Astronomers spot swirling, gritty clouds on remote planet

An illustration of exoplanet VHS 1256 b shows a dimly glowing, Jupiter-like planet with cloud features in its atmosphere.

This illustration conceptualizes the swirling clouds identified by the James Webb Space Telescope in the atmosphere of exoplanet VHS 1256 b. The planet is about 40 light-years away and orbits two stars that are locked in their own tight rotation. Its clouds, which are filled with silicate dust, are constantly rising, mixing, and moving during its 22-hour day.

Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI)

A research team led by Brittany Miles of the University of Arizona has pinpointed silicate, or sand-like, cloud features in a distant planet's atmosphere. Observing with NASA's James Webb Space Telescope, the astronomers observed an atmosphere that is constantly rising, mixing and moving during the planet's 22-hour day, bringing hotter material up and pushing colder material down.

The resulting brightness changes are so dramatic that it is the most variable planetary mass object known to date. The team also made extraordinarily clear detections of water, methane and carbon monoxide with Webb's data, and found evidence of carbon dioxide. This is the largest number of molecules ever identified all at once on a planet outside our solar system, according to the team's findings, which are published in The Astrophysical Journal Letters. 

Cataloged as VHS 1256 b and classified as a brown dwarf, the planet is about 40 light-years away and orbits not one, but two stars. It takes the planet a staggering 10,000 years to complete one trip around its two suns. Brown dwarfs are more massive and hotter than planets but lack the mass required to become stars. Their atmospheres can be similar to that of giant gas planets such as Jupiter.

The light spectrum shows the signatures of the various molecules discovered in the planet's atmosphere.

The team used two instruments known as spectrographs aboard the James Webb Space Telescope, one on its Near Infrared Spectrograph (NIRSpec) and another on its Mid-Infrared Instrument (MIRI), to observe a vast section of near- to mid-infrared light emitted by planet VHS 1256 b. Plotting the light on the spectrum, the researchers identified the signatures of silicate clouds, water, methane and carbon monoxide. They also found evidence of carbon dioxide.

Image: NASA, ESA, CSA, J. Olmsted (STScI); Science: Brittany Miles (University of Arizona), Sasha Hinkley (University of Exeter), Beth Biller (University of Edinburgh), Andrew Skemer (University of California, Santa Cruz)

"VHS 1256 b is about four times farther from its stars than Pluto is from our sun, which makes it a great target for Webb,” said Miles, a 51 Pegasi b Fellow and Presidential Postdoctoral Fellow at UArizona's Steward Observatory. "That means the planet's light is not mixed with light from its stars, which makes it easier for us to image directly."

Higher up in its atmosphere, where the silicate clouds are churning, temperatures reach a scorching 1,500 degrees Fahrenheit (830 degrees Celsius).

Within those clouds, Webb detected silicate dust grains ranging in size from 1 to 10 micrometers, smaller than would be visible to the unaided eye.

"The finer silicate grains in its atmosphere may be more like tiny particles in smoke," said study co-author Beth Biller of the University of Edinburgh in Scotland. "The larger grains might be more like very hot, very small sand particles."

VHS 1256 b has low gravity compared to more massive brown dwarfs, which means that its silicate clouds can appear and remain higher in its atmosphere where Webb can detect them. Another reason its skies are so turbulent is the planet’s age: In astronomical terms, it’s quite young. Only 150 million years have passed since it formed – and it will continue to change and cool over billions of years.

"As brown dwarfs get older, they collapse in size," Miles said. "A more compact size leads to higher surface gravity, which hinders motions in the atmosphere. In other words, once brown dwarfs get older, their atmospheres calm down quite a bit."

In many ways, the researchers consider their findings to be the first "coins" pulled out of a "treasure chest" of spectral data; they've only begun identifying the chest's contents.

"We've identified silicates, but better understanding which grain sizes and shapes match specific types of clouds is going to take a lot of additional work," Miles said. "This is not the final word on this planet – it is the beginning of a large-scale modeling effort to fit Webb's complex data."

Although all of the features the team observed have been spotted on other planets elsewhere in the Milky Way by other telescopes, other research teams typically identified only one at a time.

"No other telescope has identified so many features at once for a single target," said study co-author Andrew Skemer, a UArizona alumnus who is now at the University of California, Santa Cruz. "We're seeing a lot of molecules in a single spectrum from Webb that detail the planet's dynamic cloud and weather systems."

The team came to these conclusions by analyzing data known as spectra gathered by two instruments aboard Webb, the Near-Infrared Spectrograph and the Mid-Infrared Instrument. Since the planet orbits at such a great distance from its stars, the researchers were able to observe it directly, rather than using the transit technique or a coronagraph to take this data.

There will be plenty more to learn about VHS 1256 b in the months and years to come as this team – and others – continue to sift through Webb's high-resolution infrared data.

"There's a huge return on a very modest amount of telescope time," Biller said. "With only a few hours of observations, we have what feels like unending potential for additional discoveries."

The researchers observed VHS 1256 b as part of Webb's Early Release Science program, which is designed to help increase the astronomical community's ability to characterize planets and the disks where they form.

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