Webb detects ammonia and unexpected chill on a distant giant planet

Using the James Webb Space Telescope (JWST), a team of astronomers led by the University of Maryland identified water, methane and ammonia in the atmosphere of HATS-6 b, a giant planet 500 light-years from Earth. The observations also revealed that HATS-6 b may be significantly cooler than standard calculations predict—suggesting that the planet’s relationship with its star may be far more complicated than previously thought.

The team’s findings, published in the Astronomical Journal, raise new questions about how giant planets form and evolve.

A giant around a small star

HATS-6 b is roughly the size of Jupiter and completes an orbit every three days around an M dwarf—a small, cool, reddish star. Planets form from the disk of gas and dust left over after a star forms, and smaller stars have smaller disks. But HATS-6 b’s gigantic size doesn’t quite fit that rule, considering how small its star is.

“These smaller stars don’t have enough material or enough time to create planets as big as Jupiter and as big as Saturn,” explained the study’s lead author, Giannina Guzmán Caloca, an astronomy Ph.D. candidate at UMD. “So, the fact that HATS-6 b can exist is really interesting because it shouldn’t be possible with what we know.”

Astronomers know of only about 40 such planets. HATS-6 b is one of seven being studied in a JWST program called “Giant Exoplanets around M-dwarf Stars” (GEMS). The program is designed to compare these outliers with better-understood giants circling stars like our sun.

“Every one of these planets is a challenge to formation theory,” Guzmán Caloca said. “By measuring what their atmospheres are made of, we can start to ask whether they were built the same way as the hot Jupiters around sun-like stars or whether something different is going on.”

Ammonia points to cooler chemistry

Using a technique called transmission spectroscopy, which involves watching starlight filter through a planet’s atmosphere, the team identified four molecules in HATS-6 b’s atmosphere: water, methane, ammonia and carbon dioxide. The discovery marks only the second time the technique has detected ammonia on a distant world.

“Carbon, hydrogen and oxygen are all things that have previously been found in atmospheres of giant planets outside our solar system, but ammonia is something almost never detected before,” Guzmán Caloca explained. “It’s an entirely new molecule to think about.”

Because nitrogen-bearing molecules like ammonia should be more abundant in cooler giant planets than in scorching hot Jupiter-like ones, the discovery supports the theory that planets orbiting M-dwarf stars may be a chemically distinct population.

A temperature that defies expectations

HATS-6 b’s unexpected temperature raises another major question. The commonly cited temperature for HATS-6 b—near 800 degrees Fahrenheit (425 degrees Celsius)—is not an exact measurement but a calculation that assumes the planet absorbs all the light its star delivers and spreads that heat evenly. But scientists’ early analyses returned temperatures closer to 250 degrees Fahrenheit (120 degrees Celsius), a figure that’s physically improbable for a planet orbiting its star every three days.

“If the planet is genuinely that cool, it means that something is probably reflecting a great deal of starlight back into space before it can warm anything,” Guzmán Caloca explained. “The likeliest explanation is cloud and haze wrapping the planet the way they wrap Venus.”

The team’s results have implications beyond HATS-6 b. Because a planet’s temperature is factored into every calculation of what its atmosphere contains, a discrepancy this large raises questions about how reliably astronomers can read the atmospheres of planets orbiting small, active stars.

Further observations, wider questions

For Guzmán Caloca and her team, many mysteries remain. Longer-wavelength observations could identify other unexplained signals and test whether clouds explain the planet’s unexpectedly low temperature.

Astronomers have found more than 6,000 planets beyond our solar system, and many look nothing like the ones closest to Earth. Reading their atmospheres—what they’re made of, how they formed, which ones resemble Jupiter and which resemble nothing at all—can help scientists determine whether other solar systems formed through mechanisms similar to our own.

“What is our context? And how rare or how common are we?” Guzmán Caloca asked. “This is one planet out of thousands, but the way I like to think about it is that Earth is also one planet out of thousands and yet it holds everything that ever lived here.”

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

Full profile →

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

Full profile →