As this first growing satellite orbited within the dusty disk, it would have left a spiral wake in its path. This wake, as well as increased drag from the remaining gas in the disk, steadily pulled the moon closer to Jupiter. Eventually, the newborn moon reached the inner edge of the disk, exited the feeding zone, and, as the paper describes, halted its “migratory trek” inward.
Finally, this same process repeated, leading to the sequential creation of Jupiter’s four Galilean moons, working from inside to out. According to Batygin, Jupiter’s innermost moons — Io and Europa, respectively — formed in only about 6,000 years. Ganymede, the next closest, took about 30,000 years.
However, according to the theory, by the time icy Callisto started to coalesce, the strengthening Sun had evaporated much of the gas that was initially in Jupiter’s disk. So, although Callisto reached about half of its final mass in just 50,000 years, it took nearly 9 million years to accumulate the rest.
Same, but different
While elements of this theory have been suggested before, Batygin says their version includes a new understanding of how dust trap work. This, they claim, resolves the longstanding difficulty of accounting for all, not just some, of the matter needed to form the Galilean moons we see today.
The theory also explains how the orbits of Io, Europa, and Ganymede developed their striking orbital resonance, which has fascinated scientists for centuries. For every single orbit outer Ganymede makes, Europa makes two, and innermost Io makes four — returning to their initial setup every 172 hours. According to Batygin, their new theory explains how this relationship could have developed in a stable way: Europa first locks into a pattern with Io, then Ganymede later syncs up with Europa.
Although the team has been working on the theory since 2018, it wasn’t until last year that observations confirmed their logic and calculations.
In 2019, astronomers observing the PDS 70 system, located some 370 light-years away, found the first example of a moon-forming disk circling an exoplanet. And, as Batygin says, “It was far dustier than anyone could have guessed.” The theory was further bolstered by additional observations in 2019 that showed signs of gas circulating through the HD 163296 system, much like the new model predicts.
Searching for exoplanets isn’t just about looking for ET. Another large motivator is that learning about distant worlds can reveal a lot about our own solar system, as well as Earth itself.
For instance, Miki Nakajima, an assistant professor at the University of Rochester, tells Astronomy she believes insights from exoplanets also inspired Batygin’s well-known prediction of Planet Nine, a hypothesized super-Earth lurking in the outer solar system.
“Observations of exoplanet systems have provided us windows to observe events that likely happened in the solar system in the past,” she says, noting that this is motivating scientists to explore new formation mechanisms. “One of the novel parts of this work is to provide a complete history of the moons,” Nakajima says. She doesn’t think it would have been possible to test these ideas without the numerical simulations that exist now.
Jonathan Lunine, a planetary scientist at Cornell University who put forward one of the earlier influential models of how Jupiter’s moons may have formed, tells Astronomy he’s interested in knowing more about the formation of Ganymede and Callisto. Though these two moons are similar in size, they have drastically different geological histories. Lunine says future missions to the moons could take more detailed measurements of their gravitational fields, revealing more about their interiors and providing insights into the differences between the two.
Extending the new moon model
Though the new research focuses specifically on spelling out the history of Jupiter’s icy moons, Batygin believes its concepts could also apply to Saturn, as well as distant gas giant exoplanets. Nakajima, for one, says she would love to see simulations replicating the formation of Saturn’s moons.
And while further developing our understanding of the origins of the solar system’s icy moons could lead in many interesting directions, Batygin says it’s particularly intriguing due to their potential — however unlikely — of hosting extraterrestrial life.
Batygin says that new theories and planned missions — such as ESA’s JUpiters ICy moons Explorer (JUICE) mission and NASA’s Dragonfly mission to Saturn’s largest moon Titan — will reveal many unexpected and fascinating insights moving forward.
“In terms of what we will learn over the next decade,” he says, “the icy moons will come into remarkably sharp focus.”
