Astronomers Probe Mysteries of ‘Super Puff’ Planet

Gas giant planets need much less core mass than previously thought to create the immense gas envelope surrounding it.

Artistic rendition of the exoplanet WASP-107b and its star, WASP-107. Some of the star’s light streams through the exoplanet’s extended gas layer. (Credit: ESA/Hubble, NASA, M. Kornmesser)

(CN) — Scientists exploring space revealed Monday that gas-giant planets such as Jupiter and Saturn form more easily than previously thought.

Researchers at the Université de Montréal found that the core mass of WASP-107b, a giant exoplanet, is much lower than scientists thought necessary to create the immense gas envelope surrounding it and other super-sized planets.

Discovered by Ph.D. student Caroline Piaulet of UdeM’s Institute for Research on Exoplanets, the finding, published in the Astronomical Journal, is based on new analysis of WASP-107b’s internal structure.

Piaulet is part of the research team led by UdeM astrophysics professor Björn Benneke that detected water on an exoplanet located in its star’s habitable zone in 2019.

“This work addresses the very foundations of how giant planets can form and grow,” Benneke said. “It provides concrete proof that massive accretion of a gas envelope can be triggered for cores that are much less massive than previously thought.”

Known as a “super-puff” planet to astrophysicists, WASP-107b is an exoplanet that was first detected in 2017 around WASP-107, a star more than 200 light years from Earth. The planet, which is as big as Jupiter but 10 times lighter, exists near its star at a distance that is 16 times closer than the Earth is to the Sun.

Conducting their work at the Keck Observatory in Hawaii, Piaulet and her team first assessed the mass of WASP-107b using the radial velocity method, which determines a planet’s mass by measuring the wobble of its host star due to the planet’s gravitational pull. Researchers concluded that the mass of WASP-107b is about one-tenth that of Jupiter, or about 30 times that of Earth.

Next, Piaulet and her colleagues determined that WASP-107b’s internal structure must have a solid core of no more than four times the mass of Earth, making about 85% of the planet’s mass included in the thick layer of gas that surrounds this core. By comparison, Neptune has a similar mass to WASP-107b but no more than 15% of its total mass resides in its gas layer.

“We had a lot of questions about WASP-107b,” Piaulet said. “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?”

Such questions motivated researchers to analyze WASP-107b’s formation history.

Planets form in the layer of dust and gas surrounding a young star. Historical models of gas-giant planet formation are based on Jupiter and Saturn, where a solid core massive enough to dwarf Earth is needed to build up enough gas before the disc dissipates.

Without a massive core, scientists assumed that gas-giant planets could not create and retain their large gas envelopes. Until Piaulet’s discovery about WASP-107b, that is.

“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,” said fellow researcher Eve Lee of 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.”

The research team also included Daniel Thorngren and Merrin Peterson, as well as 19 other co-authors from Canada, the United States, Germany and Japan.

Mysteries still surround WASP-107b, inspiring Piaulet to continue studying the planet. Future analysis could involve the soon-to-launch James Webb Space Telescope, providing a more precise idea of the planet’s atmosphere.

“Exoplanets like WASP-107b that have no analogue in our solar system allow us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets,” Piaulet said. “It motivates us to study them in great detail.”

During the research, Piaulet and her colleagues made a second discovery: a second planet named WASP-107c with a mass considerably denser than its sister planet. But this planet operates differently, existing much farther from the central star and following a trajectory more ovular than circular.

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

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