Jess Speedie, a PhD candidate at the University of Victoria (UVic), has helped solve one of the universe's many mysteries.
Most physicists and astronomers have believed that planets form via core-accretion, whereby particles in a star's protoplanetary disk – a rotating, Frisbee-shaped cloud of gas and dust – stick together over millions of years.
“The stuff that the planets form out of is basically the leftovers of the star’s formation," said Speedie. "You have many small particles and they ... conglomerate into larger and larger objects until, eventually, you get a planet.”
For decades, however, others have theorized that planets can form in another way, as result of gravitational instability in a star’s disk. In these cases, some physicists and astronomers argued that the gaseous Frisbees can be so massive and dense that matter can drift away from stars, resulting in the formation of spiral arms.
"In the spiral arms, that's where you have regions of higher density, and those spiral arms basically fragment into clumps, and those clumps go on to further collapse into giant planets," said Speedie, who added that these planets can form in just thousands of years, as opposed to millions.
However, there has been no proof to support this theory – until now.
In 2020, Dr. Cassandra Hall, an assistant professor of computational astrophysics at the University of Georgia, predicted that the disks surrounding gravitationally unstable stars emit detectable radio-wave signatures.
“She discovered the equivalent of a COVID test, but to tell whether or not a disc is gravitationally unstable,” said Speedie.
To put her prediction to the test, in 2022, Hall and a team of 10, which included Speedie and Ruobing Dong, an assistant professor of astronomy and physics at UVic, secured time on the Atacama Large Millimeter Array (ALMA), a telescope located in a high-elevation desert in northern Chile.
ALMA’s operators soon aimed the 66-antenna machine at AB Origae – a star 529 light years away from Earth. Not long before, physicists and astronomers discovered planets circling the distant A-type star whose formation couldn’t be explained by core accretion – an indicator that the star’s disk is gravitationally unstable.
For two years, Speedie poured over the radio waves ALMA collected in search of gravitational instability signatures. Then, two years later, in 2024, Speedie found what she was looking for, proving Hall’s test accurate. This confirmed that the planets in AB Origae’s disk formed via gravitational instability – a discovery of huge significance.
"To understand the planet-formation process will help us understand how we get this diversity of planets around other stars," said Speedie, who added that the next step for future research is to observe other young stars and build statistics about how often planets form via gravitational instability.
The finding could also shed light on other mysteries, including those closer to home.
“[Planets are] where the life could be,” she said. “That's where you could have consciousness. It's not going to happen on the surface of stars, it's not going to happen on the surface of black holes. Planets are where the stories happen.”
"How did Earth form? How did Jupiter form? Is our solar system unique in this diversity of planets in the Milky Way?” Speedie added. “We want to know our own origin stories. We want to know how we got here."
