"The nature of sub-Neptunes is one of the hottest topics in exoplanetary science," said Dr. Christopher Glein of SwRI, lead author of the study accepted by the Astrophysical Journal. "These are the most common planets in the galaxy, yet we have none close to home. Their potential habitability, especially for those in the temperate zone, makes them fascinating."
Sub-Neptunes, such as TOI-270 d, are larger than Earth but smaller than Neptune. Some orbit in zones that could allow liquid water to exist, raising hopes they might support life. These worlds are sometimes theorized to be "Hycean" planets-water-rich bodies with hydrogen-dominated atmospheres. However, new James Webb Space Telescope (JWST) data challenges that model for TOI-270 d, instead supporting the idea of a large rocky planet with a superheated atmosphere.
"The JWST data on TOI-270 d collected by Bjorn Benneke and his team are revolutionary," Glein explained. "I was shocked by the level of detail they extracted from such a small exoplanet's atmosphere... With molecules like carbon dioxide, methane and water detected, we could start doing some geochemistry to learn how this unusual world formed."
The telescope observed gas signatures indicating atmospheric temperatures exceeding 1,000 degrees Fahrenheit-hotter than Venus. Glein's team developed a geochemical model that illustrates how gases undergo chemical equilibrium under such conditions before ascending to altitudes where instruments like JWST can detect them.
While these conditions rule out the planet's habitability, they reveal valuable information about alternative planetary formation pathways. One mystery the study addresses is the apparent absence of ammonia in the atmosphere-a substance that should be prevalent in hydrogen-rich, high-temperature environments.
The researchers propose that ammonia is chemically transformed into nitrogen gas at high temperatures or dissolves into a possible ocean of molten rock at the surface. They also suggest the planet's building materials were inherently poor in nitrogen, echoing the composition of nitrogen-deficient 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."
This research marks a step change in exoplanet studies, bringing the analysis of alien worlds closer to the geochemical precision long applied to planets within our solar system. The geochemical modeling of TOI-270 d paves the way for deeper insights into atmospheric formation, internal processes, and the thermochemical behaviors of sub-Neptunes.
"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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