Helium in the Eroding Atmosphere of an Exoplanet

Creative rendition of the exoplanet WASP-107 b and its star, WASP-107 A few of the star’s light streams through the exoplanet’s prolonged gas layer. ESA/Hubble, NASA, M. Kornmesser

The core mass of the giant exoplanet WASP-107 b is much lower than what was thought required to develop the immense gas envelope surrounding huge planets like Jupiter and Saturn, astronomers at Université de Montréal have actually discovered.

This intriguing discovery by Ph.D. student Caroline Piaulet of UdeM’s Institute for Research on Exoplanets (iREx) suggests that gas-giant worlds form a lot more easily than formerly believed.

Piaulet belongs to the groundbreaking research team of UdeM astrophysics teacher Björn Benneke that in 2019 revealed the first detection of water on an exoplanet situated in its star’s habitable zone.

Released today (January 18, 2021) in the Astronomical Journal with colleagues in Canada, the U.S., Germany and Japan, the brand-new analysis of WASP-107 b’s internal structure “has big implications,” said Benneke.

” This work deals with the extremely structures of how giant planets can form and grow,” he stated. “It offers concrete evidence that enormous accretion of a gas envelope can be triggered for cores that are much less huge than formerly believed.”

As big as Jupiter but 10 times lighter

The world is really close to its star– over 16 times closer than the Earth is to the Sun. As huge as Jupiter however 10 times lighter, WASP-107 b is one of the least dense exoplanets understood: a type that astrophysicists have actually called “super-puff” or “cotton-candy” worlds.

Piaulet and her group first used observations of WASP-107 b obtained at the Keck Observatory in Hawai’i to assess its mass more accurately. They utilized the radial velocity approach, which allows researchers to figure out a world’s mass by observing the wobbling movement of its host star due to the world’s gravitational pull.

The group then did an analysis to figure out the planet’s most likely internal structure. They concerned a surprising conclusion: with such a low density, the planet needs to have a solid core of no more than four times the mass of the Earth. This implies that more than 85 percent of its mass is included in the thick layer of gas that surrounds this core. By contrast, Neptune, which has a similar mass to WASP-107 b, just has 5 to 15 percent of its overall mass in its gas layer.

” We had a great deal of concerns about WASP-107 b,” stated Piaulet. “How could a planet of such low density type? And how did it keep its substantial layer of gas from getting away, particularly given the planet’s close distance to its star?

” This motivated us to do an extensive analysis to determine its development history.”

A gas giant in the making

Classical designs of gas-giant world formation are based on Jupiter and Saturn.

Without a massive core, gas-giant planets were not believed able to cross the crucial limit necessary to build up and retain their large gas envelopes.

How then do describe the existence of WASP-107 b, which has a much less enormous core? McGill University teacher and iREx member Eve Lee, a world-renowned expert on super-puff planets like WASP-107 b, has a number of hypotheses.

” For WASP-107 b, the most plausible situation is that the world formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very rapidly,” she said. “The world was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.”

Discovery of a 2nd planet, WASP-107 c

The Keck observations of the WASP-107 system cover a lot longer amount of time than previous research studies have, permitting the UdeM-led research group to make an extra discovery: the presence of a second world, WASP-107 c, with a mass of about one-third that of Jupiter, substantially more than WASP-107 b’s.

WASP-107 c is also much further from the main star; it takes 3 years to finish one orbit around it, compared to just 5.7 days for WASP-107 b. Also interesting: the eccentricity of this 2nd planet is high, indicating its trajectory around its star is more oval than circular.

” WASP-107 c has in some aspects kept the memory of what took place in its system,” stated Piaulet. “Its terrific eccentricity mean a rather chaotic past, with interactions between the worlds which could have resulted in considerable displacements, like the one suspected for WASP-107 b.”

Numerous more questions

Beyond its development history, there are still many secrets surrounding WASP-107 b. Research studies of the world’s atmosphere with the Hubble Area Telescope released in 2018 revealed one surprise: it consists of extremely little methane.

” That’s odd, because for this type of world, methane ought to be plentiful,” said Piaulet. “We’re now reanalysing Hubble’s observations with the new mass of the planet to see how it will impact the results, and to analyze what systems may describe the destruction of methane.”

The young researcher plans to continue studying WASP-107 b, ideally with the James Webb Space Telescope set to introduce in 2021, which will provide a much more precise idea of the composition of the planet’s environment.

” Exoplanets like WASP-107 b that have no analogue in our Planetary system permit us to much better comprehend the systems of world formation in general and the resulting range of exoplanets,” she said. “It encourages us to study them in fantastic detail.”

Recommendation: “WASP-107 b’s density is even lower: a case study for the physics of gas envelope accretion and orbital migration” by Caroline Piaulet, Björn Benneke, Ryan A. Rubenzahl, Andrew W. Howard, Eve J. Lee, Daniel Thorngren, Ruth Angus, Merrin Peterson, Joshua E. Schlieder, Michael Werner, Laura Kreidberg, Tareq Jaouni, Ian J. M. Crossfield, David R. Ciardi, Erik A. Petigura, John Livingston, Courtney D. Dressing, Benjamin J. Fulton, Charles Beichman, Jessie L. Christiansen, Varoujan Gorjian, Kevin K. Hardegree-Ullman, Jessica Krick and Evan Sinukoff, 18 January 2021, Astronomical Journal
DOI: 10.3847/1538-3881/ abcd3c

In addition to Piaulet (iREx Ph.D. student, Université de Montréal) and professors Björn Benneke (iREx, Université de Montréal) and Eve Lee (iREx, McGill Space Institute, McGill University), the research study team consists of Daniel Thorngren (iREx Postdoctoral Fellow, Université de Montréal) and Merrin Peterson (iREx M.Sc trainee), and 19 other co-authors from Canada, the United States, Germany and Japan.


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