“Cotton-Candy” Exoplanet WASP-107b Apparently Has Less Core Mass Than Previously Thought

Astronomers from the University of Montreal (UdeM) discovered that the core mass for exoplanet WASP-107b is a lot lower than previously thought necessary to form giant gas planets such as Saturn and Jupiter.

From UdeM's Institute for Research on Exoplanets (iREx), Ph.D. student Caroline Piaulet made the discovery that suggests that gas giants - planets mostly made of gaseous elements - might actually require less to form and develop. Piaulet is also a part of the research team that includes professor Björn Benneke who, in 2019, was among the first who reported finding water on an exoplanet situated in the star' Goldilocks Zone: a specific range from its host star that can support life as we know it. 

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Their findings are published in the Astronomical Journal, and according to Benneke, the new analysis of WASP-107b "has big implications." He explains that the new discovery addresses how giant planets form and grow.

As Large as Jupiter But Not As Massive

WASP-107b was first found in 2017, orbiting its host star WASP-107 - a star about 212 light-years away from Earth, at the Virgo constellation. The distance between 107b and its star is more than 16 times closer than the distance between Earth and the Sun. Almost as big as Jupiter but about ten times lighter, WASP-107b is among the least dense exoplanets ever discovered so far - being called "super-puff" or "cotton-candy" planets by astrophysicists.

Piaulet and her collaborated first used the cotton-candy planet's observations obtained at the Keck Observatory in Hawai'i, examining its mass more accurately. Researchers then used a technique called the radial velocity method, allowing scientists to determining the mass of a planet by observing the "wobbling motion" of its host star - an indication of the planet's mass interacting with the host star. From this method, they found out that WASP-107b has a mass only one-tenth of Jupiter, yet still 30 times more than Earth's.

Afterward, researchers inquired about the planet's internal structure and made a surprising discovery: it has such a low density that it must have a solid core of at least four times the Earth's mass. More than 85 percent of the planet's mass is found in the thick gaseous layer immediately surrounding the core. As a reference, Neptune, which has a mass closer to WASP-107b, finds 5 to 15 percent of its total mass in the gas layer.

Formation of Gas Giants

Planets start as discs of dust and gas surrounding a young star - with the materials called planetary discs. Classical models of gas giant formations are based on Jupiter and Saturn's data, leading to the estimated mass required for the creation of these planets. Without a massive core, gas giants were previously thought to be unable to cross that critical threshold to build up and retain gas envelopes as the discs take on their signature spherical form.

However, the discovery of WASP-107b, with its less massive core, challenges previously-established notions on gas-giant formations. "For WASP-107b, the most plausible scenario is that the planet formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly," explains Eve Lee, McGill University professor, and iREx member, in a statement.

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