An exoplanet located 212 light-years away is roughly the same size as Jupiter, but it’s 10 times lighter. The discovery is challenging our conceptions of how gas giants form and grow and the types of planets that can exist.
New research published in The Astrophysical Journal suggests it’s easier for gas giants to emerge within a protoplanetary disk than previously assumed. By “easier,” the authors of the new paper, led by astronomer Björn Benneke and PhD student Caroline Piaulet from the University of Montreal, mean that, in some special cases, the embryonic cores needed to kick-start the formation of gas giants can be lighter than current models predict.
Benneke and Piaulet just completed a four-year survey of WASP-107b, a gas giant with a mass in the Neptune range but a radius the size of Jupiter’s. This gas giant was previously known to astronomers, but the group wanted to better understand how such an object, with its extremely low density, could have formed from its protoplanetary disk. These types of planets have been detected and studied before, earning nicknames like “super-puff planets” and “cotton-candy planets.”
This world is very close to its host star, so a year on WASP-107b lasts just 5.7 days. Using the Keck Observatory in Hawai’i, the group sought to improve estimates of the object’s mass. To do so, the team measured the degree to which the exoplanet caused its host star to wobble—a technique astronomers refer to as the radial velocity method. The astronomers found that WASP-107b contains just 1.8 Neptune masses, or 30 Earth masses. That’s means it has just one-tenth the mass of Jupiter, with a comparable waistline. You can see where the cotton-candy comparison comes in.
The updated figure allowed the team to estimate the composition of the object’s internal structure. The core had to be heavy enough to prevent gas from escaping into space but light enough to maintain the extreme low density observed on the planet. Thus, the solid core, the scientists estimate, can be no heavier than 4 Earth masses. What’s more, 85% of the planet’s entire mass is packed into the thick layer of gas immediately surrounding the solid core, according to the paper. By comparison, 5% to 15% of Neptune’s mass is contained within its thick gas layer.
This was an unexpected result, as it’s “significantly lower than what is traditionally assumed to be necessary to trigger massive gas envelope accretion,” as the authors wrote in their paper (my wife accuses me of the opposite problem). In other words, the core of WASP-107b doesn’t appear to have sufficient mass, and thus gravitational influence, to facilitate the formation of a gas giant inside the protoplanetary disk—the gigantic disk of dust and gas that encircles a star during the planet formation process. But, obviously, WASP-107b exists, so our theories about such things must be wrong or at least in need of refinement.
Indeed, the new paper “addresses the very foundations of how giant planets can form and grow,” said Benneke in a University of Montreal statement. “It provides concrete proof that massive accretion of a gas envelope can be triggered for cores that are much less massive than previously thought.”
Current models of gas giant formation are biased towards the formation of Jupiter- and Saturn-like objects, and they suggest embryonic cores need to be at least 10 times heavier than Earth. Any lighter, and the core is unable to gather, or accrete, sufficient amounts of gas and dust prior to the dissipation of the protoplanetary disk. With the new data, the researchers were forced to entertain alternative scenarios.
“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,” said Piaulet in the statement. “The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”
Interesting hypothesis, but it’s exactly that. Future work will be needed to further validate this assumption.
During the course of this research, the team happened to stumble upon another exoplanet inside the same star system, which is now named WASP-107c. Encouragingly, this planet—with its exaggerated orbit—suggests Piaulet and her colleagues are on the right track with their newly proposed formation scenario.
WASP-107c has approximately one-third the mass of Jupiter, so it’s considerably heavier than its companion, WASP-107b. It takes three years for this newly detected exoplanet to make a single orbit of its host star. That’s not very interesting, but the elongated shape of its orbit very much is.
“WASP-107c has in some respects kept the memory of what happened in its system,” said Piaulet. “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.”
Nice, right? Always good to see corroborating evidence. Looking ahead, the team will seek to better understand the chemical composition of WASP-107b, including its inexplicable lack of methane. Perhaps another clue to its weirdness? We’ll be interested to find out.