Climate Regimes Across the Habitable Zone: a Comparison of Synchronous Rocky M- and K-dwarf Planets

TL;DR

This study compares climate regimes of Earth-like synchronous planets around M- and K-dwarf stars, revealing differences in habitability factors such as water distribution, temperature patterns, and the likelihood of moist or terminator habitability.

Contribution

It provides a detailed analysis of how stellar type influences climate regimes and habitability potential on synchronous exoplanets, highlighting differences between M- and K-dwarf systems.

Findings

K-dwarf planets have lower nightside cold-trapping risk.

K-dwarf planets exhibit higher dayside precipitation and less moisture transport.

Terminator habitability is more common in M-dwarf systems.

Abstract

M- and K-dwarf stars make up 86% of the stellar population and host many promising astronomical targets for detecting habitable climates in the near future. Of the two, M dwarfs currently offer greater observational advantages and are home to many of the most exciting observational discoveries in the last decade. But K dwarfs could offer even better prospects for detecting habitability by combining the advantage of a relatively dim stellar flux with a more stable stellar environment. Here we explore the climate regimes that are possible on Earth-like synchronous planets in M- and K-dwarf systems, and how they vary across the habitable zone. We focus on surface temperature patterns, water availability, and implications for habitability. We find that the risk of nightside cold-trapping decreases with increased orbital radius and is overall lower for K-dwarf planets. With reduced atmospheric shortwave absorption, K-dwarf planets have higher dayside precipitation rates and less day-to-night moisture transport, resulting in lower nightside snow rates. These results imply a higher likelihood of detecting a planet with a moist dayside climate in a habitable "eyeball" climate regime orbiting a K-dwarf star. We also show that "terminator habitability" can occur for both M- and K-dwarf land planets, but would likely be more prevalent in M-dwarf systems. Planets in a terminator habitability regime tend to have slightly lower fractional habitability, but offer alternative advantages including instellation rates more comparable to Earth in regions that have temperatures amenable to life.