Stars can be seen with the naked eye, but planets orbiting other stars are incredibly difficult to see, even with the most powerful telescopes ever made. It wasn't until 1992 that astronomers imaged an exoplanet, meaning a planet in another solar system; prior to that, some astronomers thought our solar system may be unique, and thought planets were perhaps rare.
By the 2010s, however, exoplanet discoveries were so frequent as to become quotidian; now, there are over 9,000 likely or confirmed exoplanets, according to NASA. Because of a selection bias that favors large bodies that are close to their parent stars, many of these planets are larger than Jupiter. Smaller bodies are much harder to see — and when it comes to bodies smaller than planets, like moons or asteroids, there are candidate bodies though none that are actually confirmed.
That makes the discovery of an astroid belt orbiting the star Fomelhaut all the more exciting. Thanks to new observations made using the James Webb Space Telescope (JWST), we have a good sense of the nature of the solar system around the star Fomelhaut, which appears as one of the brightest stars in the Southern Hemisphere night sky and which sits some 25 light years away from Earth. Indeed, while astronomers expected to see a lot of similar intricate details between our solar system and the Fomalhaut star system, they were surprised to find that it is more complicated than previously thought, in part because they discovered it has an unexpectedly convoluted asteroid belt.
The observations were officially published in a report on May 8 in Nature Astronomy. Previously, the Hubble Space Telescope and Herschel Space Observatory, as well as the Atacama Large Millimeter/submillimeter Array (ALMA), have taken detailed images of the outermost asteroid belt in the Fomalhaut star system. Thanks to JWST, astronomers are now able to see the inner belts for the first time, revealing a slew of surprises.
In an interview with Salon, András Gáspár, lead author of the paper and an astronomer at the University of Arizona in Tucson, said there is "so much to unpack" from this one observation.
"We originally assumed that we would see a very narrow asteroid belt," Gáspár said. But instead they found three nested belts that extend out for around 23 billion kilometers (14 billion miles), which is 150 times the distance between the Sun and Earth. "There likely aren't any gas giants orbiting close," he added, noting that in solar system there's Jupiter and Saturn. "But we do see a gap, much further [out], but that corresponds to our Neptune's orbit roughly ... it's a good sign that there could be an ice giant orbiting there."
"We originally assumed that we would see a very narrow asteroid belt."
Harvard astronomer Avi Loeb, who was not involved in the study, called the observation "exciting."
"[They show] for the first time three nested belts in the dusty debris disk around a young hot star beyond the scale of our own planetary system," Loeb said. "The dust particles in the belts are most likely shepherded gravitationally by embedded planets that had not been seen as of yet."
Loeb said the situation in space is similar to noticing waves "on the surface of a muddy pond" on Earth, which would indicate the existence of fish "hidden from view under the water."
Indeed, our own solar system's asteroid belt is considered to be a "failed planet" of sorts; there are multiple small spherical bodies that make up the asteroid belt, including Ceres, Vesta, and Pallas. But none has enough gravity to coagulate into a larger planet. That process of planetary formation, in which debris slowly accumulates enough to accure and then clear its orbital lane, is believed to be what took place during the formation of all other planets in our solar system.
Yet our own asteroid belt is influenced by planets, including as the slew of asteroids that orbit in Jupiter's shadow, which are known as trojans. Astronomers believe that the debris disks outside the Fomalhaut star system are likely shaped by unseen planets, too.
Gáspár told Salon that this star system is "really chaotic" with a lot of "dynamical activity going on."
"We definitely didn't expect the more complex structure with the second intermediate belt and then the broader asteroid belt," said Schuyler Wolff, another member of the team at the University of Arizona, in a media statement. "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!'"
Part of JWST's mission is to peer into distant corners of the universe, and survey the atmospheres of Earth-like exoplanets — or at the very least, identify them. When it comes to Fomalhaut, there are lessons to be learned regarding how our own solar system and its planets formed.
Gáspár told Salon that, based on JWST's observations, this star system is "really chaotic" with a lot of "dynamical activity going on."
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"We see this inner disc that's slightly angled away from the other one, so these are all signs [that] the system's dynamically very active," Gáspár said. "Similarly for our solar system, there are hypotheses that early on in its life, roughly at the same age, it also underwent some dynamical reorientation causing a lot of heavy bombardment happening — so it could be an analogue to what happened in the solar system early on."
One of the biggest takeaways, Gáspár said, is that the early stages of planetary formation can be a violent time that can last billions of years.
"And that's interesting, because some life evolved on Earth after the first billion years,"Gáspár said.
In other words, if there's still a lot of chaotic activity late in a planetary system's evolution, it could inhibit the ability of life to form.
Loeb said the next step is to take a more detailed image of the star system, and see if the hidden planets creating the nested belts can be identified.
"The planetary system is likely very different from the solar system, because it is likely to have more than one giant planet," he said.