A UMD-led team found water, methane and a rare ammonia signal in the atmosphere of HATS-6 b, a large planet orbiting a tiny star—raising new questions about how giant planets form.
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. Observations captured by JWST also revealed that HATS-6 b may be significantly cooler than expected compared to standard calculations of its temperature—suggesting that the planet’s relationship with its star may be far more complicated than previously thought.
The team’s findings published in the Astronomical Journal on September 8, 2026, 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. But HATS-6 b’s gigantic size doesn’t quite fit into that rule, considering how small the star it orbits is.
“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, an astronomy Ph.D. candidate at UMD. “So, the fact that HATS-6 b can exist is really interesting because it shouldn’t be possible with what we know.”
Astronomers know only about 40 such planets. HATS-6 b is one of seven being studied in a JWST program called Giant Exoplanets around M-dwarf Stars (GEMS). The program is designed to compare these outliers against the better-understood giants circling stars like our sun.
“Every one of these planets is a challenge to formation theory,” Guzmán Caloca said. “By measuring what their atmospheres are made of, we can start to ask whether they were built the same way as the hot Jupiters around sun-like stars or whether something different is going on.”
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 air: water, methane, ammonia and carbon dioxide. The discovery marks only the second time the technique detected ammonia on a distant world.
“Carbon, hydrogen and oxygen are all things that have been previously found in atmospheres of giant planets outside our solar system, but ammonia is something almost never detected before,” Guzmán Caloca explained. “It’s an entirely new molecule to think about.”
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. The commonly cited temperature for HATS-6 b—near 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 Fahrenheit[GCG(6OMAC1] , 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.
For Guzmán Caloca and her team, many mysteries remain. Longer-wavelength observations could identify other unexplained signals and test whether clouds explain a planet's unexpectedly low temperature. Astronomers have found more than 6,000 planets beyond our solar system, and many of them look nothing like the ones closest to Earth. Reading their atmospheres—what they're made of, how they formed, which ones resemble Jupiter and which resemble nothing at all—can help scientists determine whether other solar systems were created through formation mechanisms similar to our own.
“What is our context? And how rare or how common are we?” Guzmán Caloca asked. “This is one planet out of thousands, but the way I like to think about it is that Earth is also one planet out of thousands and yet it holds everything that ever lived here.”
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The study, “GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmosphere,” was published in the Astronomical Journal on September 8, 2026.
This research was supported by NASA through a grant from the Space Telescope Science Institute for JWST program GO 3171, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract (NAS 5-03127). The observations were obtained from the Mikulski Archive for Space Telescopes (MAST) at STScI, with support for MAST for non-HST data provided by the NASA Office of Space Science (NNX09AF08G) and other grants and contracts. Additional support was provided by NASA Headquarters through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by ORAU under contract with NASA, and under NASA award (80GSFC24M0006); by the National Science and Technology Council of Taiwan (114-2112-M-001-065-MY3); and by an Academia Sinica Career Development Award (AS-CDA-115-M03). The text does not necessarily represent the views of these organizations.
