In some respects, making a solar system might seem childishly simple. A cloud of gas and dust collapses under its own gravity, forming a whirling “protoplanetary” disk of debris that has a star at its center. Despite its name, though, a protoplanetary disk is a challenging environment in which to form worlds—at least, according to theorists who model the process. Now, however, new observations are revealing surprising details of how planets emerge from disks: the first worlds to form can give rise to whirlpool-like vortices, which create subsequent generations of planets. The findings were reported this month in The Astrophysical Journal Letters.
Stefan Kraus, an astronomer at the University of Exeter in England, led the team that observed a pair of vortices—as well as spiraling density waves—in a protoplanetary disk around a star more than a thousand light-years from Earth. Both features are likely due to an unseen planet, and the team’s models suggest they could boost further bouts of planet formation within the disk.
For a long time, theoretical astrophysicists have struggled to create models that accurately predict the outcomes of planet formation. Planets are made in protoplanetary disks: turbulent environments filled with blobs and streams of gas and dust, which jostle against each other while spinning around a still-forming star. The exact dynamics of planetary birth in such places are messy and not fully understood. To make things even harder, planet-formation theorists are stuck in a field that is, right now, severely lacking observations.
“Observation tells you what reality is,” says Wladimir Lyra, a theorist at California State University, Northridge. “But you need theories to explain what's happening. We feed the observations to our models in order to build theories and explain the observations. It's this synergy that advances science.”
The trouble is that observations of the innards of protoplanetary disks are hard to come by; the gas and dust in a disk block most of the light that would otherwise show the planet-forming action as it happens. But millimeter and sub-millimeter radio waves emanating from cold dust in the disk’s outer regions can escape relatively unscathed, offering astronomers a way to look inside. Arguably the best facility in the world to perform such observations is the Atacama Large Millimeter Array (ALMA) observatory, a network of radio telescopes in Chile’s Atacama Desert that came fully online in early 2013. Before ALMA, Lyra explains, astronomers had “an incomplete view.”
The latest study from Kraus and his team used ALMA to peer deep into the protoplanetary disk of V1247 Orionis, a young star around 1,044 light-years away. ALMA’s radio eyes revealed curious vortices spinning among density waves that revolved around the star, a little like the spiral arms around the Milky Way. The team’s subsequent simulations suggest that these features arose from an unseen planet—and that they could give rise to more worlds. In this scenario the birth of just one planet in a disk could spawn new features, such as vortices, which would catalyze further planet formation.
A vortex is simply a swirling mass of fluid rotating around itself. Jupiter’s Great Red Spot, a storm that is wider than Earth and has persisted for hundreds of years, is perhaps the most famous planetary example of a vortex. Earthly examples include whirlpools in Earth’s oceans and tornados in the atmosphere. Vortices can form spontaneously in any turbulent fluid, but in protoplanetary disks they can also arise as the direct consequence of a planet’s motion as it carves out a turbulent wake. The spiral arms in the ALMA images of V1247 Orionis are probably a planet’s wake, spinning out vortices like small eddies in the wake of a speeding motorboat.
According to Anders Johansen, a theorist at Lund Observatory, the study is particularly notable because it is the first to use ALMA “to match the observation of a vortex structure in a protoplanetary disk to a simulation [of planet formation].” That is, it offers a robust link between theory and observation that has, until now, eluded researchers.
The first theories of planet formation lacked detailed treatments of vortices and other complex features that modern approaches include. Yet those early theories still managed to nicely replicate the observed architecture of our solar system, with its inner retinue of rocky worlds and outer realm of giant planets. This layout mirrors basic understandings of protoplanetary disks, which are hotter near their centers and cooler at their edges. Far enough from young stars, ices would contribute substantially to planet growth, leading to larger worlds. Closer in those ices would evaporate, leaving limited amounts of rocky material that could only form modest-sized planets. Problems with this overly simplistic model developed when astronomers began discovering an abundance of exoplanets like hot Jupiters—massive, close-in worlds that, according to prevailing theories, should not exist.
In “the early theories, in the sixties and seventies,” Kraus says, “they thought it was a relatively straightforward process.” Now, he and other researchers realize that “it’s much more complicated than we originally thought.”
In order to build planets, a fledgling system first needs to make planetesimals: mountain-sized building blocks that smash together to form full planets. We can see leftover planetesimals in our own backyard—they constitute the bulk of our solar system’s supply of asteroids and comets. To make planetesimals, dust must form and somehow stick together within the protoplanetary disk long enough to build these kilometer-scale objects. However, to do this dust has to overcome a major obstacle called “the radial drift problem.”
This problem arises when dust grains experience a drag from gas in the disk. This drag should cause the particles to lose momentum and rapidly spiral inward like water down a drain, eventually falling into the star and leaving nothing in the disk from which to make planets. Because planets obviously exist, something must be missing from this picture. Vortices, this latest work suggests, might be that “missing something.”
“[Vortices] gather material,” Lyra says. “It’s like tea leaves in a teacup. As you stir tea leaves in a teacup, the tea leaves will concentrate in the center of a cup.” Through this process dust can reach the critical densities required to gravitationally collapse to form a planetesimal.
Once planetesimals form, the rest of the process is straightforward: the planetesimals clump together to form protoplanets that are somewhere between the moon and Mars in size, and these in turn coalesce into full-sized planets. Lyra describes this as a “planet formation factory,” a conveyor belt for making planets containing upwards of 10 Earth masses of material. In this model, vortices not only manufacture planetesimals to be used to make planets—they can also make fully fledged planets themselves. Based on its age, he says, our own Mars may have formed in a vortex.
“Once a planet forms and leaves a vortex, it frees [the vortex] to form another planet,” Lyra explains. “We modeled this process, and we get hundreds of moon-sized and dozens of Mars-sized planets that later coalesce to form a couple of Earth-sized and super-Earth-sized planets.”
To fully verify this model, the unseen planet needs to be found. Kraus and his team are already pursuing follow-up observations to do just that. The hope is that this future work will finally reveal the planet, completing their picture of the system and providing crucial evidence for a key step in planet formation.
“It’s like several pieces of a puzzle just fitting together really nicely [so] you can see the picture,” Kraus says. “But the one puzzle piece where the planet should be is still missing.”