Astronomers using the James Webb Space Telescope have found ammonia in the atmosphere of a giant planet 500 light-years away, and the planet looks far cooler than it should.

The planet, HATS-6 b, is roughly the size of Jupiter and circles a small, cool reddish star once every three days. A University of Maryland-led team spotted water, methane, ammonia and carbon dioxide by watching starlight filter through the planet's air, a technique called transmission spectroscopy.

Ammonia is rarely seen this way on distant worlds; this is only the second such detection. Its presence suggests giants around small stars may be a chemically distinct family, the team wrote in the Astronomical Journal.

The bigger puzzle is the temperature. Standard calculations put the planet near 425 degrees Celsius, but early analysis suggests it may be closer to 120 degrees, strikingly cool for a planet so close to its star.

Only about 40 giant planets are known around such small stars, and each one challenges theories of how planets form.