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Very close to us in space, about 210 light years away, there is an exoplanet that has long eluded science: Its name is WASP 107b.
Now, two different teams of astronomers think they’ve found the solution, but it’s also raised a series of new questions that require further study.
WASP 107b: An exoplanet that shouldn’t exist
Artist’s impression of WASP 107b passing in front of its star. The planet is roughly the same size as Jupiter, but has only 12% of its mass. ESA/Hubble, NASA, M. Kornmesser
Astronomers first discovered the unusual exoplanet in 2017.
Their initial observations suggest that WASP 107b is roughly the same width as Jupiter but one-tenth its mass, earning it the nickname “Super Puff,” like a fluffy marshmallow or cotton candy.
For years, experts have struggled to understand how such a planet could grow so large in diameter and remain so light in mass – as far as scientific models are concerned, this exoplanet shouldn’t exist.
“People started trying really hard to figure out how to create such a planet,” David Sing, Bloomberg Distinguished Professor at Johns Hopkins University, told Business Insider.
Normally, planets get wider, just like humans do: the more matter they absorb, the wider and more massive they become, so if this exoplanet was as big as astronomers have observed, it couldn’t be that small in mass.
Gas giants form when a cloud of gas gathers around a dense, rocky core, but models suggest that WASP 107b’s core is too small to form a gas giant. NASA Goddard Space Flight Center, ESO/L. Calzada
“WASP 107b is an outlier among outliers,” said Lewis Wellbanks, a NASA Sagan postdoctoral fellow at Arizona State University.
Now, thanks to the James Webb Space Telescope, two separate research teams, one led by Singh and the other by Wellbanks, think they’ve finally solved the mystery — and what’s more, they’ve come to remarkably similar conclusions, reinforcing each other’s findings.
The teams speculate that the answer lies in the core of WASP 107b, which turns out to be much hotter and more massive than astronomers previously thought.
But it took two teams of people doing some serious space research to reach that conclusion.
The scientific model did not match the observations
Hubble Space Telescope images of WASP 107b didn’t explain its vast size and low density, so astronomers needed a more powerful instrument: JWST. Venom 82
The reason it took astronomers years to unravel the mysterious origins of WASP 107b stems from a problem many astronomers face: a lack of information due to technical limitations.
Thanks to preliminary observations by the Hubble Space Telescope, astronomers knew some information about WASP 107b when it was discovered, but not enough to answer the big questions.
So they first turned to scientific models to fill in the gaps, and one big one was the cores of exoplanets.
Scientific models suggest that the core should be relatively small and cool, Singh said.
“That was a real mystery,” he says, because gas giants like Jupiter and WASP 107b typically need huge cores to accumulate so much gas, and these cores tend to be hot, otherwise they would naturally shrink in size due to the cooler core.
JWST allowed astronomers to observe WASP 107b once again, this time measuring gases in its atmosphere to infer its internal temperature. National Aeronautics and Space Administration (NASA)
But scientific models suggest that WASP 107b’s core is smaller than it should be — less than 4.6 times Earth’s mass, Singh said.
So astronomers clearly didn’t have the whole picture.
That’s where JWST, the most powerful telescope ever launched into space, comes in. Using this tool, Singh and Wellbanks discovered that every previous assumption about the interior of WASP 107b was wrong.
Looking beneath the surface
This diagram shows the different gases that make up WASP 107b’s atmosphere. Sing and Welbanks measured methane and found it to be less than expected, which was key to understanding WASP 107b’s core. NASA, ESA, CSA, Ralph Crawford (STScI) Science: D. Singh (JHU) and the NIRSpec GTO Transiting Exoplanet Team
To solve the mystery of WASP 107b, Wellbanks, Singh and their team used the JWST to analyze the composition of the exoplanet’s atmosphere.
The teams identified familiar species such as carbon dioxide, sulfur dioxide and water vapor, but were surprised by the unusually low amount of methane.
Methane is unstable at high temperatures, but WASP 107b’s surface temperature was cold enough that there should have been more methane present than JWST observed.
The most plausible answer to the methane mystery, Wellbanks and Singh concluded, is that hot gas from deep within the exoplanet is violently mixing with cooler gas near the surface.
“These new measurements allow us to essentially use the methane as an internal thermometer and find that it’s much hotter than expected,” Singh said.
Both teams published their findings in the peer-reviewed journal Nature.
While JWST may not be able to directly observe the planet’s interior, it can gather information about its atmosphere and shed light on what’s inside. National Aeronautics and Space Administration (NASA)
The fact that JWST’s observations suggest the core is hot also means that it’s probably much larger, which could explain why WASP 107b has a larger diameter. Indeed, both Singh and Wellbanks conclude that the core’s mass is much larger than originally estimated.
But Wellbanks and Singh’s measurements of the core mass differ — a discrepancy that needs further investigation, but the two studies essentially tell the same story, said Scott Gowdy, an astronomy professor at Ohio State University.
“The object is bulging primarily because of its high internal temperature,” said Gaudi, who was not involved in the study but is a former colleague and co-author of Wellbanks’ paper.
Collaboration is key
The results of Singh and Wellbanks’ joint study could inform future studies of other gas giants like Kepler-51. NASA, ESA, L. Hustak and J. Olmsted (STScI)
This clear picture of WASP 107b’s interior doesn’t end astronomers’ questions about the strange, super-hot gas — they still have questions about how the exoplanet’s core got so hot in the first place.
“We don’t know exactly what’s causing the high temperatures inside,” Gaudi said, but Wellbanks and Singh have a theory that Gaudi thinks might be correct.
WASP 107b’s orbit around its star is an “eccentric orbit,” meaning it’s not perfectly circular. This eccentric orbit squeezes the planet periodically, “which makes it warm up, just like if you were to pick up clay and move it around,” Wellbanks explained.
The compression that produces this heat is called tidal heating, and Gowdy thinks it may be a good explanation for why WASP 107b’s core is hotter than expected.
However, Gaudi still has some questions about the mechanism behind this tidal heating.
For example, for WASP 107b’s eccentric orbit to heat its core so much, the core would need to be very efficient at dissipating tidal heating, Gowdy said.
A NASA map of all 4,003 exoplanets discovered as of 2019. Each of these planets has its own unique characteristics that scientists like Singh and Wellbanks are helping us understand. YSTEM Sound (M. Russo, A. Santaguida); Data: NASA Exoplanet Archive
This means that every time an exoplanet orbits a star, it gets “compressed,” storing a lot of energy in its core. According to Gaudi, an exoplanet’s orbit should not remain eccentric for long periods of time, but should eventually become perfectly circular.
So why is WASP 107b’s orbit still eccentric? Did Singh and Wellbanks simply catch it at the right time, or is there something else, like the gravitational pull of a nearby planet, that’s supporting the exoplanet’s erratic orbit?
Wellbanks and Singh plan to investigate WASP 107b’s eccentric orbit, tidal heating, and other remaining questions in the future.
But for Singh, Wellbanks and Gowdy, the most important lesson from the job is that collaboration brings success.
“In a time in science where a lot of things are not reproducible, it’s very encouraging that two teams came up with the same thing so quickly,” Singh said.
“When we work together, we do greater and better science,” Wellbanks said.