Impossible atmospheres? Not for lava worlds

Recent observations made with the James Webb Space Telescope (JWST) have called into question the model called cosmic shoreline, or "cosmic coast," which indicates how far from their star rocky planets can be without losing their atmosphere. As a coastline on Earth indicates, for example, the shore of a sea or lake, the cosmic shoreline represents the boundary between the region around a star where terrestrial planets are expected to have an atmosphere and the region where they should be devoid of one. However, the new data have revealed the existence of lava exoplanets, worlds with thick atmospheres of volatile gases, located in the zone where stellar radiation should lead to a severe loss of atmospheric gases. The discovery was reported last month by a group of researchers from Stanford in an article in The Astrophysical Journal Letters. The authors of the study developed a convincing explanation for the anomaly by extending the existing model. "The lava planets have shown us that something was not working in the cosmic shoreline model, but we managed to explain the maintenance of the atmosphere by suggesting to consider a new parameter," says Barron Nguyen, the first author of the study. Based on the behavior of planetary atmospheres and the interiors of planets, the team concluded that the planets in question are able to maintain thick layers of gas in their orbit for billions of years precisely because of the molten rock present on their surfaces. The lava slows down the release of gas inside the planet, a process called outgassing, and in doing so, it balances the amount present in the atmosphere by replacing what is lost due to stellar radiation. The name the researchers chose for this new zone, where the lava planets are located, is cosmic sandbar, analogous to the sand accumulations produced by the action of waves but situated at a certain distance from the shores of the terrestrial oceans. The planets located between the shoreline and the sandbar are close enough to the star to have their atmospheres swept away, but after their formation they cool down too quickly to be able to replenish their orbit with gas by balancing its loss. This third region identified is called airless valley, literally "valley without air." "The position of the cosmic shoreline has long been a big unanswered question in the field of planetary science," explains Laura Schaefer, co-author of the article and professor at the Doerr School of Sustainability. "The new model expands our awareness of this boundary and the factors that influence its placement for specific stars and planets." This new theory could have important implications for the search for life and habitable planets outside the Solar System. For this purpose, with regard to planets orbiting our own star, the strategy has long been to follow water, but for extrasolar worlds a new approach is necessary. "Scientists have begun to be interested in a way to discover which planets have an atmosphere and which do not because that is the first step to study their habitability," states Nguyen. The concept of cosmic shoreline emerged over the last decade as a promising method to identify potentially habitable worlds, but it has recently been called into question due to the discovery of planets like 55 Cancri and a super-Earth orbiting about twenty times closer to its star than Mercury is to the Sun, around which a surprisingly thick atmosphere was detected in 2024. In addition to this, in recent times, a considerable amount of new observations has allowed for the lengthening of the list of lava planets similar to 55 Cancri and, also, those equipped with thick atmospheres. To explain the existence of these unexpected planets, Nguyen and his colleagues have built a model that simulates the exchange of gaseous material between an atmosphere and a lava surface, taking into account both gas escape into space and the cooling and solidification of the lava. They then incorporated the newly discovered worlds, along with the planets of the Solar System and other known exoplanets as a comparison. What is revealed by the results is that the atmospheres are regulated mainly by two factors: gas escape into space and the outgassing phenomenon. The cosmic sandbar, therefore, includes those hot and close-to-stars planets for which the effects of the two phenomena are in balance. These are typically large super-Earths with surfaces covered in lava. The airless valley is populated by planets that are slightly farther from their star and tend to cool down and solidify more quickly: the gas remains trapped inside them and therefore is not able to replenish the atmosphere in decline. Only at distances even greater from the star, on the cosmic shoreline, are found smaller and colder planets, like Venus and Earth, whose atmospheres are not blown away by stellar radiation. "One of the most important things our study has made us understand is that the idea of the cosmic shoreline is not a lost cause," says Nguyen. "There was pessimism about it because of these lava worlds, but now we know that there are further parameters that can allow a planet to generate and maintain an atmosphere." To learn more: - Read the article “An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley” by Barron K. Nguyen, Laura K. Schaefer, Xuan Ji, Christopher A. Theissen, Fei Dai, Bo Peng, Yao Tang, Andrea Zorzi, Michelle Hill and Megan Weiner Mansfield on The Astrophysical Journal Letters