. mass for giant planets
• The Preservation of Super Earths and the Emergence of Gas Giants after Their Progenitor Cores have Entered the Pebble Isolation Phase
• Accretion of Gas Giants Constrained by the Tidal Barrier
• Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure
• ALMA observations require slower Core Accretion runaway growth
• MOA-2009-BLG-319Lb: A Sub-Saturn Planet Inside the Predicted Mass Desert
• Clearing residual planetesimals by sweeping secular resonances in transitional disks: a lone-planet scenario for the wide gaps in debris disks around Vega and Fomalhaut
• Formation of giant planets with large metal masses and metal fractions via giant impacts in a rapidly dissipating disk
• Wide Dust Gaps in Protoplanetary Disks Induced by Eccentric Planets: A Mass-Eccentricity Degeneracy
• The Demographics of Exoplanets
• Constraining protoplanetary disks with exoplanetary dynamics: Kepler-419 as an example