Astronomers spot uniquely bloated, asymmetrical exoplanet

An international group of researchers, including astronomers from the University of Arizona, has used NASA’s James Webb Space Telescope to observe the atmosphere of a hot and particularly bloated exoplanet. This exoplanet, which is the size of Jupiter but only a tenth of its mass, has an east-west asymmetry in its atmosphere, meaning that there is a significant difference between the two edges of its atmosphere.

The results are published in the journal Astronomy of nature.

“This is the first time that the east-west asymmetry of an exoplanet has been observed as it transits in front of its star, from space,” said Matthew Murphy, lead author of the study and a graduate student at the University of Alberta’s Steward Observatory. A transit occurs when a planet passes in front of its star, similar to the Moon’s solar eclipse.

“I think there are many advantages to observations from space over observations from Earth,” Murphy said.

The east-west asymmetry of an exoplanet refers to differences in atmospheric characteristics, such as temperature or cloud properties, observed between the planet’s eastern and western hemispheres. Determining whether or not this asymmetry exists is crucial to understanding the climate, atmospheric dynamics, and weather of exoplanets, planets that exist beyond our solar system.

The exoplanet WASP-107b is tidally locked around its star. This means that the exoplanet always shows the same face to the star it orbits. One hemisphere of the tidally locked exoplanet constantly faces the star it orbits, while the other hemisphere is always rotated in the opposite direction, giving the exoplanet a permanent dayside and a permanent nightside.

Murphy and his team used transmission spectroscopy with the James Webb Space Telescope. It’s the primary tool astronomers use to better understand the composition of other planets’ atmospheres, Murphy said. The telescope took a series of snapshots as the planet passed in front of its host star, encoding information about the planet’s atmosphere.

Using new techniques and the unprecedented precision of the James Webb Space Telescope, researchers were able to separate signals from the eastern and western sides of the atmosphere and gain a more precise look at the specific processes occurring in the exoplanet’s atmosphere.

“These snapshots tell us a lot about the gases in the exoplanet’s atmosphere, the clouds, the structure of the atmosphere, the chemistry and how everything changes when it receives different amounts of sunlight,” Murphy said.

The exoplanet WASP-107b is unique in that it has a very low density and relatively weak gravity, resulting in a puffier atmosphere than other exoplanets of its mass.

“We don’t have anything like this in our own solar system. It’s unique, even among the exoplanet population,” Murphy said.

WASP-107b reaches a temperature of about 890 degrees Fahrenheit, a temperature intermediate between that of the planets in our solar system and that of the hottest known exoplanets.

“Traditionally, our observational techniques don’t work as well for these intermediate planets, so there are a lot of exciting open questions that we can finally start to answer,” Murphy said. “For example, some of our models have told us that a planet like WASP-107b shouldn’t have this asymmetry at all. So we’re already learning something new.”

Researchers have been studying exoplanets for nearly two decades, and numerous observations from Earth and space have helped astronomers guess what exoplanet atmospheres would be like, said Thomas Beatty, study co-author and assistant professor of astronomy at the University of Wisconsin-Madison.

“But this is really the first time we’ve observed these kinds of asymmetries directly in the form of transmission spectroscopy from space, which is the primary way we understand what exoplanet atmospheres are made of. It’s really incredible,” Beatty said.

Murphy and his team have been working with the observational data they collected and plan to look in much more detail at what’s happening with the exoplanet, including additional observations, to understand what’s driving this asymmetry.

“For most exoplanets, we can’t even observe them directly, let alone know what’s happening on one side versus the other,” Murphy said. “For the first time, we’re able to get a much more localized view of what’s happening in an exoplanet’s atmosphere.”