Asteroid Belt

Webb Examined an Asteroid Belt and Found More Than it Bargained For

One of the things astronomers would love to see is planets forming around other stars. That would help us understand our own Solar System better. But it all happens behind a veil of obscuring dust. The James Webb Space Telescope has the power to see through the veil.

A team of astronomers pointed the JWST at the well-known star Fomalhaut and its dusty debris disk. They found more complexity than they imagined, including hints of planets forming among all that dust and debris.

The star Fomalhaut is fairly well-known, even if its pronunciation is vexing. It’s a young, nearby star in the constellation Piscis Austrinus, the “Southern Fish.” It’s a main-sequence star almost twice as massive as the Sun and is about 25 light years away.

Fomalhaut’s debris disk is considered archetypal based on observations so far. Astronomers have studied Fomalhaut with other telescopes like the Hubble and ALMA and have observed the disk, but not in very satisfying detail. Still, those observations of Fomalhaut and other nearby debris disks have fleshed out our knowledge of how these disks form and evolve.

But there are unanswered questions. For example, astronomers are still puzzling over how water was distributed when our Solar System formed. Could observations of other still-forming systems and disks help us answer that?

Hopefully, they can, but our earlier observations lacked the JWST’s power and only showed us the outer disks, similar to the Kuiper Belt in our Solar System. Did astronomers working with the Hubble know that their images of Fomalhaut’s disk were just a tantalizing taste of what the JWST would reveal?

Hubble captured this image of Fomalhaut in 2008. At the time, scientists thought they had identified an exoplanet in the disc named Fomalhaut b. Subsequent studies suggest there’s no planet there. But it’s almost a certainty that somewhere in that dusty disk, a planet or planets are forming. Image Credit: NASA, ESA and P. Kalas (University of California, Berkeley, USA)

When astronomers pointed the JWST at Fomalhaut and its asteroid belt, they were surprised by what they saw. The belt has a more complex structure than expected, and some of the complexity could signal the presence of planets.

“I think it’s not a very big leap to say there’s probably a really interesting planetary system around the star.”

George Rieke, science team lead for the JWST’s MIRI

The belts are more correctly called debris disks and, in some respects, are similar to the asteroid belt in our Solar System. Both are populated by debris left over from collisions and the Solar System’s formation. It’s the first debris disk seen outside of our Solar System, and the Webb observations are the first infrared images of the disk.

Hubble has been a workhorse that’s changed the history of astronomy, but the Webb was built to take us deeper, and in this case, it has.

The JWST image of Fomalhaut reveals more detail in the disk, and more detail means a better idea of what’s taking place in the young solar system. The space telescope’s powerful infrared instruments caught the glow of dust in the disk’s inner regions, revealing the presence of multiple rings and other features, something that is beyond the power of other telescopes.

This image of the Fomalhaut system, captured by Webb’s Mid-Infrared Instrument (MIRI), shows compass arrows, a scale bar, and a colour key for reference. Labels indicate the various structures. It reveals the presence of an inner disk, an intermediate belt, and an outer ring, as well as a halo. Scientists think that the highlighted dust cloud could be from a collision between two protoplanets rather than a still-forming planet. Image Credit: NASA, ESA, CSA, A. Pagan (STScI), A. Gáspár (University of Arizona)

A team of astronomers from the University of Arizona, JPL, the Space Telescope Science Institute, and other institutions presented these new images and results in a paper in Nature Astronomy. It’s titled “Spatially resolved imaging of the inner Fomalhaut disk using JWST/MIRI.” The lead author is András Gáspár, an Assistant Research Professor at the Steward Observatory at the University of Arizona.

“I would describe Fomalhaut as the archetype of debris disks found elsewhere in our galaxy because it has components similar to those we have in our own planetary system,” said lead author Gáspár. “By looking at the patterns in these rings, we can actually start to make a little sketch of what a planetary system ought to look like — if we could actually take a deep enough picture to see the suspected planets.”

Schuyler Wolff, also an Assistant Research Professor at Steward Observatory, is one of the paper’s co-authors. “Where Webb really excels is that we’re able to physically resolve the thermal glow from dust in those inner regions. So you can see inner belts that we could never see before,” she said in a press release.

Unseen, still-forming planets are likely behind the multiple rings. One line of thinking says that as a planet forms, it takes up an orbit and sweeps out a gap in the dust. These are the telltale gaps between rings in the image. ALMA has imaged these gaps in many other young systems, too.

ALMA’s high-resolution images of nearby protoplanetary disks are the results of the Disk Substructures at High Angular Resolution Project (DSHARP). Astronomers think that the gaps in the disks are from planets still forming and carving gaps in the disk. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

Even though the JWST is the most powerful new telescope in astronomers’ toolkits, its predecessors are still contributing. Each telescope is different from the others, and the best results arise from observing with multiple telescopes. This provides a holistic view beyond the capability of a single telescope. It’s certainly true in this case, where Hubble and ALMA can see the cooler outer disks but not the inner, warmer region.

“With Hubble and ALMA, we were able to image a bunch of Kuiper Belt analogues, and we’ve learned loads about how outer disks form and evolve,” said Wolff. “But we need Webb to allow us to image a dozen or so asteroid belts elsewhere. We can learn just as much about the inner warm regions of these disks as Hubble and ALMA taught us about the colder outer regions.”

“We definitely didn’t expect the more complex structure with the second intermediate belt and then the broader asteroid belt,” added Wolff. “That structure is very exciting because any time an astronomer sees a gap and rings in a disk, they say, ‘There could be an embedded planet shaping the rings!'”

Earlier Hubble images of Fomalhaut showed a dusty clump that suggested the presence of a planet, which astronomers called Fomalhaut b, following planet-naming convention. At the time, some thought it was the first directly-imaged exoplanet. But that conclusion was always contentious, and others thought it could’ve been something else, like a collision between two planetesimals.

Astronomers call it the Great Dust Cloud (GDC), and follow-up images showed that the GDC was fading and broadening, indicating that it was an expanding dust cloud. The JWST observations also pour cold water on the exoplanet explanation. The authors explain that the GDC could’ve resulted from a “catastrophic collision between two objects of 355 km radius at a velocity of 360 m s-1.”

This panchromatic gallery of Fomalhaut images shows how different telescopes can contribute to an overall, holistic study of an object. Depending on the wavelength of the observations, Fomalhaut looks much different, and each telescope tells astronomers something different. It’s easy to see how the JWST is advancing our understanding of Fomalhaut and other objects it images. It has the unique ability to resolve more detail. Image Credit: Gaspar et al. 2023.

While the GDC may have turned out to be a bit of a mirage, there’s no mistaking the disks, rings, and gaps seen in the Fomalhaut system. Gravitational forces from unseen planets can create these gaps in the same way that Jupiter shepherds asteroids in our Solar System. Jupiter’s powerful gravity keeps the asteroid belt contained the way it is, and Neptune has a similar effect, sculpting the Kuiper Belt’s inner edge.

One of the JWST’s stated science themes is to look inside still-forming solar systems to learn more about the planet formation process. The Webb will image more of these disks in other systems, and we’ll slowly learn more about the configuration of planets in young systems.

But right now, it’s still a bit of a mystery.

George Rieke is the US science lead for the JWST’s Mid-Infrared Instrument (MIRI) and a co-author of the new paper. Rieke likens the situation to a mystery novel. “The belts around Fomalhaut are kind of a mystery novel: Where are the planets?” asked Rieke. “I think it’s not a very big leap to say there’s probably a really interesting planetary system around the star.”

Even though there’s some mystery involved, and even though the planet formation process is not fully understood, there are intriguing similarities between the Fomalhaut system and our own Solar System, as well as the disks in other systems. Astronomers have found that debris disks are separated into a warmer inner disk and a colder outer disk. Our Solar System conforms to this, with the relatively warm asteroid belt and the colder Kuiper Belt. A third, more distant structure called the Oort Cloud may also exist.

The relative sizes of the inner Solar System, Kuiper Belt and the Oort Cloud. The Oort Cloud is theoretical and might be the home of long-period comets, but nobody’s ever seen it. Ironically, solid evidence for its existence might be found in other solar systems with similar structures. (Credit: NASA, William Crochot)

While there are marked similarities between Fomalhaut and our Solar System, there are also differences, especially when it comes to the inner disk. “Our observations show that the Fomalhaut inner disk system is quite different from that of our own Solar System,” the paper states. Our main asteroid belt is narrow and spans from about 2.1 astronomical units (AU) to 3.3 AU. As mentioned earlier, the massive Jupiter plays a role by shepherding the belt.

But Fomalhaut’s inner region hosts an extended disk. Which arrangement is more common? An answer to that question will tell astronomers a lot about how Solar Systems form. And one might be forthcoming, thanks to the JWST and research like this. “Our upcoming observations of Vega and Epsilon Eridani system and other JWST observations of nearby debris disks should help to steer the discussions on this subject,” the authors write.

Epsilon Eridani is known to have a disk structure similar to the Kuiper Belt. It also has two other belts: an inner and an outer asteroid belt. The disk has a clumpy structure, an indication that planets are in their somewhere, carving out rings and shepherding debris. The tentative exoplanet, Epsilon Eridani b, appears to help shape the inner belt by defining that belt’s outer boundary. Other proposed planets lie at the inner and outer boundaries of the Eridani’s outermost belt, the analogue of the Kuiper Belt.

Comparison of the planets and debris belts in the Solar System to the Epsilon Eridani system. At the top are the asteroid belt and the inner planets of the Solar System. Second from the top is the proposed inner asteroid belt and planet b of Epsilon Eridani. The lower illustrations show the corresponding features of the two stars’ outer systems. The JWST is poised to reveal a clearer picture of the Epsilon Eridani system and its structure. Image Credit: By NASA/JPL-Caltech –, Public Domain,

It seems certain that the JWST’s powerful infrared imaging capabilities will tell us more about Epsilon Eridani. But that work is still in the future. For now, the researchers are still digesting what Webb revealed about Fomalhaut.

To the authors of this paper, Fomalhaut’s intermediate belt was also a surprise. “Perhaps our most intriguing result is the detection of an intermediate belt in the Fomalhaut system,” the authors write. Its width spans from about 7 to 20 AU. Compare that to our Solar System, where the main asteroid belt spans from 2 to 3.25 AU, and the Kuiper Belt spans from about 30 to 50 AU. Fomalhaut’s intermediate belt has another odd property: its inclination is offset between the inner and outer regions.

This figure from the study shows the offset in the intermediate belt. The straight blue and red vector lines highlight the offset between the intermediate belt’s inner and outer regions. Image Credit: Gaspar et al. 2023.

The Fomalhaut system is full of surprises. One of the most interesting aspects of these results is that astronomers have been looking at Fomalhaut for a long time, and with each new telescope, something new is revealed. It’s like a case study for scientific progress. What will astronomers find next?

At the end of their paper, the authors summarize the impact the JWST observations have had and how they flesh out our understanding of this fascinating system.

“The structures of the massive debris belts, their alignment offsets, and the indications of massive collisional events (Fomalhaut b and the GDC) all highlight that the 440 Myr old star Fomalhaut is surrounded by a complex planetary system undergoing dynamical perturbations.”


Evan Gough

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