With a mass of 1.8 Neptune masses and a radius of Jupiter, WASP-107b presents a challenge to planet formation theories.
An artist’s impression of WASP-107b. Image credit: NASA / ESA / Hubble / M. Kornmesser.
WASP-107 is a highly active K-type main sequence star located about 212 light-years away in the constellation of Virgo.
First detected in 2017, WASP-107b is one of the least dense exoplanets known — a type that astrophysicists have dubbed ‘super-puff’ or ‘cotton-candy’ planets.
It orbits very close to the star — over 16 times closer than the Earth is to the Sun — once every 5.7 days.
It has one of the coolest atmospheres of any of the exoplanets discovered, although at 500 degrees Celsius (932 degrees Fahrenheit) is still radically hotter that Earth.
In the new study, Université de Montréal’s Professor Björn Benneke and colleagues used observations of WASP-107b obtained at the Keck Observatory in Hawai’i to assess the planet’s mass more accurately.
They concluded that the mass of WASP-107b is about one tenth that of Jupiter, or about 30 times that of Earth.
They then did an analysis to determine the planet’s most likely internal structure.
“This work addresses the very foundations of how giant planets can form and grow,” Professor 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.”
The astronomers came to a surprising conclusion: with such a low density, the planet must have a solid core of no more than 4 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.
“We had a lot of questions about WASP-107b,” said Caroline Piaulet, a Ph.D. student in the Institute for Research on Exoplanets at the Université de Montréal.
“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?”
“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 that gas accretion can occur very quickly, said Professor Eve Lee, an astronomer in the Department of Physics and the McGill Space Institute at 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.”
Beyond WASP-107b, the team also detected a second, more massive planet on a wide eccentric orbit.
The planet has a mass of 0.36 Jupiter masses and orbits the star once every 3 years.
Named WASP-107c, it may have influenced the orbital migration and spin-orbit misalignment of WASP-107b.
“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.”
The findings were published in the Astronomical Journal.
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Caroline Piaulet et al. 2021. WASP-107b’s Density Is Even Lower: A Case Study for the Physics of Planetary Gas Envelope Accretion and Orbital Migration. AJ 161, 70; doi: 10.3847/1538-3881/abcd3c
