Gas giants might form with easier conditions than previously believed possible. This new insight comes from a detailed investigation of a peculiar “super-puff” planet, a gas giant so close to its star it is inflated. These planets are often referred to as "cotton-candy" planets due to their low density.
As reported in The Astronomical Journal, planet WASP-107b orbits its star 16 times closer than the Earth is to the Sun. This proximity heats up the planet significantly. Astronomers estimate it to be as large as Jupiter, having just one-tenth of its mass.
Estimations on this planet suggest that it has a very low density – its core has the mass of at most around four Earths, and the remaining 26 Earth's worth of mass is located in its puffy atmosphere. That’s over 85 percent of its mass being a gassy envelope – for comparison, only between 5 and 15 percent of Neptune’s mass is in its gas layer. This striking difference was a crucial point for this work.
"We had a lot of questions about WASP-107b," lead author Caroline Piaulet, a graduate researcher at the University of Montreal, said in a statement. "How could a planet of such low density form? And how did it keep its huge layer of gas from escaping, especially given the planet's close proximity to its star? This motivated us to do a thorough analysis to determine its formation history."
The expected formation for a gas giant like Saturn or Jupiter sees a massive solid core, roughly 10 times the Earth, accumulating a lot of gas from the disk of material that surrounds a fledgling star. Having a massive core was considered an indispensable requirement, but WASP-107b shows that this might not be the case.
"For WASP-107b, the most plausible scenario is that the planet formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly," co-author Professor Eve Lee added. "The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system."
Finding a planet like this provides solid evidence that a large solid core is not always necessary when forming gas giants – there are processes that bypass that. And in this case, the answer comes from its planetary sibling: WASP-107c. This second planet is orbiting the star every three years (compared to the 5.7 days of WASP-107b) and it is about three times more massive. But the key evidence of past disturbances is that its eccentricity is higher, meaning the planet's orbit is oval rather than circular.
"WASP-107c has in some respects kept the memory of what happened in its system," explained Piaulet. "Its great eccentricity hints at a rather chaotic past, with interactions between the planets which could have led to significant displacements, like the one suspected for WASP-107b."
WASP-107b was already notorious, as it was the first exoplanet where scientists found helium in its atmosphere. The team hopes to collect new observations of this fascinating planet once JWST comes online at the end of the year.
