Five hundred light-years away, a Saturn-mass planet whips around a red dwarf star so tightly that its year lasts about three days. That alone makes HATS-6 b an oddity — small stars aren't supposed to have an easy time building planets this big. Now a team led by University of Maryland astronomy Ph.D. candidate Giannina Guzman Caloca has used the James Webb Space Telescope to pull apart the planet's atmosphere wavelength by wavelength, and what came out is stranger than the setup suggested: water, methane and carbon dioxide, all more or less expected — and ammonia, which almost never is.
The ammonia detection, described in a paper posted to arXiv and published Sept. 8, 2026 in The Astronomical Journal, is only the second time the molecule has ever been picked out of an exoplanet atmosphere using transmission spectroscopy. It arrived alongside a second surprise: the planet's measured temperature came in at roughly 250°F, far below the ~800°F figure commonly cited for planets in its class.
How you take a distant planet's temperature
HATS-6 b never gets photographed directly — at 500 light-years, JWST doesn't resolve a disk. Instead, Guzman Caloca's team, part of the GEMS survey (Giant Exoplanets around M-dwarf Stars), watched the planet cross in front of its host star during two separate transits. As starlight grazed through the planet's atmosphere on its way to Webb's NIRSpec instrument, specific molecules absorbed specific wavelengths, leaving fingerprints in the spectrum. NIRSpec's PRISM mode captured that signal continuously across 0.6 to 5.3 microns — a wide enough net to catch water, methane and CO2 cleanly, and ammonia at the edges of detectability.
Fitting that spectrum also yields the atmosphere's temperature and composition. The team derived a low overall metallicity for the planet (log[M/H] of −1.99, meaning heavy elements are underrepresented relative to the Sun by roughly a factor of 100) and a sub-solar carbon-to-oxygen ratio. Both numbers, together with the temperature, feed directly into models of how and where the planet assembled.
Why ammonia is the headline
"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," Guzman Caloca said. That rarity isn't for lack of looking — ammonia is chemically unstable in the hot, irradiated atmospheres typical of the giant exoplanets astronomers have studied most, where ultraviolet starlight and high temperatures tend to break it down or convert it into other nitrogen-bearing molecules before it can build up to detectable levels.
HATS-6 b's chill may be exactly why the ammonia survived. A cooler atmosphere favors different chemical equilibria than a scorching one, and nitrogen that might otherwise end up as something else can instead stay locked up as NH3. Guzman Caloca called it "an entirely new molecule to think about" for scientists modeling these atmospheres — one more data point in the periodic table's worth of chemistry that giant-planet atmospheres can host, and a reminder that the standard hot-Jupiter playbook doesn't automatically transfer to planets orbiting cooler, smaller stars.
The temperature problem
The ~800°F figure long attached to planets like HATS-6 b comes from equilibrium-temperature estimates — back-of-the-envelope calculations based on how much starlight a planet absorbs and how it's assumed to redistribute that heat. The actual measured temperature, drawn directly from the transmission spectrum rather than inferred from stellar brightness and orbital distance, landed hundreds of degrees cooler. That gap matters: atmospheric chemistry, cloud formation and even a planet's day-night heat transport all hinge on getting the temperature right, and a few-hundred-degree miss can change which molecules a model predicts should be visible at all — including, apparently, ammonia.
A planet that shouldn't be this big
The deeper puzzle sits with the host star itself. M-dwarfs are the most common stars in the galaxy, but they're also small and comparatively light on the raw material needed to build worlds. "These smaller stars don't have enough material or enough time to create planets as big as Jupiter," Guzman Caloca said — yet HATS-6 b, at roughly 0.3 Jupiter masses with a Jupiter-scale radius, is a substantial planet by any standard, orbiting one of those undersized stars on a three-day clock.
Astronomers currently know of roughly 40 planets like HATS-6 b circling small stars, out of more than 6,000 confirmed exoplanets overall — a small enough sample that each new atmospheric measurement carries outsized weight. The GEMS survey is built to grow that sample methodically: it's studying seven such planets with JWST, using consistent instruments and analysis to compare atmospheres across the group rather than relying on one-off observations that are hard to cross-check.
Why It Matters
Giant planets around red dwarfs are a formation puzzle: the leading models of planet-building predict these small, low-mass stars shouldn't have enough solid material in their protoplanetary disks to grow a Saturn- or Jupiter-sized world before the disk disperses. Every confirmed example, like HATS-6 b, is a data point that current formation theory has to explain rather than dismiss as a fluke. Its low metallicity and sub-solar carbon-to-oxygen ratio give theorists specific numbers to test against different formation pathways — whether the planet built up gradually via core accretion or collapsed more directly from the disk. The ammonia detection adds a practical dividend beyond the formation question: it demonstrates that Webb's spectroscopy is sensitive enough to catch chemically fragile molecules in favorable (cooler) conditions, widening the toolkit for characterizing other atmospheres — including, eventually, smaller and more temperate worlds where such chemistry could matter for habitability. And the temperature discrepancy is a caution flag for the field at large: if a well-studied class of planets has been running several hundred degrees cooler than assumed, other equilibrium-temperature estimates across the exoplanet catalog may need a second look.
