A Distinct Population of Small Planets: Sub-Earths

Abstract

The sizes of small planets have been known to be bi-modal, with a gap separating planets that have lost their primordial atmospheres (super-Earths), and the ones that retain them (mini-Neptunes). Here, we report evidences for another distinct population at smaller sizes. By focussing on planets orbiting around GK-dwarfs inward of 16 days, and correcting for observational completeness, we find that the number of super-Earths peak around 1.4 Earth radii and disappear shortly below this size. Instead, a new population of planets (sub-Earths) appear to dominate at sizes below ~ 1 Earth radius, with an occurrence that rises with decreasing size. This pattern is also observed in ultra-short-period planets. The end of super-Earths supports earlier claims that super-Earths and mini-Neptunes, planets that likely form in gaseous proto-planetary disks, have a narrow mass distribution. The sub-Earths, in contrast, can be described by a power-law mass distribution and may be explained by the theory of terrestrial planet formation. We therefore speculate that they are formed well after the gaseous disks have dissipated. The extension of these sub-Earths towards longer orbital periods, currently invisible, may be the true terrestrial analogues. This strongly motivates new searches.

arXiv commentAccepted to the Astronomical Journal

References in corpus (46)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• emcee: The MCMC Hammer
• A Unified Approach to the Classical Statistical Analysis of Small Signals
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Kepler Mission Design, Realized Photometric Performance, and Early Science
• Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler
• Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler planets: a tale of evaporation
• The evaporation valley in the Kepler planets
• 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
• Core-powered mass loss and the radius distribution of small exoplanets
• False positive probabilties for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives
• Planetary Candidates Observed by Kepler. VIII. A Fully Automated Catalog With Measured Completeness and Reliability Based on Data Release 25
• How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond
• Terrestrial Planet Occurrence Rates for the Kepler GK Dwarf Sample
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• The Derivation, Properties and Value of Kepler's Combined Differential Photometric Precision
• Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth radii in the Habitable Zone
• A Study of the Shortest-Period Planets Found With Kepler
• The Exoplanet Population Observation Simulator. I - The Inner Edges of Planetary Systems
• Density and Eccentricity of Kepler Planets
• The Exoplanet Census: A General Method, Applied to Kepler
• Occurrence Rates of Planets orbiting FGK Stars: Combining Kepler DR25, Gaia DR2 and Bayesian Inference
• The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age
• A Statistical Reconstruction of the Planet Population Around Kepler Solar-Type Stars
• Evolution of the radius valley around low mass stars from $Kepler$ and $K2$
• A Catalog of Kepler Habitable Zone Exoplanet Candidates
• The Roche limit for close-orbiting planets: Minimum density, composition constraints, and application to the 4.2-hour planet KOI 1843.03
• Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short and Ultra-Short Period Planets
• Searching the Entirety of Kepler Data. II. Occurrence Rate Estimates for FGK Stars
• Mass and Mass Scalings of Super-Earths
• Kepler-78 and the Ultra-Short-Period Planets
• A Probabilistic Approach to Kepler Completeness and Reliability for Exoplanet Occurrence Rates
• Ultra-short-period planets from secular chaos
• Detection of Potential Transit Signals in 17 Quarters of Kepler Data: Results of the Final Kepler Mission Transiting Planet Search (DR25)
• Low-Eccentricity Migration of Ultra-Short Period Planets in Multi-Planet Systems
• Formation and accretion history of terrestrial planets from runaway growth through to late time: implications for orbital eccentricity
• Formation of Ultra-Short-Period Planets by Obliquity-Driven Tidal Runaway
• A Joint Mass-Radius-Period Distribution of Exoplanets
• Measuring Transit Signal Recovery in the Kepler Pipeline. III. Completeness of the Q1-Q17 DR24 Planet Candidate Catalogue, with Important Caveats for Occurrence Rate Calculations
• Measuring Transit Signal Recovery in the Kepler Pipeline IV: Completeness of the DR25 Planet Candidate catalog
• Kepler-62f: Kepler's First Small Planet in the Habitable Zone, but Is It Real?
• Ultra Short Period Planets in K2 III: Neighbors are Common with 13 New Multi-Planet Systems and 10 Newly Validated Planets in Campaigns 0-8, 10

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• Scaling K2. VI. Reduced Small Planet Occurrence in High Galactic Amplitude Stars
• Super-Earths and sub-Neptunes are Insensitive to Stellar Metallicity
• Formation of super-Earths and mini-Neptunes from rings of planetesimals
• The high-energy environment of the heavy sub-Earth GJ 367 b indicates likely complete evaporation of its atmosphere
• The New Generation Planetary Population Synthesis (NGPPS) VIII. Impact of host star metallicity on planet occurrence rates, orbital periods, eccentricities, and radius valley morphology
• Diversity of Exoplanets