A team led by the University of Maryland pointed the James Webb Space Telescope at HATS-6 b, and the results are in: the atmosphere contains water, methane, carbon dioxide — and ammonia.
The ammonia alone is remarkable. It is only the second time transmission spectroscopy has detected ammonia (beyond water vapor) in an exoplanet atmosphere. Across all exoplanet atmospheres, carbon, hydrogen, and oxygen compounds have been seen; ammonia almost never. Lead author Giannina Guzmán Caloca, an astronomy PhD student at the University of Maryland, described it as an entirely new molecule that previously never had to be considered.
But what really makes you sit up is a different number.
The planet's equilibrium temperature — long cited in the literature as about 425°C (~800°F) — was never a measured value. It is an inference: assume the planet absorbs all the radiation from its star, then spreads it evenly across the surface. The preliminary analysis of this data yields a value close to 120°C.
A gap of 305 degrees.
A planet that orbits its star every 3.3 days is physically very hard to keep that cold.
Why this discrepancy matters
Temperature is not a side parameter. It is folded into every atmospheric composition calculation. Absorption features seen in transmitted starlight must be converted into physical quantities — how much water, how much ammonia — using a temperature.
So a 305-degree gap affects more than this one planet. It shakes the reliability of how astronomers interpret atmospheres of planets around low-mass active stars. The team's own statement is blunt: if the planet really is this cold, something must be reflecting a large amount of starlight back into space before it can be converted to heat.
The planet itself shouldn't exist, either. HATS-6 b has a mass close to Saturn's and a radius closer to Jupiter's, orbiting an M-dwarf star of roughly 0.6 solar masses every 3.3 days. Under core accretion — the mainstream formation model — the protoplanetary disks of M dwarfs contain too little gas and dust to build gas giants of this scale. Astronomers currently know of only about 40 such planets; each one is a challenge to formation theory.
| Item | Value | |---|---| | Distance | ~500 light-years | | Host star | M-type red dwarf, ~0.6 solar masses | | Orbital period | ~3.3 days | | Mass and radius | Mass near Saturn, radius near Jupiter | | Observation mode | NIRSpec Prism, transmission spectroscopy | | Observation windows | Two transits, 2024-01-15 and 2024-01-21 | | Molecules detected | Water, methane, ammonia, plus carbon dioxide |
Why ammonia points to a new population
Ammonia is a nitrogen-bearing molecule. In theory, nitrogen-bearing molecules should be more abundant in cooler giant planets than in hot Jupiters. This detection therefore supports the idea that giant planets orbiting M dwarfs may form their own chemical class.
Measuring the ammonia abundance also allows reconstructing the planet's nitrogen-to-oxygen ratio. Read alongside the carbon-to-oxygen ratio, it offers clues about formation conditions — a key path to answering "how did this planet actually grow?"
What comes next
The team outlined two concrete directions.
Longer-wavelength observations may identify other, currently unresolved signals and test whether the cloud hypothesis holds. If a Venus-like haze really shrouds the planet, albedo measurements could balance the temperature ledger.
There is a broader point worth keeping in mind. Over 6,000 exoplanets have been confirmed, and most bear little resemblance to the planets nearest Earth. Reading their atmospheric compositions — what they are made of and how they formed — is a prerequisite for judging whether other systems formed like ours. Which ones resemble Jupiter, and which resemble nothing at all, are branches of the same question.
Caveats
The paper was published in The Astronomical Journal on September 8, 2026; Chinese-language coverage appeared September 30 and October 1. The ~120°C figure is described in reports as "early analyses," and the cloud-haze layer as the "likeliest explanation" — both still claims awaiting verification. The 0.6-solar-mass host figure comes from ScienceOfficial's report and is consistent with "about 0.6," but was not checked against the paper's tables. The orbital period appears in different reports as both 3 and 3.3 days, from different sources.
Most important to note: the temperature issue is currently an unexplained discrepancy, not one already explained away by clouds. The team's proposed verification path has not yet been completed.
Sources: The Astronomical Journal (2026-09-08); reports by Astrobiology (2026-09-29) and ScienceOfficial (2026-09-29, including lead-author quotes); Chinese coverage from Science and Technology Daily (Zhang Mengran), republished by People's Daily Online and China Daily.