On the Origin of Worlds: Ruobing Dong’s Quest to Explain the Rise of All Planets

That the cosmos has a penchant for spawning planets, there is no doubt. Taking our solar system, for instance, the planet:star ratio is an aggressive 8:1—or even more lopsided if you include dwarf planets like Pluto. The past few decades have suggested this bounty extends to other stars, astonishingly suggesting that trillions of exoplanets exist just in our galaxy alone.

How exactly the universe’s plethora of planets come into being, however, remains a scientific mystery. This mystery has deeply intrigued Ruobing Dong ever since his undergraduate years in the mid-aughts at Peking University in China. After advancing his studies and research abroad, Dong has just recently returned to Peking University as a full professor and a member of the Kavli Institute for Astronomy and Astrophysics (KIAA), where he plans to continue unraveling the origins of worlds.

“I want to understand how all kinds of planets form—from small, rocky, Earth-like worlds to the grandest gas giant Jupiters and beyond,” says Dong, the Cheung Kong Distinguished Professor at KIAA. “And to do that,” Dong adds, “we are focusing on the birth cradle of all the planets, which are protoplanetary discs.”

These disks of material develop around fledgling stars as the stars themselves form from the gravity-induced collapse of vast interstellar clouds of gas and dust. Within these disks, planets can plausibly arise through two contrasting pathways, simply dubbed bottom-up and top-down. The former pathway has traditionally been more popular and scientifically supported, seeming to ably explain the rise of solar system’s clutch of worlds. Bottom-up, as its name implies, proceeds as tiny grains of dust glom onto each other, growing in size and aggregating—akin to the proverbial snowball rolling down a hill—until a substantial, rocky core has taken shape. In cooler environments farther out from stars where hydrogen and helium gases can remain plentiful, those rocky cores can build up further, ultimately attracting voluminous gassy atmospheres, thus giving us gas and ice giants, à la Jupiter and Neptune, respectively.

A recent study from Dong, though, has now offered some of the strongest support yet for the alternate top-down pathway. The study—published in Nature in September 2024 and led by a Ph.D. student of Dong’s at the University of Victoria in Canada, where he previously served as a professor—has reported observational evidence of “wiggles” in a protoplanetary disk. The wiggles are caused by the motion of gas in the disk. That motion is, in turn, caused by the gravitational interactions between different parts of the disk itself. “Because of gravitational instabilities, the gas in the disk is no longer moving with a Keplerian velocity on an orbit around the star,” Dong explains. “That introduces all these ‘wiggles’ into the disk that we can hope to measure.”

The wiggles serve as evidence that portions of the disk can become unstable and collapse—similar, though on smaller scales, to how stars are born from the gravitational collapse of cosmic matter clouds. “It's the same basic physics as forming a star, where gravity works to pull things together and there’s a collapsing-together effect,” says Dong.

This top-down formation pathway would generally produce large, gassy planets, rather than work as a way for cooking up small, rocky worlds. Such a formation mechanism could help explain “super-Jupiter” exoplanets that in some cases skirt right up to the line between a maximally hefty planet and a minimally massive, so-called brown dwarf, or failed star. Yet in the right conditions, Dong says, some researchers have suggested that even worlds as relatively small as Saturn, or perhaps even Neptune—a fifth as massive as Saturn, and 17 times as massive as Earth—could arise via top-down processes.

Making the matter even more fascinatingly complex, the bottom-up and top-down pathways may co-occur within a single emerging solar system, or even operate in tandem for crafting a single world. “The real frontier of our field is figuring out if planets can form not only from bottom-up, but also top-down, and if both processes can happen in some instances,” says Dong.

Forging into this frontier has become possible thanks to leaps in instrumentation, with more on the way. For the new study, Dong and colleagues relied on observations from the Atacama Large Millimeter/submillimeter Array (ALMA)—a vast array of 66 radio antennas in the remote Atacama Desert in Chile. ALMA’s exquisite sensitivity allowed the researchers to isolate wiggle signals produced by gas in the protoplanetary disk of a system dubbed AB Aurigae. For the top-down pathway to fundamentally work, researchers have modeled that the disks in question must be particularly massive in comparison to the star they encircle. Furthermore, in order to generate a detectable signal of subtle gas motion, the disk must be a real whopper amongst whoppers. Helpfully, AB Aurigae fit the bill. “You need a superbly sensitive instrument, like ALMA, to observe a massive system with a strong enough signal, and we were fortunate to locate such a system,” says Dong.

Other observatories that will help Dong and his fellow planetary scientists peer more deeply into the workings of world-forming include JWST, NASA’s latest giant space telescope that launched a couple years ago. Looking ahead, the thirty-meter-class ground telescopes, such as the aptly named European Extremely Large Telescope (E-ELT), could also crucially move the field forward. “E-ELT should be able to get really close to detecting solar planet analogs, meaning rocky worlds like those in our Solar System, which we aren’t quite able to do now,” says Dong.

Apropos, it would seem that in the quest to understand the origins of worlds, progress is happening both bit-by-bit, analogously to the bottom-up model, and sometimes all at once, as with the top-down pathway. Dong is eager to continue the quest at KIAA and establish a planetary science community at the institute.

“I’m very excited to have come back to Peking University and KIAA and to be able to pursue my research interests here,” says Dong. “We’re learning so much about the ways that planets form, and that’s helping us understand why we have this place called Earth to call our home.”