May 08, 2023
Astronomers have used the James Webb Space Telescope to take an unprecedented look at the warm dust around a young star called Fomalhaut. The purpose of this observation was to study the first asteroid belt ever seen outside of our solar system in infrared light. However, the observations revealed that the dusty structures around Fomalhaut are much more complex than the asteroid and Kuiper dust belts of our solar system.
According to the observations, there are three nested belts extending out to 14 billion miles (23 billion kilometers) from the star. To put this into perspective, it's 150 times the distance between Earth and the Sun. The scale of the outermost belt is roughly twice the scale of our solar system's Kuiper Belt of small bodies and cold dust beyond Neptune. This discovery is significant because the inner belts, which had never been seen before, were revealed by the James Webb Space Telescope for the first time.
The dusty belts around Fomalhaut encircle the young hot star, which can be seen with the naked eye as the brightest star in the southern constellation Piscis Austrinus. These belts are the debris from collisions of larger bodies, analogous to asteroids and comets, and are frequently described as "debris disks". The lead author of a new paper describing these results, András Gáspár of the University of Arizona in Tucson, described 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.
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 structure interior to it. The inner belts have been resolved for the first time by Webb in infrared light. Schuyler Wolff, another member of the team at the University of Arizona, said that "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."
The discovery of these belts is significant because they are most likely carved by the gravitational forces produced by unseen planets. Similarly, inside our solar system, Jupiter corrals the asteroid belt, the inner edge of the Kuiper Belt is sculpted by Neptune, and the outer edge could be shepherded by as-yet-unseen bodies beyond it. As Webb images more systems, we will learn about the configurations of their planets.
In addition to these belts, Webb also imaged what Gáspár dubs "the great dust cloud," which may be evidence for a collision occurring in the outer ring between two protoplanetary bodies. This is a different feature from a suspected planet first seen inside the outer ring by Hubble in 2008. Subsequent Hubble observations showed that by 2014 the object had vanished. A plausible interpretation is that this newly discovered feature, like the earlier one, is an expanding cloud of very fine dust particles.
These findings are exciting for astronomers because any time a gap and rings are seen in a disk, they say, "There could be an embedded planet shaping the rings!" Webb's observations have added new complexities to the story of Fomalhaut's planetary system, and it's clear that there is much more to learn. With the continued use of the James Webb Space Telescope and the work of other observatories, astronomers hope to gain a more complete understanding of the structure and formation of planetary systems like Fomalhaut's.
The Fomalhaut star is relatively close to our solar system, at a distance of about 25 light-years. It is about twice as massive as the sun and about 16 times brighter. It is also very young, estimated to be only about 440 million years old, compared to our sun's age of around 4.6 billion years.
The three dusty belts around Fomalhaut were found to be much more complex than the asteroid and Kuiper belts in our own solar system. The outermost belt, which had been previously imaged by the Hubble Space Telescope and the Herschel Space Observatory, is roughly twice as large as our own Kuiper Belt, which extends from Neptune's orbit out to about 55 astronomical units (AU), or 5.1 billion miles (8.2 billion kilometers) from the sun. The newly discovered inner belts are located closer to the star and were only visible in infrared light, which is why they had not been seen before.
The dusty debris disks around stars like Fomalhaut are believed to be the remnants of collisions between larger bodies, such as asteroids and comets. By studying the structure of these disks, astronomers can learn more about the formation and evolution of planetary systems. The fact that Fomalhaut's disks are so complex suggests that there may be unseen planets in the system shaping the structure of the debris belts.
The James Webb Space Telescope is a powerful new tool for studying these debris disks, as it is able to resolve the thermal glow from dust in the inner regions of the disks. This allows astronomers to see inner belts that were previously invisible. By combining observations from Webb with data from other telescopes, such as the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers hope to gain a better understanding of the structure and evolution of these disks.
Overall, the discovery of these three nested belts around Fomalhaut is a significant step forward in our understanding of planetary systems beyond our own. As Schuyler Wolff, a member of the University of Arizona team studying the Fomalhaut system, put it: "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." With the James Webb Space Telescope now in operation, we can expect many more exciting discoveries like this one in the years to come.