Researchers have developed a new model that explains how lava-covered planets can keep their atmospheres. This discovery could help in the search for life beyond our solar system.
Some planets orbit very close to their stars. The intense radiation from these stars should strip away their atmospheres. However, several hot, lava-covered planets still have thick atmospheres. This challenges a key idea scientists use to predict which rocky planets can hold onto their air.
The Cosmic Sandbar
Stanford researchers now offer an explanation. Their model, published in The Astrophysical Journal Letters, suggests that molten surfaces can control how quickly gases escape from a planet's inside. By slowing this release, lava allows new atmospheric gases to replace those lost to stellar radiation. This can preserve an atmosphere for billions of years.
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Start Your News DetoxThis finding expands the idea of the "cosmic shoreline." This concept describes the boundary between rocky worlds that can keep atmospheres and those that cannot. The cosmic shoreline marks where an atmosphere can survive. However, some lava worlds are much closer to their stars than this boundary predicts, yet they still have atmospheres.
Barron Nguyen, a graduate student at Stanford, explained that these lava worlds showed something was wrong with the cosmic shoreline idea. He noted that their new model proposes a "cosmic sandbar" beyond this boundary, allowing these planets to keep their atmospheres.
The researchers call this new area the "cosmic sandbar," similar to sandy ridges in Earth's oceans. Between the traditional shoreline and this sandbar is the "airless valley." Planets in this valley orbit close enough to lose their atmospheres. They also cool and solidify too quickly after forming to replace the escaping gas.
Laura Schaefer, a professor at Stanford and senior author of the study, said the new model helps us understand this boundary better. It shows the factors that determine where this boundary lies for different stars and planets.
Scientists are interested in which planets have atmospheres because it's the first step in looking for habitable worlds.
How Lava Worlds Keep Their Air
The cosmic shoreline has been a useful tool for finding potentially habitable planets. But recent discoveries have made the picture more complex.
One example is 55 Cancri e, a "super-Earth" much larger than Earth. It orbits its star about 20 times closer than Mercury orbits our Sun. Despite this extreme closeness, observations from the James Webb Space Telescope in 2024 showed that 55 Cancri e has a very thick atmosphere. More similar lava worlds with substantial atmospheres have been found recently.
To understand how these planets keep their air, Nguyen and his team created a model. It tracks gas moving between a planet's atmosphere and its molten surface. The simulations also considered gas escaping into space and the cooling and solidifying of the lava. They then compared these atmosphere-retaining lava worlds with other known exoplanets and bodies in our solar system.
The model showed that atmospheric changes are mainly controlled by two competing processes: gas escaping into space and gas released from inside the planet into the atmosphere, called "outgassing." The cosmic sandbar describes hot worlds near their stars where these two processes can balance each other.
Planets in this "cosmic sandbar" area, like 55 Cancri e, are often massive super-Earths with lava-covered surfaces. Worlds a bit farther from their stars, like Mercury, cool and solidify faster. This traps gases inside them. Without enough outgassing to replace lost atmospheric gases, they fall into the "airless valley."
Even farther out, along the traditional cosmic shoreline, smaller and cooler planets like Venus and Earth receive less stellar radiation. This allows them to avoid losing their atmospheres entirely.
The Stanford researchers believe future exoplanet surveys will test and improve this expanded cosmic shoreline framework. Nguyen noted that the cosmic shoreline isn't a "lost cause" and that there's a broader range of conditions that allow a planet to create and keep an atmosphere.
Deep Dive & References
An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley - The Astrophysical Journal Letters, 2026
