An exoplanet with 212 points of lightyears away is about the same size as Jupiter, but it is 10 times lighter. The discovery challenges our views on how gas giants form and grow and the types of planets that can exist.
New Research published in The Astrophysical Journal suggests it is easier for gas giants to appear in a protoplanetary disk than before supposed. By ‘easier’, the authors of the new paper, led by astronomer Björn Benneke and PhD student Caroline Piaulet of the University of Montreal, mean that in some special cases the embryonic nuclei needed to initiate the formation of gas giants are lighter than predict current models.
Benneke and Piaulet have just completed a four-year study of WASP-107b, a gas giant with a mass in the range of Neptune but a ray the size of Jupiter. This gas giant was previously known to astronomers, but the group wanted to better understand how such an object, with its extreme low density, could be formed from its protoplanetary disk. These types of planets have been discovered and studied before and have been given nicknames as ‘super puff planets’ and ‘cotton candy planets’.
This world is very close to its host star, so a year on WASP-107b lasts only 5.7 days. Using the Keck Observatory in Hawai’i, the group sought to improve estimates of the object’s mass. To do this, the team measured the extent to which the exoplanet made its host star wobble – a technique astronomers call the radial speed method. The astronomers found that WASP-107b only 1.8 masses of Neptune, or 30 Earth’s masses. That means it has only a tenth of the mass of Jupiter, with a similar waist. You can see where the cotton candy comparison comes in.
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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 extremely low density seen on the planet. The solid core, the scientists estimate, therefore cannot be heavier than 4 Earth’s masses. In addition, 85% of the planet’s total mass is packed in the thick layer of gas that surrounds the solid core, the paper said. For comparison: 5% up to 15% of Neptune’s mass is in its thick gas layer.
This was an unexpected result, as it is “significantly lower than what is traditionally believed to be necessary to induce massive gas envelope build-up,” as the authors wrote in their paper (my wife accuses me of the opposite problem). In other words, the core of WASP-107b does not appear to have enough mass, and thus gravitational influence, to facilitate the formation of a gas. huge in the protoplanetary disk – the gigantic disk of dust and gas that encircles a star during the planet formation process. But, apparently, WASP-107b exists, so our theories about such things must be wrong or at least need refinement.
Indeed, the new paper “covers the basics of how giant planets can form and grow,” said Benneke of a University of Montreal. statement. “It provides concrete evidence that massive gas envelope growth can be activated for nuclei that are much less massive than previously thought.”
Current models of gas giant formation focus on the formation of Jupiter- and Saturn-like objects, and they suggest that embryonic nuclei must be at least 10 times heavier than Earth. Any lighter, and the core is unable to collect or generate sufficient amounts of gas and dust prior to dissipation from the protoplanetary disk. With the new data, the researchers were forced to use alternative scenarios.
“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 for gas accretion to occur very quickly,” Piaulet said in the statement. “The planet was later able to migrate to its current position, either through interactions with the disk or with other planets in the system.”
Interesting hypothesis, but it is just that. Future work will be needed to further validate this assumption.
During this investigation, the team encountered another exoplanet in the same galaxy, now called WASP-107c. Encouragingly, this planet – with its exaggerated orbit – suggests that Piaulet and her colleagues are on the right track with their newly proposed formation scenario.
WASP-107c is about one-third the mass of Jupiter, so it is significantly heavier than its companion, WASP-107b. It takes three years for this newly discovered exoplanet to make a single orbit around its host star. That’s not terribly interesting, but the elongated shape of its track is.
“WASP-107c, in some ways, has preserved the memory of what happened in its system,” said Piaulet. “Its great eccentricity indicates a rather chaotic past, with interactions between the planets that could have resulted in significant displacements, as was suspected for WASP-107b.”
Nice is not it? Always good to see supporting evidence. Looking ahead, the team will try to better understand the chemical makeup of WASP-107b, including its inexplicable lack of methane. Maybe another clue to the madness of it? We are curious.