Why is HATS-6 b hard to explain?

HATS-6 b is roughly the size of Jupiter but circles a small, cool, reddish star called an M dwarf. It completes an orbit every three days. Planets grow from the disk of gas and dust left over after a star forms, and small stars produce small disks. A Jupiter-scale planet around such a tiny star breaks that rule.

"These smaller stars don't have enough material or enough time to create planets as big as Jupiter and as big as Saturn," Guzmán Caloca said. "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 oversized worlds. HATS-6 b is one of seven planets being studied under a JWST program called GEMS, short for Giant Exoplanets around M-dwarf Stars.

"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."

What does the temperature mismatch mean?

The widely cited temperature for HATS-6 b, near 800°F (425°C), is not a measurement. It is a calculation that assumes the planet absorbs all incoming starlight and spreads that heat evenly. Early analyses by the team returned temperatures closer to 250°F (120°C), a figure physically unlikely for a world hugging its star on a three-day orbit.

"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 said. "The likeliest explanation is cloud and haze wrapping the planet the way they wrap Venus."

A discrepancy that large carries weight beyond one planet. Atmospheric composition calculations depend on temperature. An unreliable temperature can throw off readings of what a planet's air actually contains.

What's next?

The team plans longer-wavelength JWST observations to test whether clouds explain the chill and to search for other unexplained signals. So far, astronomers have confirmed more than 6,000 planets beyond the solar system.

"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."