Astronomers used NASA’s James Webb Space Telescope to image the warm dust around the nearby young star Fomalhaut to study the first asteroid belt ever observed outside our solar system in infrared light. But to their surprise, the dusty structures are much more complex than the asteroids and Kuiper dust belts of our solar system.
In total, there are three nested belts extending 14 billion miles (23 billion kilometers) from the star; which is 150 times the distance of the Earth from the Sun. The extent of the outermost belt is roughly twice the scale of our Solar System’s Kuiper belt of small bodies and cold dust beyond Neptune. For the first time, Webb revealed inner bands that had never been seen before.
The belts surround a young, hot star that can be seen with the naked eye as the brightest star in the southern constellation Piscis Austrinus. Dust belts are debris from the collisions of larger bodies, analogous to asteroids and comets, and are often described as ‘debris disks’.
we have in our own planetary system,” said András Gáspár of the University of Arizona in Tucson and lead author of a new paper describing these results. “By looking at the patterns in these rings, we can actually start to make a small sketch of what the planetary system should look like if we could actually get a deep enough image to see the suspected planets.”
The Hubble Space Telescope and Herschel Space Observatory, as well as the Atacama Large Millimeter/submillimeter Array (ALMA), have previously taken sharp images of the outermost belt. However, none of them found any internal structure.
The inner bands were first resolved by Webb in infrared light. “Webb really excels in being able to physically resolve the heat glow from the dust in these inner regions. So you can see inner bands that we’ve never seen before,” said Schuyler Wolff, another team member from the University of Arizona.
Hubble, ALMA and Webb have teamed up to compile a holistic view of debris disks around a number of stars. “With Hubble and ALMA, we’ve been able to image a lot of analogues of the Kuiper belt, and we’ve learned a lot about how the outer disks form and evolve,” Wolff said.
Hubble and ALMA have cooler outer regions
“But we need Webb to allow us to image a dozen or so asteroid belts elsewhere. We can learn just as much about the hot inner regions of these disks as Hubble and ALMA have taught us about the cooler outer regions.
These bands are most likely carved by the gravitational forces produced by unseen planets. Similarly, inside our solar system, Jupiter bounds the asteroid belt, the inner edge of the Kuiper belt is shaped by Neptune, and the outer edge could be protected by as-yet-unseen bodies beyond. As Webb images more systems, we will learn about the configurations of their planets.
Fomalhaut’s dust ring was discovered in 1983 during observations made by NASA’s Infrared Astronomy Satellite (IRAS). The ring’s existence was also inferred from previous observations and observations at longer wavelengths using the submillimeter telescopes on Mauna Kea in Hawaii, NASA’s Spitzer Space Telescope, and Caltech’s Submillimeter Observatory.
“The belts around Fomalhaut are a kind of mystery novel: Where are the planets?” said George Rieke, another team member and US science chief for Webb’s Mid-Infrared Instrument (MIRI), which made the observations. “I think it’s not too much of a leap to say that there’s probably a really interesting planetary system around the star.”
“We certainly didn’t expect a more complex structure with a second middle belt and then a wider asteroid belt,” Wolff added. “This structure is very exciting because whenever an astronomer sees a gap and rings in the disk, he says, ‘There could be an embedded planet that shapes the rings!'”
Webb also imaged what Gáspár calls a “large cloud of dust,” which may be evidence of a collision that occurred in the outer ring between two protoplanetary bodies. This is a different feature from the supposed planet first seen inside the outer ring by Hubble in 2008.
The idea of a protoplanetary disk around a star dates back to the late 18th century, when astronomers Immanuel Kant and Pierre-Simon Laplace independently developed the theory that the Sun and planets formed from a rotating cloud of gas that collapsed and flattened due to gravity.
Debris disks develop later, after the planets form and the primordial gas is dispersed in the systems. They show that small bodies such as asteroids collide catastrophically, pulverizing their surfaces into huge clouds of dust and other debris. Observations of their dust provide unique clues to the structure of the exoplanetary system, which extends to Earth-sized planets and even asteroids too small to see individually.
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