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This strange ‘super-puff’ planet is more atmosphere than core

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An artist’s impression of WASP-107b.
An artist’s impression of WASP-107b. NASA / ESA / Hubble / M. Kornmesser

While most planets we’ve discovered outside our solar system have been rocky worlds like Earth, gas giants like Jupiter, or ice giants like Uranus, these aren’t the only types of planets out there. There are also some strange “super-puff” planets that are so low in density that they are also called cotton candy planets. Now, new research into one of these super puffs shows it is even odder than previously thought.

Planet WASP-107b has a mass just one-tenth that of Jupiter, even though it is around the same size. It also orbits very close to its star, with a year there lasting only 5.7 days.

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But the really strange thing about this planet has to do with its atmosphere. The layer of gas around the planet makes up more than 85% of its mass, meaning the planet’s core must be tiny, with a mass of just four times the mass of Earth.

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This raises questions about what is keeping the atmosphere in place.

“We had a lot of questions about WASP-107b,” said Caroline Piaulet, a Ph.D. student in the Institute for Research on Exoplanets at the Université de Montréal, 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?”

The researchers think that the only way such an odd planet could exist is if it formed elsewhere and moved over time. “For WASP-107b, the most plausible scenario is that the planet formed far away from the star, where the gas in the disk is cold enough that gas accretion can occur very quickly,” explained Professor Eve Lee, an astronomer in the Department of Physics and the McGill Space Institute at McGill University. “The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”

This theory is supported by the detection of another planet in the system, called WASP-107c, which has an eccentric orbit. This suggests that the system has a “chaotic past,” according to Piaulet, which pushed the planets into different positions.

The research is published in the Astronomical Journal.

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