Puffy Venuses: the Mass-Radius Impact of Carbon-Rich Atmospheres on Lava Worlds

TL;DR

This paper models carbon-rich atmospheres on lava worlds, showing they can significantly inflate planetary radii and offering new interpretations for underdense exoplanets like TOI-561 b and 55 Cancri e.

Contribution

It introduces a theoretical model for puffy Venus-like exoplanets with thick, carbon-dominated atmospheres, exploring their impact on planet radius and structure.

Findings

Carbon-rich atmospheres can inflate planet radii by 0.16-0.3 R_earth.

A carbon content of 1200 ppm can produce significant atmospheric inflation.

TOI-561 b and 55 Cancri e are potential puffy Venus candidates.

Abstract

The recent advancements in exoplanet observations enable the potential detection of exo-Venuses, rocky planets with carbon-rich atmospheres. How extended these atmospheres can be, given high carbon abundances, has not been studied. To answer this, we present a model for a theoretical class of exoplanets - puffy Venuses - characterized by thick, carbon-dominated atmospheres in equilibrium with global magma oceans. Our model accounts for carbon and hydrogen partition between the atmosphere and the magma ocean, as well as the C-H-O equilibrium chemistry throughout a semi-grey, radiative-convective atmosphere. We find that radius inflation by puffy Venus atmospheres is significant on small and irradiated planets: carbon content of 1200 ppm (or that of ordinary chondrites) can generate an atmosphere of ~0.16 - 0.3 $R_{\oplus}$ for an Earth-mass planet with equilibrium temperatures of 1500 to 2000 K. We identify TOI-561 b as an especially promising puffy Venus candidate, whose under-density could be attributed to a thick C-rich atmosphere. We also advocate for a puffy Venus interpretation of 55 Cancri e, where recent JWST observation indicates the presence of a CO/CO2 atmosphere. Puffy Venuses may thus constitute a testable alternative interpretation for the interior structure of underdense low-mass exoplanets.