Artistic version of the exoplanet WASP-107b and its star, WASP-107. Some of the star’s light flows through the exoplanet’s extensive gas layer. ESA / Hubble, NASA, M. Kornmesser
The core mass of the giant exoplanet WASP-107b is much lower than what was needed to build the enormous gas envelope around giant planets Jupiter and Saturn, found astronomers at the Université de Montréal.
This intriguing discovery by Ph.D. student Caroline Piaulet of UdeM’s Institute for Exoplanets Research (iREx) suggests that gas giant planets form much easier than previously believed.
Piaulet is part of the pioneering research team of UdeM professor of astrophysics, Björn Benneke, who in 2019 announced the first detection of water on an exoplanet in the habitable area of its star.
Today (January 18, 2021) in the Astronomical Journal with new colleagues in Canada, the US, Germany and Japan, the new analysis of the internal structure of WASP-107b has ‘major implications’, Benneke said.
“This work addresses the foundation of how giant planets can form and grow,” he said. “This provides concrete evidence that massive growth of a gas envelope can be caused by cores that are much less massive than previously thought.”
As big as Jupiter but 10 times lighter
WASP-107b was first detected in 2017 around WASP-107, a star about 212 light-years from Earth in the Virgo constellation. The planet is very close to its star – more than 16 times closer to the earth than the earth. As large as Jupiter, but ten times lighter, WASP-107b is one of the least dense exoplanets known: a species that astrophysicists call ‘super-puff’ or ‘cotton-candy’ planets.
Piaulet and her team first used observations of the WASP-107b obtained at the Keck Observatory in Hawai’i to more accurately determine its mass. They used the radial velocity method, which enables scientists to determine the mass of a planet by observing the wobbly motion of its host due to the gravity of the planet. They concluded that the mass of WASP-107b is about one tenth of Jupiter, or about 30 times the mass of Earth.
The team then conducted an analysis to determine the most likely internal structure of the planet. They came to a surprising conclusion: with such a low density, the planet must have a solid core of no more than four times the mass of the earth. This means that more than 85 percent of its mass is absorbed in the thick layer of gas that surrounds this core. For comparison, Neptune, which has a mass similar to WASP-107b, has only 5 to 15 percent of its total mass in its gas layer.
“We had a lot of questions about WASP-107b,” Piaulet said. ‘How can a planet with such a low density come into being? And how did the enormous layer of gas not escape from it, especially given the planet’s proximity to its star?
“This motivated us to do a thorough analysis to determine its formation history.”
A gas giant in the making
Planets form in the disk of dust and gas surrounding a young star, a protoplanetary disk. Classical models of gas giant planet formation are based on Jupiter and Saturn. In this, a solid core at least ten times more massive than the earth is needed to store a large amount of gas before the disk disappears.
Without a massive core, it was not thought that gas giant planets could cross the critical threshold needed to build and maintain their large gas envelopes.
How then do I explain the existence of WASP-107b, which has a much less massive core? McGill University professor and iREx member Eve Lee, a world-renowned expert on super-puff planets such as WASP-107b, has several hypotheses.
“For WASP-107b, the most plausible scenario is that the planet has formed far away from the star, where the gas in the disk is cold enough for gas accumulation to occur rapidly,” she said. “The planet could later migrate to its current position, either through interactions with the disk or with other planets in the system.”
Discovery of a second planet, WASP-107c
The Keck observations of the WASP-107 system cover a much longer period than previous studies did, allowing the UdeM-led research team to make an additional discovery: the existence of a second planet, WASP-107c , with a mass of about a third of Jupiter, significantly more than WASP-107bs.
WASP-107c is also much further away from the central star; it takes three years to complete one lane on it, compared to only 5.7 days for WASP-107b. Also interesting: the eccentricity of this second planet is high, which means that its orbit around its star is more oval than circular.
“WASP-107c has in some ways preserved the memory of what happened in its system,” Piaulet said. “Its great eccentricity points to a rather chaotic past, with interactions between the planets that could have led to significant shifts, such as those suspected of WASP-107b.”
A few more questions
Besides the formation history, there are still many mysteries surrounding WASP-107b. Studies of the planet’s atmosphere with the Hubble Space Telescope published in 2018 revealed one surprise: it contains very little methane.
“This is strange, because for this type of planet, methane must be abundant,” Piaulet said. “We are now reviewing Hubble’s observations with the new mass of the planet to see how it will affect the results, and to investigate what mechanisms may explain the destruction of methane.”
The young researcher plans to continue the study of WASP-107b, hopefully with the James Webb Space Telescope launched in 2021, which will give a much more precise idea of the composition of the planet’s atmosphere.
“Exoplanets like WASP-107b that have no analogue in our solar system enable us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets,” she said. “It motivates us to study them thoroughly.”
Reference: “The density of WASP-107b is even lower: a case study for the physics of gas envelope supply and orbital migration” by Caroline Piaulet, Björn Benneke, Ryan A. Rubenzahl, Andrew W. Howard, Eve J. Lee , Daniel Thorngren Ruth Angus, Merrin Peterson, Joshua E. Schlieder, Michael Werner, Laura Kreidberg, Tareq Jaouni, Ian JM Crossfield, David R. Ciardi, Erik A. Petigura, John Livingston, Courtney D. Dressing, Benjamin J. Fulton, Charles Beichman, Jessie L. Christiansen, Varoujan Gorjian, Kevin K. Hardegree-Ullman, Jessica Krick and Evan Sinukoff, January 18, 2021, Astronomical Journal.
DOI: 10.3847 / 1538-3881 / abcd3c
In addition to Piaulet (iREx Ph.D. student, University of Montreal) and professors Björn Benneke (iREx, University of Montreal) and Eve Lee (iREx, McGill Space Institute, McGill University), the research team includes 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.