JWST images the disk around a young star

One of the brightest stars in the night sky, Fomalhaut is 25 light-years from Earth and 16 times as luminous as the Sun. It’s also just a few hundred million years old. The star’s most famous feature is a massive disk of dust and debris that surrounds it. Analogous to our own solar system’s asteroid belt and the Kuiper belt, the disk was first detected by NASA’s Infrared Astronomical Satellite in 1983 and, in visible light, by the Hubble Space Telescope in 2004.

The presence of such disks around young stars is no surprise. They are the by-products of angular-momentum conservation when stars form and are created when a rotating cloud of gas collapses under its own gravity upon reaching a critical size, mass, or density. As the collapsing cloud, known as a solar nebula, becomes denser, it flattens into a disk. The solids inherited from the gas cloud are initially small—perhaps a few microns across. But after a few million years, the dust and rocky material grow into planets. (See the article by Sean Andrews, Physics Today, August 2021, page 36 .)

Hubble captured sharp details of Fomalhaut’s outermost belt, largely because the star’s high luminosity lights up the debris around it. But the telescope missed the disk’s inner structure entirely. András Gáspár (University of Arizona) and his colleagues have now used the Mid-Infrared Instrument of the James Webb Space Telescope (JWST) to fill in those inner details. As shown above, the dusty structures it imaged are much more complex than the asteroid belts in our own solar system. In particular, at an IR wavelength of 25.5 µm, it resolved three nested belts of dust—separated by gaps—that extend out to 23 billion km from the star. That’s 150 times the distance from the Sun to Earth.

NASA’s earlier IR telescopes hinted at the existence of those inner belts, but spectral analysis couldn’t resolve where they would be located. Gáspár and his colleagues argue that the belts are likely carved by gravitational forces that are produced by unseen planets in the disk. The JWST image also reveals a large dust cloud (insets, through the 23 µm and 25.5 µm filters) that was likely caused by a collision between protoplanetary bodies in the outermost ring. The researchers plan to follow this work with a systematic planet search. (A. Gáspár et al., Nat. Astron., 2023, doi:10.1038/s41550-023-01962-6 .)

Editor’s note, 15 May: The article has been updated to correct the distance of the dust belts from the star. The belts extend as far as 23 billion km from Fomalhaut, not 23 million km.