The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains

Abstract

The size frequency distribution of exoplanet radii between 1 and 4$R_{\oplus }$ is bimodal with peaks at $\sim$1.4 $R_{\oplus }$ and $\sim$2.4 $R_{\oplus }$, and a valley at $\sim$1.8$R_{\oplus }$. This radius valley separates two classes of planets -- usually referred to as "super-Earths" and "mini-Neptunes" -- and its origin remains debated. One model proposes that super-Earths are the outcome of photo-evaporation or core-powered mass-loss stripping the primordial atmospheres of the mini-Neptunes. A contrasting model interprets the radius valley as a dichotomy in the bulk compositions, where super-Earths are rocky planets and mini-Neptunes are water-ice rich worlds. In this work, we test whether the migration model is consistent with the radius valley and how it distinguishes these views. In the migration model, planets migrate towards the disk inner edge forming a chain of planets locked in resonant configurations. After the gas disk dispersal, orbital instabilities "break the chains" and promote late collisions. This model broadly matches the period-ratio and planet-multiplicity distributions of Kepler planets, and accounts for resonant chains such as TRAPPIST-1, Kepler-223, and TOI-178. Here, by combining the outcome of planet formation simulations with compositional mass-radius relationships, and assuming complete loss of primordial H-rich atmospheres in late giant-impacts, we show that the migration model accounts for the exoplanet radius valley and the intra-system uniformity ("peas-in-a-pod") of Kepler planets. Our results suggest that planets with sizes of $\sim$1.4 $R_{\oplus }$ are mostly rocky, whereas those with sizes of $\sim$2.4 $R_{\oplus }$ are mostly water-ice rich worlds. Our results do not support an exclusively rocky composition for the cores of mini-Neptunes.

arXiv commentAccepted for publicationn in ApJL

References in corpus (113)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler
• The false positive rate of Kepler and the occurrence of planets
• Planetary Candidates Observed by Kepler, III: Analysis of the First 16 Months of Data
• Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits
• Rapid growth of gas-giant cores by pebble accretion
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler planets: a tale of evaporation
• Architecture and Dynamics of Kepler's Candidate Multiple Transiting Planet Systems
• The evaporation valley in the Kepler planets
• Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates
• Understanding the Mass-Radius Relation for Sub-Neptunes: Radius as a Proxy for Composition
• Probabilistic Forecasting of the Masses and Radii of Other Worlds
• The effect of gas drag on the growth of protoplanets -- Analytical expressions for the accretion of small bodies in laminar disks
• Growth Model Interpretation of Planet Size Distribution
• The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii
• A torque formula for non-isothermal Type I planetary migration - II. Effects of diffusion
• The California Kepler Survey VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• The Role of Core Mass in Controlling Evaporation: the Kepler Radius Distribution and the Kepler-36 Density Dichotomy
• Core-powered mass loss and the radius distribution of small exoplanets
• Separating gas-giant and ice-giant planets by halting pebble accretion
• Mass-Radius Relation for Rocky Planets based on PREM
• Collisions between Gravity-Dominated Bodies: 2. The Diversity of Impact Outcomes during the End Stage of Planet Formation
• Collisions Between Gravity-Dominated Bodies: 1. Outcome Regimes and Scaling Laws
• Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
• How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond
• Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities
• Breaking the Chains: Hot Super-Earth systems from migration and disruption of compact resonant chains
• Migration and the formation of systems of hot super-Earths and Neptunes
• Statistical Study of the Early Solar System's Instability with 4, 5 and 6 Giant Planets
• A seven-planet resonant chain in TRAPPIST-1
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
• The California-Kepler Survey V. Peas in a Pod: Planets in a Kepler Multi-planet System are Similar in Size and Regularly Spaced
• The California-Kepler Survey. I. High Resolution Spectroscopy of 1305 Stars Hosting Kepler Transiting Planets
• Pebble isolation mass --- scaling law and implications for the formation of super-Earths and gas giants
• Super-Earth Atmospheres: Self-Consistent Gas Accretion and Retention
• Planetesimal formation starts at the snow line
• Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation
• The Derivation, Properties and Value of Kepler's Combined Differential Photometric Precision
• The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores
• About 30% of Sun-like Stars Have Kepler-like Planetary Systems: A Study of their Intrinsic Architecture
• Formation of planetary systems by pebble accretion and migration: How the radial pebble flux determines a terrestrial-planet or super-Earth growth mode
• Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planets
• The orbital eccentricity of small planet systems
• The Exoplanet Population Observation Simulator. I - The Inner Edges of Planetary Systems
• Three-dimensional simulations of multiple protoplanets embedded in a protostellar disc
• Revisited Mass-Radius relations for exoplanets below 120 Earth masses
• To Cool is to Accrete: Analytic Scalings for Nebular Accretion of Planetary Atmospheres
• Toward a Deterministic Model of Planetary Formation VI: Dynamical Interaction and Coagulation of Multiple Rocky Embryos and Super-Earth Systems around Solar Type Stars
• Ocean Planet or Thick Atmosphere: On the Mass-Radius Relationship for Solid Exoplanets with Massive Atmospheres
• A resonant chain of four transiting, sub-Neptune planets
• Signatures of the Core-Powered Mass-Loss Mechanism in the Exoplanet Population: Dependence on Stellar Properties and Observational Predictions
• Six transiting planets and a chain of Laplace resonances in TOI-178
• On the formation of planetary systems via oligarchic growth in thermally evolving viscous discs
• Formation of planetary systems by pebble accretion and migration: Growth of gas giants
• The California-Kepler Survey. II. Precise Physical Properties of 2025 Kepler Planets and Their Host Stars
• Observed Properties of Extrasolar Planets
• Volatile-Rich Earth-Mass Planets in the Habitable Zone
• Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs
• Minimum Radii of Super-Earths: Constraints from Giant Impacts
• Reduced gas accretion on super-Earths and ice giants
• Hot super-Earths and giant planet cores from different migration histories
• The nature and origins of sub-Neptune size planets
• Contemporary formation of early solar system planetesimals at two distinct radial locations
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• Architectures of Exoplanetary Systems. I: A Clustered Forward Model for Exoplanetary Systems around Kepler's FGK Stars
• Irradiated ocean planets bridge super-Earth and sub-Neptune populations
• Planetary evolution with atmospheric photoevaporation I. Analytical derivation and numerical study of the evaporation valley and transition from super-Earths to sub-Neptunes
• Planetesimal rings as the cause of the Solar System's planetary architecture
• Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets
• Photoevaporation vs. core-powered mass-loss: model comparison with the 3D radius gap
• A reassessment of the in situ formation of close-in super-Earths
• Grain opacity and the bulk composition of extrasolar planets. II. An analytical model for the grain opacity in protoplanetary atmospheres
• Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities
• Planetesimal Interactions Can Explain the Mysterious Period Ratios of Small Near-Resonant Planets
• Migration-driven diversity of super-Earth compositions
• Occurrence and core-envelope structure of 1--4x Earth-size planets around Sun-like stars
• Two Empirical Regimes of the Planetary Mass-Radius Relation
• Effect of the rotation and tidal dissipation history of stars on the evolution of close-in planets
• Giant Impact: An Efficient Mechanism for the Devolatilization of Super-Earths
• On the formation of hot Neptunes and super-Earths
• Formation of close-in super-Earths in evolving protoplanetary disks due to disk winds
• Rocky super-Earths or waterworlds: the interplay of planet migration, pebble accretion and disc evolution
• On the formation of compact planetary systems via concurrent core accretion and migration
• Testing exoplanet evaporation with multi-transiting systems
• Atmospheric mass loss due to giant impacts: the importance of the thermal component for hydrogen-helium envelopes
• Spin-orbit Misalignment as a Driver of the Kepler Dichotomy
• Terrestrial Planet Formation in the Presence of Migrating Super-earths
• Global models of planetary system formation in radiatively-inefficient protoplanetary discs
• Hiding Planets Behind a Big Friend: Mutual Inclinations of Multi-Planet Systems with External Companions
• Architectures of Compact Super-Earth Systems Shaped by Instabilities
• Two Views of the Radius Gap and the Role of Light Curve Fitting
• The eccentricity distribution of giant planets and their relation to super-Earths in the pebble accretion scenario
• To cool is to keep: Residual H/He atmospheres of super-Earths and sub-Neptunes
• The Kepler Peas in a Pod Pattern is Astrophysical
• An upper limit on late accretion and water delivery in the Trappist-1 exoplanet system
• Envelopes of embedded super-Earths II. Three-dimensional isothermal simulations
• New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State
• Breaking Resonant Chains: Destabilization of Resonant Planets due to Long-term Mass Evolution
• Formation of compact systems of super-Earths via dynamical instabilities and giant impacts
• Recycling of the first atmospheres of embedded planets: Dependence on core mass and optical depth
• Properties of the radius valley around low mass stars: Predictions from the core-powered mass-loss mechanism
• Impact of photoevaporative mass loss on masses and radii of water-rich sub/super-Earths
• The "Breaking The Chains" migration model for super-Earths formation: the effect of collisional fragmentation
• Inner rocky super-Earth formation: distinguishing the formation pathways in viscously heated and passive discs
• Identifying inflated super-Earths and photo-evaporated cores
• Formation of short-period planets by disk migration
• Losing Oceans: The Effects of Composition on the Thermal Component of Impact-driven Atmospheric Loss
• The origins of nearly coplanar, non-resonant systems of close-in super-Earths
• Signatures of impact-driven atmospheric loss in large ensembles of exoplanets

Cited by in corpus (40)

• A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067
• The mass-radius relation of exoplanets, revisited
• LHS 1140 b is a potentially habitable water world
• The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets
• Giants are bullies: how their growth influences systems of inner sub-Neptunes and super-Earths
• Super-Earths and Earth-like Exoplanets
• Shallower radius valley around low-mass hosts: Evidence for icy planets, collisions or high-energy radiation scatter
• Unveiling the internal structure and formation history of the three planets transiting HIP 29442 (TOI-469) with CHEOPS
• Intra-system uniformity: a natural outcome of dynamical sculpting
• When, where, and how many planets end up in first-order resonances?
• Bioverse: The Habitable Zone Inner Edge Discontinuity as an Imprint of Runaway Greenhouse Climates on Exoplanet Demographics
• Resonant sub-Neptunes are puffier
• Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape
• TOI-1801 b: A temperate mini-Neptune around a young M0.5 dwarf
• Constraining exoplanet interiors using observations of their atmospheres
• Evolution of steam worlds: energetic aspects
• Using Lyman-$\alpha$ transits to constrain models of atmospheric escape
• Building Wet Planets through High-Pressure Magma-Hydrogen Reactions
• Photo-dynamical characterisation of the TOI-178 resonant chain
• Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap
• A primordial radius valley as a consequence of planet formation
• Assessing the spin-orbit obliquity of low-mass planets in the breaking the chain formation model: A story of misalignment
• Formation of super-Earths and mini-Neptunes from rings of planetesimals
• Possible Evidence for the Presence of Volatiles on the Warm Super-Earth TOI-270 b
• Searching for Hot Water World Candidates with CHEOPS: Refining the radii and analysing the internal structures and atmospheric lifetimes of TOI-238 b and TOI-1685 b
• A formation pathway for terrestrial planets with moderate water content involving atmospheric-volatile recycling
• Diversity of disc viscosities can explain the period ratios of resonant and non-resonant systems of hot super-Earths and mini-Neptunes
• Radius valley scaling among low mass stars with TESS
• Long-term evolution of the temperature structure in magnetized protoplanetary disks and its implication for the dichotomy of planetary composition
• A joint effort to discover and characterize two resonant mini Neptunes around TOI-1803 with TESS, HARPS-N and CHEOPS
• Close-in compact super-Earth systems emerging from resonant chains: slow destabilization by unseen remnants of formation
• A transiting rocky super-Earth and a non-transiting sub-Neptune orbiting the M dwarf TOI-771
• Evidence that Planets in the Radius Gap Do Not Resemble Their Neighbors
• Stability of a cluster-disrupted mean-motion resonance (chain) in HR 8799 and PDS 70
• A distant brown dwarf coplanar to a warm Jupiter and a hot super-Earth
• A Validated Low-to-Intermediate Mass Planetary Interior Structure Model and New Mass-Radius Relations
• Cosmic Hydrogen and Ice Loss Lines
• Sibling Sub-Neptunes Around Sibling M Dwarfs: TOI-521 and TOI-912
• Retention of primordial atmospheres through giant impacts
• Re-accretion of Giant Impact Ejecta Can Drive Significant Atmospheric Erosion on Terrestrial Planets