- September 24, 2026
- By Georgia Jiang
Using the James Webb Space Telescope (JWST), a team of astronomers led by the University of Maryland identified water, methane and ammonia in the atmosphere of HATS-6 b, a giant planet 500 light-years away from Earth. The observations also revealed that HATS-6 b may be significantly cooler than expected—suggesting that the planet’s relationship with its star may be far more complicated than previously thought.
The multi-institutional team’s findings published this month in the Astronomical Journal raise new questions about how giant planets form and evolve.
HATS-6 b is roughly the size of Jupiter, completing an orbit every three days around an M dwarf—a small, cool, reddish star. Planets are built from the leftover disk of gas and dust that surrounds such stars after their formation, and a smaller star leaves behind only small disks.
“These smaller stars don’t have enough material or enough time to create planets as big as Jupiter and as big as Saturn,” explained the study’s lead author, Giannina Guzmán Caloca, a UMD astronomy Ph.D. candidate. “The fact that HATS-6 b can exist is really interesting because it shouldn’t be possible with what we know.”
Using a technique called transmission spectroscopy, which involves watching starlight filter through a planet’s atmosphere, the team identified four molecules in HATS-6 b’s atmosphere: water, methane, ammonia and carbon dioxide. It’s only the second time the technique has detected ammonia on a distant world. Because nitrogen-bearing molecules like ammonia should be more abundant in cooler giant planets than in scorching-hot Jupiter-like ones, the discovery supports the theory that planets that orbit M-dwarf stars may be a chemically distinct population.
HATS-6 b’s unexpected temperature raises another major question. Its commonly cited temperature—nearly 800 degrees Fahrenheit—is not an exact measurement but a calculation that assumes the planet absorbs all the light its star delivers and spreads that heat evenly. But scientists’ early analyses returned temperatures closer to 250 degrees, a figure that’s physically improbable for a planet orbiting its star every three days.
“If the planet is genuinely that cool, it means that something is probably reflecting a great deal of starlight back into space before it can warm anything,” Guzmán Caloca explained. “The likeliest explanation is cloud and haze wrapping the planet the way they wrap Venus.”
The team’s results have implications beyond just HATS-6 b. Because a planet’s temperature is folded into every calculation of what its atmosphere contains, a discrepancy this large raises questions about how reliably astronomers can read the atmospheres of planets orbiting small, active stars.