Scorching world offers clues to planetary evolution

A recent study has offered an in-depth look at an exoplanet called TOI-270 d, located just 73 light-years away from Earth.

Although it’s between Earth and Neptune in size, models suggest this alien world is a hot rocky planet with a thick atmosphere. It may not be the watery, hydrogen-rich “Hycean” planet some scientists originally envisioned.

This study, led by the Southwest Research Institute (SwRI), may help researchers unravel the mystery of sub-Neptunes.

“The nature of sub-Neptunes is one of the hottest topics in exoplanetary science,” said lead author Christopher Glein, an astronomer at SwRI.

“These sub-Neptunes are the most abundant size range of planets in the galaxy, yet none exist in our solar system. They are exotic. Temperate sub-Neptunes are of even higher interest because some could be habitable.”

The mystery of sub-Neptune planets

Ever since scientists began detecting exoplanets en masse, planets between Earth and Neptune in size have stood out as particularly puzzling. Since these sub-Neptune planets are common in surveys but absent in our solar system, researchers created new theories about their formation and composition.

One popular proposal is that some sub-Neptunes orbiting in a star’s habitable zone might be massive ocean worlds. These planets could have hydrogen-rich atmospheres, leading to the term “Hycean” (short for hydrogen-ocean) worlds.

However, observations of TOI-270 d with the James Webb Space Telescope (JWST) suggest a simpler composition. Instead of being swathed in a relatively thin hydrogen atmosphere over a deep ocean, this planet appears to have a superheated rocky interior enclosed within a thick, hot atmosphere.

“The search for habitable worlds continues! The JWST data on TOI-270 d collected by Björn 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.”

New paths of planetary evolution

Based on JWST’s measurements, the atmospheric temperatures on TOI-270 d exceed 1,000 degrees Fahrenheit – much hotter than anything Earth-like. This finding helps explain why the planet likely lacks the expansive oceans once suggested by the Hycean hypothesis.

Instead, the new geochemical model shows how gases could undergo high-temperature reactions at the molten surface, then rise into cooler atmospheric layers for JWST to detect.

“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,” Glein said. “We are learning much more about the crazy configurations of planets that nature comes up with.”

The curious case of missing ammonia

One puzzle within the sub-Neptune class has been the absence of ammonia in many atmospheres, despite theoretical models suggesting that thick, hydrogen-rich environments should produce it.

In their integrated perspective on TOI-270 d, Glein’s team illustrates how multiple processes could combine to deplete ammonia. These include nitrogen gas produced at high temperatures and ammonia dissolving in a molten rock ocean. A nitrogen shortage in the planet’s building blocks also contributes.

“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.”

Molecules on scorching planets

The study exemplifies how methods traditionally used to explore our own solar system are now being applied to exoplanets. Researchers can detect molecules such as carbon dioxide, methane, and water vapor and piece together how those elements and compounds interact on scorching exoplanets.

With technological advances like JWST, exoplanetary geochemistry is rapidly approaching the sophistication once reserved for spacecraft missions to local worlds like Venus or Mars.

The team says these studies reveal magma oceans, atmospheric temperatures, and how planetary atmospheres form and evolve. By examining TOI-270 d, scientists learn more about the possibilities for sub-Neptune formation and the diversity of planetary outcomes.

“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 explained. “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.”

Clues to abundant exoplanets

TOI-270 d’s status as a “Rosetta Stone” could help decode a class of exoplanets that dominates our galaxy but defies easy categorization. Just 73 light years away, this rocky world shows how alien planetary systems can differ greatly from our own Sun’s.

Through precise observations and advanced modeling, scientists are learning to identify exoplanetary signatures of chemical and geologic processes. These processes may appear alien compared to those found on Earth.

Each new exoplanet studied brings scientists closer to understanding where habitable environments might emerge in the cosmos.

The study has been accepted for publication in The Astrophysical Journal.

Image Credit: NASA

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