A super-puff planet like no other

The heart mass of the giant exoplanet WASP-107b is much lower than what was thought necessary to build the giant gas envelope around large planets like Jupiter and Saturn, astronomers at Université de Montréal have discovered.

This interesting discovery with a Ph.D. student Caroline Piaulet from the UdeM Research Institute on Exoplanets (iREx) suggests that large gas planets are making it much easier than previously thought.

Piaulet is part of the innovative research team of UdeM astrophysics professor Björn Benneke who announced in 2019 the first water discovery of an exoplanet located in its star’s residential zone.

Published today in the Astronomical Iris with colleagues in Canada, the US, Germany and Japan, the new study has a major impact on the internal structure of WASP-107b, “Benneke said.

“This work addresses the fundamental foundations of how large planets can grow and flourish,” he said. “It provides concrete proof that a large accumulation of a gas envelope can be stimulated for unpredictable rights. not as big as expected. “

WASP-107b was first discovered in 2017 around WASP-107, a star about 212 light-years from Earth in the constellation Virgo. The planet is very close to its star – more than 16 times closer than Earth to the Sun. As big as Jupiter but 10 times lighter, WASP-107b is one of the most densely known exoplanets: a type that astronauts have named “super-puff” or “cotton-candy” planets.

Piaulet and her team first used observations of WASP-107b obtained at the Keck Observatory in Hawai’i to more accurately estimate its mass. They used the radial distance method, which allows scientists to determine the mass of a planet by observing the moving motion of its host star as a result of the gravitational pull of the planet. They determined that the mass of WASP-107b is about one-tenth the size of Jupiter, or about 30 times the size of Earth.

The team then performed an analysis to determine the most likely internal structure on the planet. They came to a surprising conclusion: with such a low density, the planet must have a solid core no larger than four times the mass of the Earth. This means that more than 85 percent of its mass is absorbed into the thick layer of gas that surrounds this core. In contrast, Neptune, which has a mass similar to WASP-107b, has only 5 to 15 percent of the total mass in its gas series.

“We had a lot of questions about WASP-107b,” Piaulet said. “How could a planet with such a low density exist? And how did it keep most of its gas from escaping, especially since the planet was so close to its star?

“This inspired us to do a detailed study to find out what the creative history was.”

Planets form in the disk of dust and gas that surrounds a young star called a protoplanetary disk. Classic models of giant gas planet formation are based on Jupiter and Saturn. In these, a hard core needs to be at least 10 times larger than the Earth to collect a lot of gas before the disk spreads.

Without a large core, large gas planets were not thought to be capable of exceeding the critical threshold required to build and maintain their large gas envelopes.

How do you explain the presence of WASP-107b, which has a much smaller heart rate? McGill University professor and iREx member Eve Lee, a world-renowned expert on super-puff planets like WASP-107b, has a number of ideas.

“For WASP-107b, the most plausible scenario is that the planet created far away from the star, where the gas in the disk is cold enough that gas accumulation can occur very quickly,” she said. “The planet has been able to migrate into its normal state, either through interaction with the disk or with other planets in the system.”

Keck’s observations of the WASP-107 system cover a much longer time than previous studies, allowing the UdeM-led research team to further discover: the existence of a second planet, WASP-107c, with a mass of about a third of Jupiter, much more than WASP-107b’s.

WASP-107c is also far from the main star; it will take three years to complete one orbit around it, compared to just 5.7 days for WASP-107b. Also interesting: the remoteness of this second planet is high, meaning that the orbit around its star is more oval than a sphere.

“WASP-107c is in some ways a memory of what happened in its system,” Piaulet said. “Its great wonder is advertising at a very controversial time, with interactions between the planets that could have caused major shifts, such as the suspect for WASP-107b.”

Beyond its creative history, many mysteries remain about WASP-107b. Studies of the planet’s atmosphere with the Hubble Space Telescope unveiled in 2018 have revealed one surprise: it contains very little methane.

“That’s weird, because for this kind of planet, methane should be plentiful,” Piaulet said. “We are now reiterating Hubble’s ideas with the planet’s new mass to see how it will affect the results, and explore the possible ways to explain methane destruction.”

The young researcher plans to continue studying WASP-107b, hopefully with the James Webb Space Telescope launched in 2021, which will give a much more detailed idea of ​​the planet’s atmospheric shape.

“Exoplanets such as WASP-107b which do not have analogue in our Solar System allow us to better understand the mechanisms involved in planet formation in general and the resulting mix of exoplanets, “She said.” We are encouraged to examine them in detail. “

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“The density of WASP-107b is even lower: a case study for the physics of gas casing accumulation and orbital migration,” by Caroline Piaulet et al., Was posted today in the Astronomical Iris. DOI: 10.3847 / 1538-3881 / abcd3c. In addition to Piaulet (iREx Ph.D. student, Université de Montréal) and professors Björn Benneke (iREx, Université de Montréal) and Eve Lee (iREx, McGill Space Institute, McGill University), the research team including Daniel Thorngren (iREx Postdoctoral Fellow, Université de Montréal) and Merrin Peterson (iREx M.Sc student), and 19 other co-authors from Canada, the United States, Germany and Japan.