UMD-Led Study Lifts Veil on Common but Mysterious Type of Exoplanet
Webb Telescope Provides Rare Glimpse of Distant Planet Unobscured by Haze and Clouds
This artist’s concept shows what the hot sub-Neptune exoplanet TOI-421b could look like.
Image courtesy of NASA, ESA, CSA, Dani Player (STScI)
Though they don’t orbit our sun, sub-Neptunes are the most common type of planet throughout the galaxy. Larger than Earth but smaller than Neptune, these gassy exoplanets are shrouded in mystery—and often, a lot of atmospheric haze—but a new University of Maryland-led study provides a new level of clarity.
The researchers theorized that TOI-421b, which orbits a star nearly 245 light years from Earth, would essentially have clear skies because of its relatively toasty temperatures—and they were right. Using NASA’s James Webb Space Telescope (JWST), they were able to get an unimpeded view, enabling them to detect water vapor and possibly carbon monoxide and sulfur dioxide in its atmosphere. They published the results Monday in Astrophysical Journal Letters.
“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said Eliza Kempton, a UMD astronomy professor and principal investigator of the new study. “By studying their atmospheres, we’re getting a better understanding of how sub-Neptunes formed and evolved, and part of that is understanding why they don't exist in our solar system.”
Before JWST, most sub-Neptune atmospheres appeared to have flat or featureless transmission spectra, meaning that scientists couldn’t discern specific chemical fingerprints revealing the atmosphere’s composition. This led astronomers to conclude that certain sub-Neptunes were likely highly obscured by clouds or hazes.
In their latest study of TOI-421b, researchers used Webb to see features that were previously difficult to capture.
A transmission spectrum captured by NASA’s James Webb Space Telescope reveals the presence of water and the possible presence of sulfur dioxide and carbon monoxide, but no signs of carbon dioxide or methane, in the atmosphere of the hot sub-Neptune exoplanet TOI-421b. (Image courtesy of NASA, ESA, CSA, Joseph Olmsted [STScI])
“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” said the study’s first author, UMD astronomy Ph.D. student Brian Davenport. “Whereas with many of the other sub-Neptunes that had been previously observed, we know their atmospheres are made of something, but they're being blocked by haze.”
In addition to Kempton and Davenport, co-authors include astronomy Research Professor Drake Deming, former postdoc Matej Malik, postdoc Matt Nixon, Guangwei Fu M.S. ’19, Ph.D. ’22 and Jegug Ih Ph.D. ’24.
The team found water vapor in the exoplanet’s atmosphere, as well as tentative signatures of carbon monoxide and sulfur dioxide. There are also molecules they didn’t detect, such as methane and carbon dioxide. From the data, they inferred that a large amount of hydrogen is present—a lightweight atmosphere that caught the researchers by surprise.
“We had recently wrapped our minds around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmospheres, so that had become our expectation,” Kempton said. “And then we found the opposite.”
The findings suggest that TOI-421b may have formed and evolved differently from the cooler sub-Neptunes observed previously.
TOI-421b’s hydrogen-dominated atmosphere is also interesting because it mimics the composition of its host star, according to Kempton.
“If you just took the same gas that made the host star, plopped it on top of a planet's atmosphere and put it at the much cooler temperature of this planet, you would get the same combination of gases,” Kempton said. “That process is more in line with the giant planets in our solar system, and it is different from other sub-Neptunes that have been observed with Webb so far.”
In addition to being hotter than other sub-Neptunes previously observed with Webb, TOI-421b is also unusual because it orbits a sun-like star. Most others observed so far orbit smaller, cooler stars called red dwarfs.
“We've unlocked a new way to look at these sub-Neptunes,” Davenport said. “These high-temperature planets are amenable to characterization. So by looking at sub-Neptunes of this temperature, we're perhaps more likely to accelerate our ability to learn about these planets.”
This article was adapted from a news release by the Space Telescope Science Institute.
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