Webb Scans a Nearby Brown Dwarf and Finds it has Clouds Made of Sand

In its first few months of operation, the James Webb Space Telescope (JWST) is already proving that it was well worth the wait! To date, it has provided astronomers with the most detailed and precise images of the cosmos, conducted observations of iconic galaxies and nebulae, peered to the very edge of the universe, and obtained spectra from distant exoplanets. These resulting images, made public through the JWST Early Release Science (ERS) program, have provided a good cross-section of what this next-generation observatory can do.

Among its many objectives, the JWST will provide valuable insights into the formation and evolution of exoplanet systems through direct imaging. Using data from the ERS, an international team of astronomers and astrophysicists conducted a direct imaging study of a brown dwarf companion (VHS 1256-1257 b) orbiting within a triple brown dwarf system approximately 69.0 light-years away. The spectra they obtained from this body provided a detailed composition of its atmosphere, which included an unexpected find – clouds made of silicate minerals (aka. sand)!

The research was conducted by the JWST Early Release Science Program for Direct Observations of Exoplanetary Systems collaboration (The ERS 1386 team, for short), led by the University of California Santa Cruz (UCSC). The paper that describes their findings is the second in a series that examines direct exoplanet observations conducted by Web, both of which are currently under review. The first paper (released concurrently) examined ERS data on the exoplanet HIP 65426 b, a super Jupiter that Webb observed in the near- and mid-infrared wavelengths.

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Artist conception of the James Webb Space Telescope. Credit: NASA GSFC/CIL/Adriana Manrique Gutierrez

The ERS 1386 collaboration comprises 120 astronomers from more than 100 institutes and universities worldwide and is dedicated to directly imaging exoplanet systems in the mid-infrared range. This will include obtaining spectra from exoplanet atmospheres to determine habitability and examining circumstellar debris disks to learn more about planet formation. As the team declared during the 2018 European Planetary Science Congress, “Humankind has never observed exoplanetary systems at these wavelengths, and our observations will be transformative for understanding the chemistries and compositions of these distant worlds.”

From a technical perspective, the program’s Early Release Program is designed to assess the performance of the JWST’s observation modes that enable the direct imaging of exoplanets, planetary-mass companions, and the circumstellar disks that form them. This includes the Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) coronagraphic modes (which blocks out starlight, so exoplanets are visible) and the Near InfraRed Imager and Slitless Spectrograph (NIRSpec) aperture masking interferometry mode (which combines light from disparate sources to create images).

Dr. Aarynn Carter, a Postdoctoral Scholar at UCSC and an ERS 1386 member, was the lead author on the collaboration’s first paper. As he explained to Universe Today via email, Webb’s observations of HIP 65426 b effectively demonstrated the direct imaging capabilities of the observatory:

“These observations demonstrated that JWST is capable of obtaining precise flux measurements of exoplanets all across the near- to mid-infrared. These measurements allow us to obtain a precise constraint on the overall emitted energy, or luminosity, of HIP 65426b. In comparison to models of planetary evolution, this has, in turn, given us very precise constraints on its bulk properties such as temperature, mass, and radius. With future work, we can start to understand what these observations mean for HIP 65426 b’s atmospheric properties.”

For their latest study, the team consulted data obtained by Webb’s MIRI and NIRSpec of VHS 1256 b, a brown dwarf companion more than twenty times as massive as Jupiter and orbits at a distance of about 150 AU. These observations were conducted on July 5th, 2022, for over two hours and at wavelengths ranging from 1 to 20 micrometers. The spectra they obtained provided detailed information on VHS 1256 b’s atmospheric composition and at wavelengths never before seen with a brown dwarf.

Dr. Britanny E. Miles, a UC Presidential Postdoctoral Fellow at UC Irvine and a member of the ERS 1386 Collaboration, was the lead author on the second paper. As she told Universe Today via email:

“The near-infrared and mid-infrared show features of methane, carbon monoxide, sodium, potassium, and water. There is evidence of carbon dioxide. All of these features have been observed before in brown dwarfs of this temperature. We have never seen carbon monoxide in such detail at 5 microns, though.

“These give us the opportunity in future studies to understand how much carbon and oxygen are in the overall object, which gives a clue to how “metal-rich” it is compared to its host star. The composition of a brown dwarf can potentially give insight into ways the object may have formed.”

Miles and her colleagues also noted the direct detection of silicate clouds, making this the first instance where such a phenomenon was made for a planetary-mass companion. This and other recent spectroscopic examinations of brown dwarfs (such as a recent study based on Spitzer data) confirm that these sub-stellar mass objects produce enough heat to vaporize minerals. It also provides insight into how planetary atmospheres work, particularly for planets that are closer in size and temperature to Earth.

Brown dwarfs are too big to be planets, but not quite stars. Credit: NASA/JPL-Caltech

These results were similar to previous observations of HR 8799 c, d, and e, three exoplanets that orbit a variable K-type star about 133 light-years from Earth. These exoplanets range from an estimated 7 and 9 solar masses, likely brown dwarfs, and have similar spectra. However, the JWST provided far greater resolution and imaging capability than previous observation campaigns, further validating the sophisticated observatory and its ability to image and characterize exoplanets directly. Said Carter:

“We also determined that JWST is up to a factor of 10 more sensitive than we anticipated in these observing modes. This means we’ll easily be able to do this type of observation across a larger number of known objects. Additionally, for some stars, we will be more sensitive than what is currently possible from the ground, meaning we may be able to discover new planets too. Particularly, so far, we’ve only directly imaged objects larger than Jupiter. JWST may allow us to detect Saturn or even Uranus/Neptune analogs.”

The richly-detailed study of exoplanets is just one more way that Web is fulfilling its scientific objectives. With its advanced optics, coronographs, and spectrometers, this next-generation observatory will confirm and characterize exoplanets like never before. This will allow astronomers to complete the census of exoplanets, detect smaller rocky planets that orbit more closely with their stars, and further constrain planetary habitability. With a little luck, it may even uncover the first evidence of life beyond our Solar System.

Further reading: arXiv

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