The mass of the core of the giant exoplanet WASP-107b is much smaller than was thought necessary to build the immense gas envelope surrounding giant planets such as Jupiter and Saturn, as discovered by astronomers at the University of Montreal.
“This work addresses the very foundations of how giant planets can form and grow,” says the author of the study.
“It provides concrete evidence that the massive accumulation of a gas envelope can be triggered by nuclei that are much less massive than previously thought,” said Björn Benneke, one of the authors of the study from the Institute for Research on Exoplanets (iREx ).
Big but light
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. WASP-107b is one of the least dense exoplanets known – a type that astrophysicists have dubbed super-inflated planets.
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Astrophysicists were able to assess its mass more precisely for the first time: it turned out to be 10 times lighter than Jupiter. They also determined its most probable internal structure.
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 Earth. This means that more than 85% of its mass constitutes the thick layer of gas that surrounds this core. By comparison, Neptune, which has a similar mass to WASP-107b, only has between 5 and 15% of its total mass in its gas shell.
“We had a lot of questions about WASP-107b,” said Caroline Piaulet, another author of the study. “How could such a low-density planet form? And how did it prevent its huge layer of gas from escaping, especially given the planet’s proximity to its star?”
The formation of the gas giant
Planets form in the disk of dust and gas that surrounds a young star called the protoplanetary disk. Classic models of gas giant planet formation are based on Jupiter and Saturn. In them, it takes a solid core at least 10 times more massive than Earth to accumulate a large amount of gas before the disk dissipates.
Without a massive core, the gas giant planets were thought to be unable to cross the critical threshold necessary to accumulate and retain their large gas envelopes.
However, the WASP-107b has a much less massive core.
One of the explanations for its existence is that the planet “formed far from the star, where the gas in the disk is cool enough that the accumulation of gas can occur very quickly,” said Eve Lee, professor at McGill University and member of the iREx. Later the planet was able to migrate to its current position, Lee added.
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Studies of the planet’s atmosphere with the Hubble Space Telescope published in 2018 revealed a surprise: it contains very little methane.
“That is strange, because for this type of planet, methane should be abundant,” said Piaulet, detailing that the team is trying to understand what mechanisms could explain the destruction of methane.
“Exoplanets like WASP-107b that have no analogs in our solar system allow us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets,” Caroline concluded.
The study was published in the Astronomical Journal.