“Puffed-up” planet unlike any other

New observations of the exoplanet WASP–107b suggest that existing models of how gas giant planets form may not be quite right.

A team of astronomers, led by Caroline Piaulet from the Université de Montréal (UdeM) in Canada, used the Keck Observatory in Hawai’i to probe the giant planet’s internal structure. They discovered that its core mass is much lower than expected – which has big implications, according to co-author Björn Benneke, also from UdeM.

“This work addresses the very foundations of how giant planets can form and grow,” he explains. “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 research appears in the Astronomical Journal.

First detected in 2017, WASP-107b is about 200 light-years away in the constellation of Virgo and orbits very close to its host star – eight times nearer than Mercury is to the Sun. The planet is the size of Jupiter but much lighter, making it one of the least dense exoplanets known. Astronomers classify these as “super-puff” or “cotton-candy” planets.

“We had a lot of questions about WASP-107b,” says Piaulet. “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?”

To answer these questions, Piaulet and team used the Keck Observatory to more accurately measure WASP-107b’s mass. They did so via the radial velocity method, observing how much the planet’s host star “wobbled” due to the planet’s gravitational influence.

Turns out WASP-107b is about 10 times lighter than Jupiter (or about 30 times more massive than the Earth).

Further calculations resulted in a surprising conclusion: the planet must have a solid core only around four times the mass of the Earth, meaning its thick, gaseous atmosphere makes up more than 85% of its total mass.

This is starkly different to the gas giants of our own solar system. Neptune, for example, has a similar mass to WASP-107b but its gas layer makes up only 5–15% of its total mass.

This finding may pose a challenge to our current models of how gas giants form, which are based on the planets in our own Solar System such as Jupiter and Saturn.

“Standard ideas of the ‘core accretion’ model for planet formation suggest you need about 10 times Earth’s mass before you can start really gathering lots of hydrogen and helium gas,” explains astronomer Jonti Horner, from the University of Southern Queensland, who was not involved in the study.

Despite being smaller, the core of WASP-107b has still attracted and retained an immense envelope of gas.

“That means that, if their estimated core mass is right, something unusual is going on,” Horner says. “Either our understanding of the formation of planets through core accretion is incomplete – which is certainly possible – or this planet formed in a totally different manner.”

Study co-author and super-puff planet expert Eve Lee, from McGill University in Canada, offers an explanation.

“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,” she says. “The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”

Horner also suggests an alternative origin for the planet’s “puffiness”.

“It could be the result of tidal interactions with the star, as a result of the planet having a slightly non-circular orbit,” he says – an explanation that “doesn’t require our understanding of core accretion itself to be flawed, but one that requires more work to be done”.

The new Keck observations led to an additional exciting discovery – that this exosolar system is also home to a second, smaller planet. Dubbed WASP-107c, it’s about one-third as massive as Jupiter and has a much longer orbit than its planetary sibling: three years compared to WASP-107b’s 5.7 days.

Next, Piaulet and team hope to study this interesting exosolar system with the James Webb Space Telescope, slated to launch this year.

“Exoplanets like WASP-107b that have no analogue in our Solar System allow us to better understand the mechanisms of planet formation in general and the resulting variety of exoplanets,” Piaulet says.

Horner agrees: “It’s all well and good finding more and more planets that fit with our current models and ideas – but the best science comes when you find something that doesn’t fit.”

Related reading: How extremely Large Telescopes will reveal exoplanets