A bold geochemical model suggests that the sub-Neptunian TOI-270 d, 73 light-years away, is likely to be a massive rocky world hidden in a hot, dense atmosphere rather than a water-filled ocean planet.
Observations by the Webb Space Telescope have discovered carbon dioxide, methane and water vapor at temperatures exceeding 1,000 degrees Fahrenheit (about 474 degrees Celsius) and proposed a new chemical framework to explain the evolution of these gases. This discovery revolutionizes our understanding of habitable zone exoplanets and marks a major leap forward in using solar system chemistry tools to explore alien worlds.
A new study led by the Southwest Research Institute (SwRI) simulated the atmosphere of TOI-270 d, a planet just 73 light-years away from Earth. Researchers have found evidence that the exoplanet, which is larger than Earth but smaller than Neptune, may be a rocky world wrapped in a thick and extremely hot atmosphere. Because of its unique properties, TOI-270 d could serve as a "Rosetta Stone" for understanding a widespread but poorly understood class of planets.
Exoplanets are planets orbiting stars outside our solar system. Among them, "sub-Neptune" refers to a planet between the size of Earth (the largest rocky planet in the solar system) and Neptune (the smallest gas giant planet).
"The nature of sub-Neptunes is one of the hottest topics in exoplanet science," said Dr. Christopher Glein of Southwest Research Institute, lead author of a paper to be published in The Astrophysical Journal. "These sub-Neptunes are the most abundant planets in the Milky Way, but there are no such planets in our solar system. They are very strange. The gentle sub-Neptunes are more interesting because some of them may be suitable for life."
A study led by Southwest Research Institute (SwRI) modeled the chemical composition of TOI-270 d. TOI-270 d, an exoplanet between the size of Earth and Neptune, has found evidence that it is a large rocky planet (super-Earth) surrounded by a thick, hot atmosphere. NASA's James Webb Space Telescope (JWST) has detected gases emanating from regions of the atmosphere that are over 1,000 degrees Fahrenheit (about 477 degrees Celsius), even hotter than the surface of Venus. The model shows that a potential magma ocean is absorbing ammonia from the atmosphere. The hot gases then go through an equilibrium process and are fed into the planet's photosphere, where they can be detected by the James Webb Space Telescope (JWST). Image source: Southwest Research Institute
Some scientists believe that sub-Neptunian planets in the habitable zone (where liquid water may exist) may be ocean-covered planets with thin atmospheres rich in hydrogen. These so-called "Hycean" planets, like the previously studied K2-18 b, are considered potential candidates for habitable planets.
However, recent observations of TOI-270 d using the James Webb Space Telescope (JWST) revealed a different story. The data suggest that a simpler model is more consistent with the planet's makeup: a large, rocky planet with a hot, dense atmosphere — more like a super-Earth than a Hercynian planet.
"The search for habitable planets continues! The JWST TOI-270 d data collected by Björn Benneke and his team is revolutionary," said Glein. "I was blown away by the richness of detail they were able to extract from the atmosphere of such a small exoplanet, and it provides an excellent opportunity to understand the story of a completely unfamiliar planet. By detecting molecules such as carbon dioxide, methane and water, we can start to do some geochemical studies to understand how this unusual world formed."
NASA's James Webb Space Telescope detected gas temperatures exceeding 1,000 degrees Fahrenheit (about 477 degrees Celsius), which is hotter than the surface of Venus. New geochemical models explain how the gas undergoes an equilibrium process at these temperatures before being lifted to a position where it can be detected by the James Webb Space Telescope.
"While it is somewhat disappointing to find that TOI-270 d is unlikely to be habitable, the planet still provides an excellent opportunity to explore alternative paths to planetary origin and evolution," said Grein. "We are gaining a deeper understanding of the strange structures that nature has created on the planet."
Scientists have been puzzled by the reason for the lack of ammonia in the atmospheres of temperate sub-Neptunes. Previous models suggested that ammonia should be produced in a thick, hot atmosphere rich in hydrogen. The new study provides a comprehensive view of how ammonia is consumed through a combination of planetary processes, including the high-temperature production of nitrogen and its dissolution in superheated molten oceans on the planet's surface. Grein's team also found that the planet itself is likely deficient in nitrogen, since solid building blocks of planets, such as chondrites, are also typically deficient in nitrogen.
“I see a lot of similarities between planetary science and biology,” Grein said. "A core set of building blocks and rules of interaction lead to bursts of various forms. We're starting to see some of this diversity in the compositional signatures of the James Webb Space Telescope."
This study shows that exoplanet geochemistry is now approaching the level of complexity found in our own solar system. Geochemical tools developed for the solar system can now be applied to exoplanet atmospheres. These studies provide new insights into atmospheric temperatures, possible magma oceans, and the origin of planetary atmospheres.
"We hope to use multiple approaches to more fully characterize the inner workings and history of exoplanets," Gline said. "Last time I checked, we have discovered over 5,800 confirmed exoplanets. TOI-270 d is just one of them. It will be very interesting to see what the next exoplanet brings to us."
Compiled from /ScitechDaily