Astronomers using the James Webb Space Telescope have characterized the atmosphere of HATS-6 b, a gas giant orbiting a red dwarf star, and found a chemical fingerprint that may set it apart from planets circling Sun-like stars. The findings, published in The Astronomical Journal, add new detail to a class of worlds that planetary formation theory struggles to explain.
HATS-6 b was discovered in 2015 by the HATSouth survey, a global network of automated telescopes that hunts exoplanets using the transit method. It is a gas giant with a mass comparable to Saturn but a radius closer to Jupiter's, completing an orbit around its host star in just three days. The star, HATS-6, is a red dwarf with about 0.6 times the mass of the Sun, located roughly 500 light-years from Earth. It is one of the lowest-mass stars known to host a close-in gas giant.
Planets like HATS-6 b belong to a category researchers call GEMS — Giant Exoplanets orbiting M-dwarf Stars. According to core accretion, the leading model for giant planet formation, M dwarfs should rarely produce such worlds. Their protoplanetary disks contain relatively little gas and dust, making it difficult to build the massive cores needed to trigger rapid gas capture. Yet astronomers have now identified about 40 such planets, and HATS-6 b is one of seven targets in JWST's GEMS observing program, which aims to compare their atmospheres with those of gas giants around Sun-like stars.
«The fact that HATS-6 b exists is really interesting: given what we know about planet formation, the planet shouldn't exist,» said Giannina Guzmán Caloca of the University of Maryland, the study's lead author. «These small stars don't have enough material or enough time to form planets as large as Jupiter and Saturn. By studying the composition of their atmospheres, we can begin to ask whether they formed the same way as hot Jupiters around Sun-like stars, or whether something different is happening.»
To study the planet, the team used the Prism mode of JWST's Near-Infrared Spectrograph, NIRSpec, observing two transits of HATS-6 b across its star's disk — one on January 15, 2024, and another on January 21. Using transmission spectroscopy, a technique that reveals an exoplanet's atmospheric composition as it passes in front of its star, the researchers identified four molecules: water, methane, carbon dioxide, and ammonia.
Ammonia is particularly notable. It is one of only a few detections of the molecule in an exoplanetary atmosphere and only the second achieved through transmission spectroscopy. Its presence strengthens the hypothesis that planets orbiting M dwarfs may represent a chemically distinct population compared with gas giants around more Sun-like stars, the researchers say. Determining ammonia's abundance could also help reconstruct the planet's nitrogen-to-oxygen ratio, which, alongside the carbon-to-oxygen ratio, offers clues about the conditions in which the planet formed.
Another result has left scientists puzzled: the planet's equilibrium temperature. HATS-6 b is commonly assigned a temperature of about 425 degrees Celsius, based on a theoretical estimate that assumes it absorbs all radiation from its star and redistributes it evenly without reflecting any. The new study instead points to much lower equilibrium temperatures, around 120 degrees Celsius — a value difficult to reconcile with a planet that completes an orbit in just three days. If confirmed, researchers would need to explain how the planet retains such a low temperature despite its close orbit.
The work is part of a broader effort to understand whether giant planets around small stars form differently from those around stars like the Sun. By comparing atmospheric compositions across the GEMS sample, astronomers hope to determine whether these rare worlds are simply scaled-down versions of familiar gas giants or a separate class shaped by their unusual birth environments.