The Outer Edge of the Venus Zone Around Main-Sequence Stars

TL;DR

This study defines the Venus zone around different star types using climate models, identifying the outer boundary where Venus-like runaway greenhouse conditions occur, and highlighting potential overlaps with habitable zones.

Contribution

It introduces a quantitative method to determine the outer edge of the Venus zone across various stellar types, enhancing understanding of planetary habitability boundaries.

Findings

Outer edge of Venus zone varies with star type

Significant overlap between Venus and habitable zones

Provides incident flux thresholds for Venus-like conditions

Abstract

A key item of interest for planetary scientists and astronomers is the habitable zone, or the distance from a host star where a terrestrial planet can maintain necessary temperatures in order to retain liquid water on its surface. However, when observing a system's habitable zone, it is possible that one may instead observe a Venus-like planet. We define "Venus-like" as greenhouse-gas-dominated atmosphere occurring when incoming solar radiation exceeds infrared radiation emitted from the planet at the top of the atmosphere, resulting in a runaway greenhouse. Our definition of Venus-like includes both incipient and post-runaway greenhouse states. Both the possibility of observing a Venus-like world and the possibility that Venus could represent an end-state of evolution for habitable worlds, requires an improved understanding of the Venus-like planet; specifically, the distances where these planets can exist. Understanding this helps us define a "Venus zone", or the region in which Venus-like planets could exist, and assess the overlap with the aforementioned "Habitable Zone". In this study, we use a 1D radiative-convective climate model to determine the outer edge of the Venus zone for F0V, G2V, K5V, and M3V and M5V stellar spectral types. Our results show that the outer edge of the Venus zone resides at 3.01, 1.36, 0.68, 0.23, and 0.1 AU, respectively. These correspond to incident stellar fluxes of 0.8, 0.55, 0.38, 0.32, and 0.3 S, respectively, where stellar flux is relative to Earth (1.0). These results indicate that there may be considerable overlap between the habitable zone and the Venus zone.