Researchers observing with the NASA/ESA/CSA James Webb Space Telescope have located silicate clouds in the atmosphere of a distant planet. The atmosphere rises, mixes and moves constantly during the 22-hour day, bringing hotter material up and pushing colder material down. The resulting changes in brightness are so dramatic that it is the most variable planetary-mass object known to date. The science team also made extremely clear detections of water, methane and carbon monoxide using Webb’s data and found evidence of carbon dioxide. This is the largest number of molecules ever identified at one time on a planet outside our solar system.

The planet, cataloged as VHS 1256 b, is about 40 light-years away and orbits not one, but two stars over a period of 10,000 years. “VHS 1256 b is about four times farther from its stars than Pluto is from our sun, making it a great target for Webb,” said science team leader Brittany Miles of the University of Arizona. “That means the light from the planet is not mixed with light from the stars.” Higher in the atmosphere, where the silicate clouds are churning, temperatures reach a scorching 830 degrees Celsius.

In those clouds, Webb detected both larger and smaller silicate dust grains, which are shown on a spectrum. “The finer silicate grains in its atmosphere may look more like tiny particles in smoke,” noted co-author Beth Biller of the University of Edinburgh in the United Kingdom. “The larger grains may look more like very hot, very small sand particles.”

VHS 1256 b has low gravity compared to more massive brown dwarfs, meaning the silicate clouds can appear higher in the atmosphere and stay where Webb can detect them. Another reason why the air is so turbulent is the age of the planet. In astronomical terms, it is quite young. Only 150 million years have passed since it formed – and it will continue to change and cool for billions of years.

Exoplanet VHS 1256 b (NIRSpec and MIRI emission spectrum)
A research team led by Brittany Miles of the University of Arizona 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 capture a huge portion of near- to mid-infrared light emitted from the planet VHS 1256 b. They plotted the light on the spectrum and identified signatures of silicate clouds, water, methane and carbon monoxide. They also found evidence of carbon dioxide.

[Image Description: Graphic titled “Exoplanet VHS 1256 b Emission Spectrum.” The label at top right reads NIRSpec and MIRI, IFU Medium-Resolution Spectroscopy. The spectrum is plotted on a graph with y- and x-axes. The graph shows jagged lines. There are labels for water, carbon monoxide, methane, and silicates.]

Credits: NASA, ESA, CSA, J. Olmsted (STScI), B. Miles (University of Arizona), S. Hinkley (University of Exeter), B. Biller (University of Edinburgh), A. Skemer (University of California, Santa Cruz)

In many ways, the team considers these findings to be the first “coins” drawn from a spectrum that researchers consider a treasure trove of data. In many ways, they have only just begun to identify the content. “We have identified silicates, but a better understanding of which grain sizes and shapes correspond to specific types of clouds will require a lot of additional work,” Miles said. “This isn’t the last word on this planet — it’s the start of a large-scale modeling effort to fit Webb’s complex data.”

While all the features the team observed have been observed by other telescopes on other planets elsewhere in the galaxy, 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 co-author Andrew Skemer of the University of California, Santa Cruz. “We see many molecules in a single spectrum from Webb that detail the planet’s dynamic cloud and weather systems.”

The team reached these conclusions by analyzing data known as spectra collected by two instruments aboard Webb, the Near-Infrared Spectrograph (NIRSpec) and the Mid-Infrared Instrument (MIRI). Because 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 collect this data.

There will be much more to learn about VHS 1256 b in the coming months and years 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,” added Bill. “With just a few hours of observations, we have a seemingly infinite potential for additional discoveries.”

What could become of this planet billions of years from now? Because it is so far from its stars, it will get colder over time and the sky may go from cloudy to clear.

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

Magazine reference

  1. The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 Micron Spectrum of the Planetary-Mass Companion VHS 1256-1257 b,” will be published in The Astrophysical Journal Letters