The James Webb Space Telescope has found ammonia in the atmosphere of a planet outside our solar system. It's only the second such find ever, and the first made with this method. The planet's temperature also doesn't add up: it's far cooler than it should be, given how close it sits to its star. Together, the two findings are making astronomers rethink how Jupiter-sized gas giants form around small red stars.
The planet is HATS-6 b, roughly the size of Jupiter but with only about a third of its mass, closer to Saturn's. It circles its star every three days. That star, HATS-6, is a red dwarf, small, cool and reddish, a type far more common in our galaxy than stars like the sun. The system sits 500 light-years from Earth. Johanna Guzmán Caloca, an astronomy PhD student at the University of Maryland, led the study, published on 8 September 2026 in the Astronomical Journal. The planet is part of the GEMS program, under which Webb is studying the atmospheres of seven gas giants around red dwarfs to compare them with the hot Jupiters already known around sun-like stars.
NIRSpec and two transits
The method is called transit spectroscopy. As the planet passes in front of its star, some starlight doesn't hit a solid surface. Instead, it filters through the thin layer of atmosphere at the edge of the planet's disc. Each molecule soaks up light at a very specific wavelength, much like tinted glass lets some colors through and blocks others. By comparing the star's light before and during the transit, astronomers can see which colors are missing and work out which molecules blocked them. For HATS-6 b, the team used Webb's NIRSpec instrument in PRISM mode, which covers a wide range of wavelengths from visible light to the near-infrared. They observed two transits.
Ammonia, water and methane at 120°C instead of 425°C
In the light signature of HATS-6 b, the scientists found water, methane, carbon dioxide and ammonia. That last molecule is rare. Astronomers have already spotted carbon and oxygen in the atmospheres of many distant planets, but ammonia almost never turns up. The only other confirmed detection of ammonia outside the solar system came from direct imaging with a coronagraph, an instrument that blocks a star's light. It let astronomers photograph a much colder object, around 240°C. At HATS-6 b, scientists detected ammonia for the first time through light that passed through a planet's atmosphere during a transit — a method with much wider reach, since most known exoplanets can't be photographed directly. Ammonia survives more easily in cooler atmospheres and breaks down quickly around scorching hot Jupiters, so its presence suggests HATS-6 b isn't as hot as such planets usually are.
Based only on its distance from the star and the amount of light it receives, the planet should sit at around 425°C. But the spectrum data show an atmosphere behaving as if it's much colder, closer to 120°C. A gap of about 300°C is huge for a planet sitting this close to its star. If the planet really is that cool, Guzmán Caloca says the most likely explanation is that something is bouncing much of its starlight back into space — the way Venus's thick clouds reflect sunlight and throw off estimates of the true surface temperature underneath. The team plans new observations at longer wavelengths to test the cloud theory and look for other, fainter signals in the spectrum.
An atmosphere with a hundred times fewer heavy elements than the sun
The standard model for how giant planets form starts with a small solid core of dust and pebbles. That core needs to grow to about ten times Earth's mass before the gas disc around the young star disperses. Only then is the core heavy enough to quickly pull in a huge amount of gas. Discs around red dwarfs tend to be sparser and shorter-lived, so by this logic, Jupiter-sized planets should be rare there. So far, scientists know of about 40 such planets, including HATS-6 b. The Webb data add a detail that doesn't fit the theory: the heavy elements in its atmosphere are about a hundred times scarcer than in the sun. That means the planet is short on exactly the raw material its core is supposed to have been built from. The combination of rare ammonia, low temperature and a metal-poor atmosphere has led the team to think gas giants around red dwarfs may be a chemically distinct group of planets, formed along a different path than the hot Jupiters found around sun-like stars.
The GEMS team will keep comparing the seven planets in its sample.