Beyond our solar system, sub-Neptunes — gassy planets larger than Earth but smaller than Neptune — reign as the most common type of exoplanet observed in our galaxy. Despite their prevalence, no such planets exist around our Sun, so these worlds have remained shrouded in mystery. NASA’s James Webb Space Telescope (JWST) recently observed exoplanet TOI-421 b, whose unique atmospheric conditions are now enabling scientists to understand sub-Neptunes in unprecedented detail.
A unique opportunity
JWST’s sharp eye on the infrared universe is uniquely suited to studying exoplanet atmospheres. “I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” explained Eliza Kempton of the University of Maryland, College Park, principal investigator and co-author of a study published on the work, in a NASA news release. “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.”
Sub-Neptunes were unknown just 10 years ago, before NASA’s Kepler space telescope discovered them. These planets, several times larger than Earth but smaller than gas giants Jupiter and Saturn, present significant observational challenges because their relatively cool atmospheres appear heavily obscured by clouds or hazes produced by methane gas. When researchers watched such a planet pass in front of its star and looked at the starlight filtering through the atmosphere for clues to its composition, those clouds produced flat, featureless spectra that provided little information. However, scientists hypothesized that planets with temperatures above 1,070 degrees Fahrenheit (580 degrees Celsius) might have clearer atmospheres due to the absence of methane at such temperatures — and thus, no methane-triggered hazes.
But TOI-421 b is different. As a so-called hot sub-Neptune with a temperature of approximately 1,340 F (730 C), TOI-421 b presented an ideal candidate for testing this theory. The gamble paid off — the space telescope successfully detected spectral features revealing the planet’s atmospheric composition as it passed in front of its star.
In the air
A team from University of Maryland identified water vapor along with possibly carbon monoxide and sulfur dioxide, while methane and carbon dioxide were notably absent.
The most surprising discovery was that TOI-421 b’s atmosphere appears dominated by hydrogen, which contrasts with previously observed sub-Neptunes. This hydrogen-rich composition mirrors that of TOI-421 b’s Sun-like host star, suggesting that the planet formed more like our solar system’s giant planets than to other sub-Neptunes we’ve found, most of which orbit smaller, cooler red dwarf stars.
Researchers now hope to observe more hot sub-Neptunes around Sun-like stars to determine whether TOI-421 b represents a broader trend or a unique case. “We’ve unlocked a new way to look at these sub-Neptunes,” said study first author and University of Maryland Ph.D. student Brian Davenport, who provided primary analysis of the exoplanet. “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.”
The team’s findings, published in The Astrophysical Journal Letters on May 5, demonstrate JWST’s transformative impact on exoplanet research, offering new insights into the formation and evolution of these common yet enigmatic worlds.
