Early Webb analyses put giant planet HATS-6 b about 300 degrees Celsius below its standard estimate

Nobody ever measured the 425 C figure usually attached to HATS-6 b. It is a shortcut with two built-in assumptions: the planet absorbs all the light its star delivers, and it spreads that heat evenly ^[11]. The early Webb analyses came in about 305 degrees below it ^[1].

Giannina Guzmán Caloca, the study's lead author and an astronomy Ph.D. candidate at the University of Maryland, goes after the first assumption. "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," she said. "The likeliest explanation is cloud and haze wrapping the planet the way they wrap Venus." ^[14] If clouds are the answer, the physics of stellar heating holds, and what fails is the assumption that the planet reflects nothing.

Formation is the older and firmer puzzle. Planets build from the disk of gas and dust left around a young star, and smaller stars have smaller disks ^[4]. "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." ^[5]

The sample is small. GEMS covers roughly one in six of the known giants around M dwarfs ^[2]. The whole class is under 1 percent of the more than 6,000 exoplanets found so far ^[17][3]. The program's comparison group is the better-understood giants around sun-like stars ^[6]. "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," she said ^[7].

Ammonia is the chemical clue in this result. Nitrogen-bearing molecules should be more abundant in cooler giant planets than in scorching ones ^[9], so finding ammonia fits a planet on the cold side of the estimate. "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 said. "It's an entirely new molecule to think about." ^[8]

The thing this doesn't tell you is how HATS-6 b formed. The data cover one planet out of seven ^[6], and its temperature comes from early analyses ^[12]. That is not yet enough to tell an unusual formation route apart from an unusually cloudy atmosphere. In my view the evidence backs the formation puzzle, which rests on the planet existing at all. It gives much weaker support to the idea that heating models fail, because the team's own leading explanation is reflective cloud ^[14].