Webb detects ammonia and unexpected chill on a distant giant planet

An artist's concept of a gas giant planet orbiting a red dwarf. Credit: NASA, ESA, and G. Bacon (STScI) 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."

• HATS-6 b is a giant planet with water, methane, and ammonia in its atmosphere.
• It is 500 light-years from Earth and orbits an M dwarf star.
• It may be significantly cooler than previously calculated.

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 lik

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