Astronomers using NASA’s James Webb Space Telescope have taken a major step toward understanding a puzzling type of planet called a “sub-Neptune.”
These planets don’t exist in our solar system, but they are the most common kind of exoplanet—planets that orbit stars beyond our Sun—found in our galaxy.
Sub-Neptunes are bigger than Earth but smaller than Neptune, and most of them are wrapped in mysterious layers of haze or clouds that make it hard for scientists to see what their atmospheres are made of.
Now, thanks to Webb, researchers have finally gotten a clear view of one of these strange worlds.
The planet, called TOI-421 b, orbits a star about 244 light-years away from Earth.
It’s much hotter than Earth, with temperatures around 1,340 degrees Fahrenheit, and it circles its star very closely—just 5.6% of the distance between Earth and the Sun.
Because it’s so hot, scientists suspected that this planet might not have the thick haze that blocks their view on cooler sub-Neptunes.
Their hunch paid off. Using two instruments aboard Webb, astronomers were able to study the light passing through the planet’s atmosphere as it moved in front of its star.
This technique, called transmission spectroscopy, reveals the “fingerprints” of gases in the planet’s atmosphere. For the first time with a sub-Neptune, they saw clear features in the spectrum instead of a flat, hazy line.
The team detected water vapor, and found hints of sulfur dioxide and carbon monoxide. They also discovered something they didn’t expect—a large amount of hydrogen.
This was surprising because other sub-Neptunes previously observed by Webb appeared to have atmospheres made of heavier molecules. Finding hydrogen, which is the lightest element, suggests that TOI-421 b may have formed in a different way than its cooler cousins.
Interestingly, the planet’s atmosphere has a similar chemical makeup to its host star, which is more like our Sun than the smaller red dwarf stars that most previously studied sub-Neptunes orbit.
This similarity in composition is also seen in the gas giants of our solar system, like Jupiter and Saturn. The discovery raises new questions: Is TOI-421 b a special case, or are hot sub-Neptunes around sun-like stars a whole different class of planet?
Scientists think that the extreme heat of TOI-421 b may prevent the chemical reactions that create haze in cooler sub-Neptunes. This allowed them to study its atmosphere in detail and make discoveries that weren’t possible before.
If this turns out to be true for other hot sub-Neptunes, it could offer a valuable new window into understanding how these planets form, evolve, and why they’re so common across the galaxy—even though we don’t have any like them close to home.
This research opens up exciting possibilities for future observations. Scientists hope to study more hot sub-Neptunes to see whether TOI-421 b is one-of-a-kind or part of a larger pattern.
Either way, the Webb Telescope is helping to peel back the layers of mystery surrounding the universe’s most common planets.
Source: KSR.
