A Metallicity Recipe for Rocky Planets

Abstract

Planets with sizes between those of Earth and Neptune divide into two populations: purely rocky bodies whose atmospheres contribute negligibly to their sizes, and larger gas-enveloped planets possessing voluminous and optically thick atmospheres. We show that whether a planet forms rocky or gas-enveloped depends on the solid surface density of its parent disk. Assembly times for rocky cores are sensitive to disk solid surface density. Lower surface densities spawn smaller planetary embryos; to assemble a core of given mass, smaller embryos require more mergers between bodies farther apart and therefore exponentially longer formation times. Gas accretion simulations yield a rule of thumb that a rocky core must be at least 2$M_{\oplus }$ before it can acquire a volumetrically significant atmosphere from its parent nebula. In disks of low solid surface density, cores of such mass appear only after the gas disk has dissipated, and so remain purely rocky. Higher surface density disks breed massive cores more quickly, within the gas disk lifetime, and so produce gas-enveloped planets. We test model predictions against observations, using planet radius as an observational proxy for gas-to-rock content and host star metallicity as a proxy for disk solid surface density. Theory can explain the observation that metal-rich stars host predominantly gas-enveloped planets.

Submitted to MNRAS on April 22, 2015. Accepted July 15, 2015. In press

References in corpus (14)

• Most 1.6 Earth-Radius Planets are not Rocky
• KEPLER's First Rocky Planet: Kepler-10b
• Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e
• Three regimes of extrasolar planets inferred from host star metallicities
• Exoplanet population inference and the abundance of Earth analogs from noisy, incomplete catalogs
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)
• Five Planets Orbiting 55 Cancri
• A Study of the Shortest-Period Planets Found With Kepler
• Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts
• Hot super-Earths and giant planet cores from different migration histories
• Formation of close in Super-Earths \& Mini-Neptunes: Required Disk Masses \& Their Implications
• The Formation of Ice Giants in a Packed Oligarchy: Instability and Aftermath
• A Continuum of Planet Formation Between 1 and 4 Earth Radii

Cited by in corpus (60)

• Origins of Hot Jupiters
• The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets
• Breeding Super-Earths and Birthing Super-Puffs in Transitional Disks
• In Situ Formation and Dynamical Evolution of Hot Jupiter Systems
• To Cool is to Accrete: Analytic Scalings for Nebular Accretion of Planetary Atmospheres
• An Increase in the Mass of Planetary Systems around Lower-Mass Stars
• The New Generation Planetary Population Synthesis (NGPPS). I. Bern global model of planet formation and evolution, model tests, and emerging planetary systems
• An Excess of Jupiter Analogs in Super-Earth Systems
• First grids of low-mass stellar models and isochrones with self-consistent treatment of rotation : From 0.2 to 1.5 M_\odot at 7 metallicities from PMS to TAMS
• The nature and origins of sub-Neptune size planets
• The In Situ Formation of Giant Planets at Short Orbital Periods
• Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities
• Correlations between compositions and orbits established by the giant impact era of planet formation
• A Super-Solar Metallicity For Stars With Hot Rocky Exoplanets
• SWEET-Cat updated. New homogenous spectroscopic parameters
• Exoplanets around Low-mass Stars Unveiled by K2
• Heavy metal rules. I. Exoplanet incidence and metallicity
• Primordial Radius Gap and Potentially Broad Core Mass Distributions of Super-Earths and Sub-Neptunes
• The Boundary between Gas-rich and Gas-poor Planets
• Planet formation and migration near the silicate sublimation front in protoplanetary disks
• Creating the Radius Gap without Mass Loss
• Predicting Exoplanets Mass and Radius: A Nonparametric Approach
• How planets grow by pebble accretion II: Analytical calculations on the evolution of polluted envelopes
• Chemical Diversity of Super-Earths As a Consequence of Formation
• A Balanced Budget View on Forming Giant Planets by Pebble Accretion
• Unresolved Binary Exoplanet Host Stars Fit as Single Stars: Effects on the Stellar Parameters
• Elemental Abundances of Kepler Objects of Interest in APOGEE. I. Two Distinct Orbital Period Regimes Inferred from Host Star Iron Abundances
• A detailed analysis of the Gl 486 planetary system
• Influence of Stellar Metallicity on Occurrence Rates of Planets and Planetary Systems
• TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes
• Heavy-metal Jupiters by major mergers: metallicity vs. mass for giant planets
• An upper boundary in the mass-metallicity plane of exo-Neptunes
• Sub-Neptune Formation: The View from Resonant Planets
• Forming Diverse Super-Earth Systems in Situ
• Formation of planetary systems by pebble accretion and migration: Hot super-Earth systems from breaking compact resonant chains
• TOI-216b and TOI-216c: Two warm, large exoplanets in or slightly wide of the 2:1 orbital resonance
• TESS Reveals a Short-period Sub-Neptune Sibling (HD 86226c) to a Known Long-period Giant Planet
• Three Pathways for Observed Resonant Chains
• Edge-of-the-Multis: Evidence for a Transition in the Outer Architectures of Compact Multi-Planet Systems
• Earths in Other Solar Systems N-body simulations: the Role of Orbital Damping in Reproducing the Kepler Planetary Systems
• TOI-532b: The Habitable-zone Planet Finder confirms a Large Super Neptune in the Neptune Desert orbiting a metal-rich M dwarf host
• When, where, and how many planets end up in first-order resonances?
• Chains of Planets in Mean Motion Resonances Arising from Oligarchic Growth
• Confirming the 3:2 Resonance Chain of K2-138
• Exoplanet Populations and their Dependence on Host Star Properties
• Magellan/PFS Radial Velocities of GJ 9827, a late K dwarf at 30 pc with Three Transiting Super-Earths
• Higher Compact Multiple Occurrence Around Metal-Poor M-Dwarfs and Late K-Dwarfs
• Size evolution of close-in super-Earths through giant impacts and photoevaporation
• The HARPS search for southern extra-solar planets XL. Searching for Neptunes around metal-poor stars
• Which type of planets do we expect to observe in the Habitable Zone?
• Sun-like stars unlike the Sun: Clues for chemical anomalies of cool stars
• An Earth-sized Planet around an M5 Dwarf Star at 22 pc
• Nature vs. nurture: a Bayesian framework for assessing apparent correlations between planetary orbital properties and stellar ages
• A Spectroscopic Analysis of a Sample of K2 Planet-Host Stars: Stellar Parameters, Metallicities and Planetary Radii
• Planet formation: key mechanisms and global models
• Diversity of disc viscosities can explain the period ratios of resonant and non-resonant systems of hot super-Earths and mini-Neptunes
• Effects of Outer Giant Planets on In Situ Formation of Inner Super-Earths
• Diversity of Exoplanets
• Scaling K2. V. Statistical Validation of 60 New Exoplanets From K2 Campaigns 2-18
• Longterm Stability of Planetary Systems formed from a Transitional Disk