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"Cotton candy" exoplanet defies models for how gas giants form

"Cotton candy" exoplanet defie...
Artist's impression of a "Super-Neptune" exoplanet
Artist's impression of a "Super-Neptune" exoplanet
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An artist's impression of the exoplanet WASP-107b, in silhouette against its star, revealing the different layers of its atmosphere
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An artist's impression of the exoplanet WASP-107b, in silhouette against its star, revealing the different layers of its atmosphere
Artist's impression of a "Super-Neptune" exoplanet
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Artist's impression of a "Super-Neptune" exoplanet

WASP-107b was already one of the weirdest exoplanets ever discovered, with its incredibly low mass for its size giving it the density of cotton candy. On closer inspection, astronomers have now found that its density is even lower than previously thought, defying our current understanding of how gas giants even form.

Discovered in 2017, WASP-107b lies about 212 light-years away in the constellation of Virgo. It orbits very close to its host star, completing a lap once every 5.7 days. It’s about as big as Jupiter but packs only one tenth of its mass, making it one of the least dense planets we know of.

In the new study, astronomers examined WASP-107b more closely and found that this exoplanet is even stranger than we thought. The team conducted an analysis of its most likely internal structure, and found that the planet’s solid core must have a mass of four Earths or less.

That might sound like a lot, but by comparison Jupiter’s core has been estimated to have up to 25 times the mass of the Earth. The team calculated that the vast majority of WASP-107b’s mass – over 85 percent – is contained in its gassy atmosphere, far more than usual. Neptune’s atmosphere, for instance, only accounts for up to 15 percent of its mass.

An artist's impression of the exoplanet WASP-107b, in silhouette against its star, revealing the different layers of its atmosphere
An artist's impression of the exoplanet WASP-107b, in silhouette against its star, revealing the different layers of its atmosphere

The weirdness might sound like nothing more than a numbers game, but it actually has some pretty major implications. Our current models for how gas giant planets form suggests that they need to start with a solid core of at least 10 Earth masses, which attracts gas from the protoplanetary disc surrounding young stars.

"We had a lot of questions about WASP-107b,” says Caroline Piaulet, lead author of the study. “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 find suggests that gas giants form more easily than previously thought, but the team has a few hypotheses for how this one in particular arose.

“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,” says Eve Lee, an author of the study. “The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”

And the team may have found other evidence to support the latter part of that idea. Their observations revealed a previously unknown planet in the system, now named WASP-107c. This newcomer is much more massive than its cotton candy sibling, containing about a third of the mass of Jupiter, and it orbits the star once every three (Earth) years. But the smoking gun may be in its highly eccentric orbit.

“WASP-107c has in some respects kept the memory of what happened in its system,” says 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.”

The WASP-107 system will no doubt be the subject of future observations, and the team says that the upcoming James Webb Space Telescope, due to launch this October, may help shed further light on it.

The research was published in the Astronomical Journal.

Source: University of Montreal

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