The central mass of the exoplanet giant WASP-107b is much smaller than previously thought to be needed to build the huge gas envelope surrounding giant planets such as Jupiter and Saturn, according to a team of astronomers led by Canada, including Professor Eve Lee of McGill University.
This intriguing discovery by Caroline Piaulet of the University of Montreal under the supervision of Björn Benneke suggests that gaseous giant planets form much more easily than previously thought. Published in the Astronomical Journal by a team of astronomers from Canada, the United States, Germany, and Japan, the new analysis of the internal structure of the WASP-107b has major implications.
“This study pushes the limits of our theoretical understanding of how giant-sized planets form. WASP-107b is one of the most inflated planets out there and we need a creative solution to explain how these small nuclei can build such massive gas envelopes, ”says co-author Eve Lee, an adjunct professor in the Department of Physics at McGill University. McGill Space Institute.
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 constellation Virgo. The planet is very close to its star, more than 16 times closer than Earth to the Sun. About as large as Jupiter but ten times lighter, WASP-107b is one of the least dense exoplanets known: a type of astrophysicist has dubbed the planet “super puffs” or “cotton candy.”
Astronomers first used WASP-107 observations obtained at the Keck Observatory in Hawaii to more accurately assess the mass of the planet. They used the radial velocity method, which allows scientists to determine the mass of a planet by observing the oscillating motion of its host star due to the gravitational attraction of the planet. They concluded that the mass of WASP-107b is about one-tenth that of Jupiter, or about 30 times that of Earth.
When analyzing 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 no more than four times the mass of the Earth. This means that more than 85% of its mass is included in the thick layer of gas that surrounds this core. In comparison, Neptune, which has a mass similar to WASP-107b, has only between 5 and 15 percent of its total mass in its gas layer.
A gas giant in process
Planets form in the disk of dust and gas that surrounds a young star called a protoplanetary disk. Classic models of the formation of gas giant planets are based on Jupiter and Saturn. In these, a solid core at least 10 times more massive than Earth is needed to accumulate a large amount of gas before the disk dissipates.
Without a massive core, gas-giant planets were not believed to be able to cross the critical threshold needed to accumulate and retain their large gas envelopes.
How can we explain the existence of WASP-107b, which has a much less massive core? Professor Lee, who is 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 formed far away from the star, where the gas in the disk is cold enough for gas accretion to occur very quickly,” he said. “The planet was later able to migrate to its current position, either through interactions with the disk or with other planets in the system,” he says.
Discovery of a second planet
Keck’s observations on the WASP-107 system cover a much longer period of time than previous studies, allowing the research team to make an additional discovery: the existence of a second planet, WASP-107c, with a mass of about one-third. that of Jupiter, considerably more than that of WASP-107b.
WASP-107c is also much further away from the central star; it takes three years to complete an orbit around it, compared to just 5.7 days for WASP-107b. It is also interesting: the eccentricity of this second planet is high, that is, its trajectory around the star is more oval than circular.
“The WASP-107c has kept in some respects the memory of what happened to its system,” Piaulet said. “Its great eccentricity hints at a rather chaotic past, with interactions between planets that could have caused significant displacements, such as the WASP-107b suspect.”
Researchers plan to continue studying WASP-107b, hopefully with the James Webb Space Telescope launched in 2021, which will provide a much more accurate idea of the composition of the planet’s atmosphere.
About the study
“The density of WASP-107b is even lower: a case study for the physics of gas envelope accretion 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 were published in the Astronomical Journal.