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New Constraints on the M Dwarf Cosmic Shoreline from a Galaxy Far, Far Away

Michael Radica

Abstract

Whether there is a cosmic shoreline that divides terrestrial planets which have atmospheres from those that don't is one of the biggest open questions in exoplanet science. Most atmosphere searches have focused on terrestrial planets around M dwarf stars, since their smaller radii compared to sun-like stars boost planet atmosphere signals. However, the higher activity levels of M dwarfs might also entirely preclude atmosphere retention for their planets. In this work we present a new hope for defining an M dwarf cosmic shoreline, leveraging not only data from exoplanets in our own galaxy, but a comprehensive survey conducted by a commission of the Galactic Republic a long, long time ago in a galaxy far, far away. In this survey, we find definitive proof that M dwarf planets can retain atmospheres, and define an M dwarf cosmic shoreline whose slope agrees well with empirical predictions for Sun-like stars. We then define atmosphere retention metrics for the planets on the JWST Rocky Worlds DDT Targets Under Consideration list. Our analysis highlights the benefits of looking beyond the Milky Way for answers to some of the field's most pressing questions.

New Constraints on the M Dwarf Cosmic Shoreline from a Galaxy Far, Far Away

Abstract

Whether there is a cosmic shoreline that divides terrestrial planets which have atmospheres from those that don't is one of the biggest open questions in exoplanet science. Most atmosphere searches have focused on terrestrial planets around M dwarf stars, since their smaller radii compared to sun-like stars boost planet atmosphere signals. However, the higher activity levels of M dwarfs might also entirely preclude atmosphere retention for their planets. In this work we present a new hope for defining an M dwarf cosmic shoreline, leveraging not only data from exoplanets in our own galaxy, but a comprehensive survey conducted by a commission of the Galactic Republic a long, long time ago in a galaxy far, far away. In this survey, we find definitive proof that M dwarf planets can retain atmospheres, and define an M dwarf cosmic shoreline whose slope agrees well with empirical predictions for Sun-like stars. We then define atmosphere retention metrics for the planets on the JWST Rocky Worlds DDT Targets Under Consideration list. Our analysis highlights the benefits of looking beyond the Milky Way for answers to some of the field's most pressing questions.

Paper Structure

This paper contains 4 sections, 2 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Data Sourcing
  3. Shoreline Analysis
  4. Discussion

Figures (2)

  • Figure 1: The Galactic Senate, circa 19 BBY, colourized.
  • Figure 2: The M dwarf cosmic shoreline in XUV irradiation and escape velocity space. Small grey points show the broader exoplanet population, whereas darker points are potentially terrestrial planets around M dwarfs. Larger orange circles denote the Rocky Worlds DDT targets under consideration (TUC) list. Red points are bona fide rocks, and blue points have atmospheres. Several key planets are labelled, both from our galaxy and another far, far away. We show three potential empirical shorelines: one anchored by Mars and Kamino (red dashed), another anchored by LTT 1445 A b and Kamino (black), and the shoreline for Sun-like stars derived by meni-gallardo_empirical_2025 (red dotted).