JWST sees an asteroid belt around the star Fomalhaut!
Three huge dusty structures surround the nearby naked-eye star
May 9, 2023 Issue #562
It’s a big Universe. Here’s a thing about it.
Well, Fomalhaut just got a lot more interesting!
New JWST observations of the nearby star were expected to show more detail on a ring of material we already knew existed around Fomalhaut, but what they wound up revealing are two more rings of dust, one of which corresponds roughly to our asteroid belt. Even more, they show a huge cloud of dust in the outer ring that looks like the expanding debris from a planetesimal collision! [Link to paper]
Yeah, a lot to unpack. Let’s take a wee step back.
Fomalhaut is a star with about twice the mass and diameter of the Sun. It’s hotter, too, making it about 16 times more luminous than the Sun. It’s only 25 light-years from us, one of the closest stars to the Sun, and therefore one of the brightest in the sky. It’s the brightest star in the constellation of Pisces Austrinus, the southern fish, and its name in Arabic literally means “the mouth of the fish (or whale)”. It’s easily visible to the naked eye in the fall months low to the southern horizon for most northern hemisphere observers.
Physically speaking, it’s a very young star, probably only a couple of hundred million years old, and that turns out to be important.
In the early 80s something fishy — haha — popped up about Fomalhaut. IRAS, an early infrared satellite, spotted more infrared light coming from the star than astronomers expected. The suspicion was that it might be circled by a ring or disk of dust: Grains of rocky material warmed by the star. Warm matter emits infrared light, so that would explain the excess.
In the late 90s a ring was directly seen by infrared telescopes, and has since been observed in detail by observatories like ALMA and Hubble Space Telescope. The images are dramatic: A sharply defined ring circling the glaring star, making it look very much like the Eye of Sauron from the “Lord of the Rings” movies.
That ring is pretty far out from the star, about 140 times the Earth-Sun distance, more than 20 billion kilometers. That’s over 4 times farther out than Neptune from the Sun! Our own solar system has a donut-shaped collection of icy, rocky bodies out past Neptune called the Kuiper Belt. Fomalhaut’s ring is an analogous feature.
Another surprise — a big one — came when JWST peered back at Sauron. JWST sees pretty far out into the infrared, a kind of light well past what our eyes are sensitive to. It easily spotted that ring, but it also saw two more structures: a disk of dusty material very close in to the star, and another ring a bit farther out, separated from the inner disk by a visible gap.
And yes, that’s a big deal. Why? Well, first of all that inner disk is analogous to our own asteroid belt! It’s not clear how close it gets to the star; it can be seen as close as 1.5 billion kilometers (roughly the distance of Saturn from the Sun) but might extend closer in; the star is too bright and overwhelms the disk to be able to tell.
An asteroid belt has never been clearly seen around another star before, so that’s exciting. The dust is very likely from collisions between asteroids in the belt. As the big rocks collide they generate shrapnel, from big chunks down to teeny grains. Warmed by the star, they emit the infrared light JWST spotted.
But that gap! There’s a clear delineation between the newly discovered inner disk and the newly discovered inner ring (seen in two different JWST filters, showing it’s real and not some observational effect). By far the most likely cause of that is a planet sculpting the dust gravitationally. As a planet orbits the star, its gravity pulls on the dust, leaving behind a gap.
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