Sub-Neptunes are planetary bodies that bridge the size gap between rocky planets like Earth and gas giants like Neptune, but they are absent from our own solar system. "The nature of sub-Neptunes is one of the hottest topics in exoplanetary science," explained Dr. Christopher Glein of SwRI, lead author of the study published in the Astrophysical Journal. "These sub-Neptunes are the most common type of planet in the galaxy, yet we know very little about them. Temperate sub-Neptunes are particularly intriguing because they may include some habitable candidates."
Previous theories have speculated that some sub-Neptunes situated in their star's habitable zone might be "Hycean worlds" - water-covered planets with hydrogen-rich atmospheres. While this hypothesis was applied to K2-18 b, recent James Webb Space Telescope (JWST) data on TOI-270 d suggests a better fit for a model involving a hot, rocky planet enveloped in a thick atmosphere.
"The search for habitable worlds continues! The JWST data on TOI-270 d collected by Bjorn Benneke and his team are revolutionary," Glein said. "I was shocked by the level of detail they extracted from such a small exoplanet's atmosphere, which provides an incredible opportunity to learn the story of a totally alien planet. With molecules like carbon dioxide, methane and water detected, we could start doing some geochemistry to learn how this unusual world formed."
The detected gases point to atmospheric temperatures exceeding 1,000 degrees Fahrenheit, surpassing even Venus's surface temperature. SwRI's new model shows how such high temperatures drive chemical equilibria in the lower atmosphere, enabling gases to rise to altitudes observable by JWST.
While TOI-270 d is deemed unlikely to host life, Glein emphasized the scientific value of the planet. "While it is a bit disappointing to find that TOI-270 d is unlikely to be habitable, this planet still offers a fantastic opportunity to explore alternative paths of planetary origins and evolution," he noted. "We are learning much more about the crazy configurations of planets that nature comes up with."
The study also provides a novel explanation for the lack of ammonia observed in many temperate sub-Neptune atmospheres. Earlier models predicted significant ammonia production in hydrogen-rich atmospheres, but Glein's team found that high temperatures could convert ammonia into nitrogen gas or dissolve it into a molten rock surface, leading to its apparent depletion. Moreover, they argue that TOI-270 d likely formed from nitrogen-poor material, consistent with the composition of chondritic meteorites.
"I see a lot of parallels between planetary science and biology," Glein said. "A core set of building blocks and rules for interactions result in an explosion of diverse forms. We're starting to see some of that diversity come through in compositional signatures from JWST."
The research marks a significant advancement in applying solar system geochemical modeling techniques to the study of distant exoplanets. By integrating these tools, scientists can now examine alien worlds with a level of sophistication once reserved for planets in our own celestial neighborhood.
"We wanted to paint a more complete picture of the inner workings and history of an exoplanet by approaching the problem in multiple ways," Glein concluded. "Last time I checked, we have discovered over 5,800 confirmed exoplanets. TOI-270 d is just one of them. It's going to be very interesting to see what the next exoplanet has in store for us."
Research Report:"Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270-d"
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