Hunting for Air: Testing the Cosmic Shoreline Around M Stars with JWST

This paper, led by Jegug Ih, tackles one of the James Webb Space Telescope’s (JWST) major science questions: do rocky planets around M stars have atmospheres? M stars, the most common type in our galaxy, are prime targets for this search because their small size makes it easier to detect planetary signals. However, their intense high-energy radiation could strip atmospheres away, leaving many planets as bare rocks. The study approaches this problem at the population level, rather than focusing on single planets, using a statistical and computational framework to figure out the best way to design observations that can answer this question.

The Cosmic Shoreline Concept

The authors start with the concept of the Cosmic Shoreline, an empirical trend in our Solar System that separates airless worlds from those with atmospheres based on escape speed and the energy they receive from their star. For M stars, the rules might differ due to their strong X-ray and ultraviolet output and long active lifetimes. The team reframes target selection for JWST as a “knapsack problem” from computer science, where the “items” are planets, each with an observation time “cost” and a potential “value” for answering the research question. Because the problem is non-linear and has no simple shortcut, they employ a genetic algorithm to search for the optimal set of planets to observe.

Hypotheses and Modeling Approach

To test strategies, the team defines four hypotheses: (1) Pessimist – no M-star rocky planets have atmospheres; (2) Random – atmospheres occur at random; (3) Cosmic Shoreline – atmospheric presence depends on a planet’s location in escape speed–instellation space; and (4) XUV Cosmic Shoreline – same as (3) but based on cumulative high-energy radiation exposure. Using simulations, they “inject” one hypothesis into a virtual planet population and try to “recover” it from synthetic observations. They also model atmospheric spectra with a radiative–convective code, simulate JWST secondary eclipse measurements, and estimate the likelihood that each planet has an atmosphere based on a benchmark CO₂-rich model.

Experiment #1 – Measuring Occurrence Rates

In their first experiment, the authors explore how well JWST could constrain the occurrence rate of atmospheres if, in reality, none exist. They find that even without optimized target selection, a “wide and shallow” survey could show that fewer than 1 in 8 M-star rocky planets have atmospheres within about 500 hours of observing time. However, pushing the upper limit lower than that would require dramatically more telescope time. The results are mostly robust to moderate surface brightness (albedo) assumptions but would be weakened if all planets had very bright, reflective surfaces.

Experiment #2 – Detecting the Cosmic Shoreline

The second experiment asks how to best detect a Cosmic Shoreline trend. Here, optimized target selection becomes crucial. If such a shoreline exists, the most effective strategy is still “wide and shallow”: quickly confirming many planets on the “dry” side as airless, while dedicating more time to a smaller number of “wet” side candidates that might host atmospheres. This approach, with ~500 hours, could provide strong statistical evidence for or against the Cosmic Shoreline hypothesis. They also test the XUV-based shoreline and find it could be distinguished from the bolometric version with similar efficiency.

Broader Implications and Future Work

The paper emphasizes that their framework is not meant to produce a final “best target list” for JWST’s ongoing Rocky Worlds program, but rather a reusable, statistically grounded method that can be adapted for future surveys, including next-generation telescopes aiming to detect biosignatures. Their work shows that population-level thinking, careful statistical design, and computational optimization can make the most of precious observation time, bringing us closer to answering whether small worlds around M stars are likely to be barren or enveloped in air.

Source: Ih