The Nominal Range of Rocky Planet Masses, Radii, Surface Gravities and Bulk Densities

Abstract

The two primary observable quantities of an exoplanet--its mass and radius--alone are not sufficient to probe a rocky exoplanet's interior composition and mineralogy. To overcome this, host-star abundances of the primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for the planet's bulk composition. The majority of small exoplanet hosts, however, do not have available abundance data. Here we present the open-source ExoPlex mass-radius-composition solver. Unlike previous open-source mass-radius solvers, ExoPlex calculates the core chemistry and equilibrium mantle mineralogy for a bulk composition, including effects of mantle FeO content, core light elements and surface water/ice. We utilize ExoPlex to calculate the planetary radii, surface gravities and bulk densities for 10${}^6$ model planets up to 2 R${}_{\oplus }$ across these geochemistries, adopting the distribution of FGK stellar abundances to estimate of the range of bulk exoplanet compositions. We outline the $99.7\%$ distribution of radii, surface gravity and bulk densities that define planets as "nominally rocky." Planets outside this range require compositions outside those expected from stellar abundance data, likely making them either Fe-enriched super-Mercuries, or volatile-enriched mini-Neptunes. We apply our classification scheme to a sample of 85 well-resolved exoplanets without available host-star abundances. We estimate only 9 planets are within the "nominally rocky planet zone" at $>70\%$ confidence, while $\sim 20\%$ and $\sim 30\%$ of this sample can be reasonably classified as super-Mercuries or volatile-rich, respectively. Our results provide observers with a self-consistent way to broadly classify a planet as likely rocky, Mercury-like or volatile-enriched, using mass and radius measurements alone.

41 pages, 21 figures, 2 tables. Accepted to ApJ

References in corpus (14)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• Mass-Radius Relationships for Solid Exoplanets
• Growth Model Interpretation of Planet Size Distribution
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• Detailed Models of super-Earths: How well can we infer bulk properties?
• Stellar Abundances in the Solar Neighborhood: The Hypatia Catalog
• Radius and Structure models for the First Super-Earth Planet
• A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system
• Coreless Terrestrial Exoplanets
• A new class of Super-Earths formed from high-temperature condensates: HD219134 b, 55 Cnc e, WASP-47 e
• The persistence of oceans on Earth-like planets: insights from the deep-water cycle
• A Catalog of Stellar Unified Properties (CATSUP) for 951 FGK-Stars Within 30 pc
• MAGRATHEA: an open-source spherical symmetric planet interior structure code
• A Concise Treatise on Converting Stellar Mass Fractions to Abundances to Molar Ratios

Cited by in corpus (16)

• Majority of water hides deep in the interiors of exoplanets
• Setting the stage for the search for life with the Habitable Worlds Observatory: Properties of 164 promising planet survey targets
• Super-Earths and Earth-like Exoplanets
• Unveiling the internal structure and formation history of the three planets transiting HIP 29442 (TOI-469) with CHEOPS
• A mineralogical reason why all exoplanets cannot be equally oxidising
• Volatile atmospheres of lava worlds
• Redox evolution of the crystallizing terrestrial magma ocean and its influence on atmosphere outgassing
• Predicting the Yield of Small Transiting Exoplanets around Mid-M and Ultra-Cool Dwarfs in the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey
• Linking the primordial composition of planet building disks to the present-day composition of rocky exoplanets
• A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop
• 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
• Can metal-rich worlds form by giant impacts?
• Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs
• Origins of Mercury's Big Heart of Iron: Exploring Pathways to Form High Core Mass Fraction (CMF) Planets via N-body Simulations
• A Validated Low-to-Intermediate Mass Planetary Interior Structure Model and New Mass-Radius Relations
• An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley