A giant exoplanet that shouldn’t exist, and it’s colder than anyone expected

Some planets break the rules just by being there. HATS-6 b is a Jupiter-sized gas giant circling a small red dwarf star about 500 light-years away, and by our best theories it should not have formed at all. Now the James Webb Space Telescope has peered into its atmosphere and turned up two surprises: the rare molecule ammonia, and a planet running far cooler than the numbers said it should. The study, led by Giannina Guzmán Caloca of the University of Maryland, appears in The Astronomical Journal.

A giant where giants shouldn’t form

Start with why HATS-6 b is a puzzle before Webb even looked. It orbits an M dwarf, a red dwarf star of only about six-tenths the Sun’s mass, the runtiest and most common kind of star in the galaxy. The leading theory of planet birth, core accretion, holds that such small stars are born with skimpy disks of gas and dust, too little raw material, over too little time, to assemble a giant. Yet here sits a full Jupiter-class world. It belongs to a rare and theory-defying class that astronomers call GEMS, for Giant Exoplanets around M-dwarf Stars, and its very existence is a challenge thrown at the models.

Reading an atmosphere in starlight

To study a planet so far away, Webb used transmission spectroscopy. As HATS-6 b passes in front of its star, a sliver of starlight filters through the planet’s atmosphere on its way to us, and the gases there stamp their chemical fingerprints onto that light. The measured result is clear: strong evidence for water vapor, methane and, most notably, ammonia. That last one matters, because ammonia has been detected in the atmosphere of an exoplanet only a handful of times; this is among the very first, and it hands scientists a new chemical clue to work with.

An unexpected chill

Then came the surprise in the numbers, and it is worth separating the measurement from its explanation. The measurement: Webb’s data imply an atmospheric temperature of roughly 514 kelvin, about 240 degrees Celsius, when calculations based on the planet’s orbit had predicted something closer to 440 to 465 degrees. That is a large gap. The interpretation the team favors is that HATS-6 b is wrapped in reflective clouds and haze that bounce a big fraction of the starlight straight back to space, before it can heat the planet. In fact, the planet appears to reflect about as much light as Venus, one of the shiniest bodies in our own solar system. In short, it runs cold because it wears a mirror.

Why it matters: a different kind of planet

The significance goes beyond one odd world. Because red dwarfs vastly outnumber Sun-like stars, understanding the giant planets that occasionally orbit them tells us how common, and how strange, such systems can be. The chemistry Webb found, including that sub-solar metallicity and the ammonia, is exactly the kind of detail that encodes a planet’s origin story, and it hints that worlds around red dwarfs may form a chemically distinct population, built along a path our textbooks don’t yet fully capture. HATS-6 b, the giant that shouldn’t exist, is now also the giant forcing theorists back to the drawing board, and Webb has handed them the first real atmospheric evidence to do it with.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Giannina Guzmán Caloca et al., GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmosphere, The Astronomical Journal (2026). DOI: 10.48550/arXiv.2608.16990