On a clear winter night, stargazers in the Northern Hemisphere can get a good look at AB Aurigae. Hovering some 509 light years away from Earth—about 190 billion miles, or 68 times the width of Neptune’s orbit—the star is part of the constellation Auriga, which in Latin means “the charioteer.”
Over her last several trips around the Sun, Cassandra Hall’s eye has been trained on AB Aurigae—or, more accurately, on a disk-shaped cloud of swirling dust orbiting around it. The cloud contains an embryonic planet, perhaps more than one, its massive, spiral-shaped arms coalescing on a cosmic time scale into something(s) more discrete.
It’s this process of planet formation that has Hall looking heavenward. The evidence she and her colleagues have collected suggests that the circumstellar disk dancing around AB Aurigae is transforming not through the canonical theory of core accretion—bits of space rock slowly accumulating into progressively larger bodies—but a long-theorized process called gravitational instability (GI).
In a September 2024 paper published in Nature, Hall and her co-investigators (on a team led by Jessica Speedie, then a Ph.D. student at Canada’s University of Victoria) presented the first hard evidence for GI from AB Aurigae. This publication landed Hall in The New York Times, just a year after she was featured in Wired magazine for putting forth the idea of a “photosynthetic habitable zone” in which planets might feature not only liquid water as a precondition for life but also whose distance from their host star could allow for photosynthesis.
“We’re small but mighty.”
– Assistant Professor Cassandra Hall on UGA’s astronomy program
Both projects made headlines around the world, particularly in stargazing circles. Pretty good work for someone from an astronomy program of only three faculty. (“We’re small but mighty,” Hall said.)
However, if researchers like Hall and her colleagues keep doing things that reshape our understanding of the formation of our universe, the reputation of UGA’s astronomy program soon will far outstrip its modest size.
Occam’s telescope
Hall is a computational astrophysicist. She applies for time at telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert of northern Chile—time that is awarded very competitively to astronomers worldwide—then runs the collected data through supercomputers to analyze and conduct simulations of cosmic events. Until the Nature paper, that’s all GI researchers had to go on: simulations.
“When I first started working on gravitational instability in 2013, it was kind of considered—I don’t want to say a fringe theory, because that’s not true,” said Hall, assistant professor of computational astrophysics in the Franklin College of Arts and Sciences Department of Physics and Astronomy. “It was like it was acknowledged that it was important, but it had never been definitively observed.”
When she arrived at UGA in 2020, Hall was already making plans to change this. That year she was lead author on a paper in The Astrophysical Journal that predicted how GI would influence movement in planet-forming disks, based on the research team’s simulations. Four years later, one reason the Nature paper made such a splash was that the data collected from ALMA matched their predictions.
“We had tantalizing hints that AB Aurigae was gravitationally unstable—it was massive, it had those spiral arms and some protoplanet candidates that seemed to be quite large, and it made us think this was probably the best chance we’d have for catching GI in action,” Hall said. “What we really needed was the kinematic evidence of how the disk was moving.”
ALMA provided just what they needed. There were the images themselves, gathered through ALMA’s array of 66 radio telescopes. Taking pictures of a star system 509 light years away, Hall and her colleagues could see features roughly the size of the distance between Earth and the sun. “That doesn’t sound like a lot,” Hall said, “but it’s like looking at your ceiling and seeing something 7,000 times smaller than the width of a human hair.”
Among the differences between GI planet formation and formation via core accretion is how the gas and debris in the circumstellar disk move. Core accretion works exactly how it sounds: Cosmically tiny particles accumulate toward the disk’s center and develop into kilometers-sized planetesimals, which themselves begin merging into larger objects. It is essentially a bottom-up or center-out process.
GI works more outside-in. Cooling temperatures in the spiral arms produce localized variations in gravity, which collapse first into themselves and then into each other. Conventional thinking has held that GI formation occurs only in very young disks and produces only gas giants akin to Jupiter, but a growing body of observational evidence contradicts this thinking. This inconsistency has led astronomers to posit all sorts of tweaks or special circumstances to both the GI and core accretion models to try to explain what they were seeing.
Hall and her colleagues helped answer some of the riddles. First, ALMA works at longer wavelengths than visible light, enabling it to see through clouds of cosmic gas that can obscure other types of telescopes. It also can detect movement through what are called molecular line observations, measuring the wavelengths of light emitted from molecules like carbon monoxide. These light emissions display a Doppler shift, meaning the wavelength is longer when the molecule is moving away from us, and vice versa. By taking multiple snapshots and measuring the light, the team could recreate how the disk was moving and compare to their predictions.
One of the key findings was the detection of a “GI wiggle” that the researchers had predicted through their simulations. This wiggle is produced by the localized gravity variations in the spiral arms, causing gas and dust in those areas to move at different speeds; planets forming in disks via core accretion typically produce only one local variation (at the center where the protoplanet is forming), but the AB Aurigae disk yielded up several.
Second, Hall’s colleague Cristiano Longarini from the University of Cambridge illustrated how GI could form smaller planets by calculating the theory in its entirety. In previous studies, the presence of cosmic dust had been discounted “for simplicity’s sake,” Hall said. Once Longarini did the math to account for dust, the team saw that “it should be possible for GI to form planets all the way down to roughly Earth-mass objects.”
“What I like about gravitational instability,” Hall said, “is it’s like an Occam’s razor thing. You don’t have to keep adding all these additional knobs and factors, and turn the dial and do this and that. It just exists. It comes naturally from the numerical simulations we do—if the disk cools fast enough, parts of it collapse. It can naturally explain some of the phenomena we’re seeing.”
The faults in our planetesimals
Meanwhile, as Hall was peering into the depths of the universe, Christian Klimczak was studying something much closer to home.
Vesta is the second-largest object in our solar system’s main asteroid belt, which encircles the sun between the orbits of Mars and Jupiter. Named for the Roman goddess of hearth and home, Vesta is a potato-shaped rock with a mean diameter of 326 miles, roughly the distance between Atlanta and Memphis. In September 2007, NASA launched the Dawn spacecraft, whose mission was to travel to the asteroid belt and send back data about Vesta and the protoplanet Ceres, the largest object in the belt.
Fast-forward 18 years, and Klimczak and colleagues have turned Dawn’s data into new insights about Vesta’s geologic history. At some point, the asteroid got walloped—twice—by interstellar rocks, transforming Vesta’s southern hemisphere (and likely causing its irregular overall shape). Another topographic feature, the Divalia Fossae, are Grand Canyon-sized surface troughs that encircle about two-thirds of Vesta’s equator.
It’s widely accepted that the Divalia Fossae’s origins are related to those earlier impacts, but how? Competing theories held that the impacts threw up a bunch of rock into orbit around Vesta before crashing back down to form the fossae, or that seismic waves from the impact shot through Vesta’s innards and destabilized the asteroid’s surface, causing the fossae to form.
Enter Klimczak, who used Dawn’s data to provide compelling evidence for a third explanation: tectonics.
An associate professor in the Franklin College of Arts & Sciences Department of Geology, Klimczak is a planetary geologist who studies geologic phenomenon on both his home planet and others. His hypothesis, supported by Dawn’s data, is that once those early impacts turned Vesta into a potato, the resulting changes in its rotation produced tectonic forces sufficient to reshape the planetesimal’s surface and create the fossae. He and his colleagues published their findings last June in Science Advances.
“Tectonics simply refers to large-scale motions on planetary solid surfaces that make recognizable structures,” said Klimczak, co-editor of the just-published “Planetary Tectonism across the Solar System, Volume 2” (Elsevier, 2026). “On Earth, plate tectonics is special because the surface of our planet is broken up into plates. Other planets don’t have plates, to our knowledge; they just have one large shell, but internally it can still be fractured.”
Now Klimczak plans to fly closer to the sun and renew the search for extraterrestrial plate tectonics on our neighboring planet Venus.
“Venus is a very interesting planet because it is so similar to Earth that it’s interesting to see how it evolved so differently,” Klimczak said. “When I look at spacecraft data we have for Venus, I see stuff I see on Earth every day. Venus may have had some form of early plate tectonics because we have no other way of explaining the structures we see.”
Interstellar Cosmographic Explorer
In February 2020—“Just before the world closed down,” as she tells it—Hall visited UGA as a prospective faculty member. She was a postdoctoral fellow at the University of Leicester in England who had already done work impressive enough that it would win her the Royal Astronomical Society’s Winton Capital Award later that year.
After interviewing at multiple universities, Hall chose the school in the small Southern town with a very appealing vibe.
“I just absolutely fell in love with the place,” Hall said of Athens and UGA. “I knew I’d be happy here. I wanted somewhere where I had students who would work hard with me, a place where I felt like I could make a difference in the department. Who wouldn’t choose that?”
Hall is one of nine tenure-track faculty in Franklin College’s Center for Simulational Physics, but she’s one of just three current UGA astrophysics professors out of 23 total in the Department of Physics and Astronomy (three other astrophysicists retired in 2025). With a third of the department’s 60 or so graduate students focusing on astrophysics, along with half of its 120 undergraduates majoring in astronomy, this reflects an imbalance. On top of this, the department’s observatory and telescope have been under repair since prior to Hall’s arrival.
But a new day may be dawning for UGA astronomy. Department head and Professor Phillip Stancil is looking to hire multiple faculty, including a cluster hire in planetary sciences. Historically UGA has focused on astrophysics and simulational astronomy, but Stancil said he is launching a planetary sciences initiative that will include other departments such as chemistry and geology (which Klimczak calls home). And Hall is working with the Franklin College development office to identify a donor to help get UGA’s observatory looking skyward again.
Still, one thing is clear as a starry night: Since arriving at UGA, Hall has thrived. A couple years after the Winton Award came the photosynthetic zone and GI papers and their subsequent media attention. In 2024 she delivered a well-received TEDx Talk about her hunt for coalescing exoplanets. Then, early last year, Hall was named a National Geographic Explorer, one of only 20 selected from 3,000 applicants. The honor will bring her additional research funding, as well as opportunities for professional development and training.
“When we hired Cass,” Stancil said, “we were looking for someone who was a star. And we got a star.”
Currently Hall is “working on predictions for future missions” and playing her cards close to the chest. But one thing she’s happy to share is how she feels about her move South.
“After my wife and I moved here—obviously, with the pandemic, everything was a bit crazy moving over—but when we settled, I asked her, ‘What do you think?’” Hall said. “And she was like, ‘Yeah, you were right. This is where we were meant to be.”
In 2024 Cassandra Hall delivered a TEDxUGA talk, “From Darkness to Discovery: The AI Hunt for Hidden Forming Exoplanets,” on her astrophysics research. “In her talk,” reads the description, “she encourages us to turn our fear of the celestial unknown into an opportunity for discovery.”
