This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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." 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. 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.
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