How did the Solar System’s planets come to be? The leading theory is something known as the “protoplanet hypothesis”, which essentially says that very small objects stuck to each other and grew bigger and bigger—big enough to even form the gas giants, such as Jupiter. But how the heck did that happen?
About 4.6 billion years ago, as the theory goes, the location of today’s Solar System was nothing more than a loose collection of gas and dust—what we call a nebula. Then something happened that triggered a pressure change in the center of the cloud. Perhaps it was a supernova exploding nearby, or a passing star changing the gravity. Whatever the change, however, the cloud collapsed and created a disc of material.
The center of this disc saw a great increase in pressure that eventually was so powerful that hydrogen atoms loosely floating in the cloud began to come into contact. Eventually, they fused and produced helium, kick-starting the formation of the Sun.
The Sun was a hungry youngster—it ate up 99% of what was swirling around, NASA says—but this still left 1% of the disc available for other things. And this is where planet formation began.
One major challenge to this theory, of course, is no one (that we know of!) was recording the early history of the Solar System. That’s because the Earth wasn’t even formed yet, so it was impossible for any life—let alone intelligent life—to keep track of what was happening to the planets around us. There are many solutions for this problem, formost among them is observation. Using powerful telescopes such as the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers can actually observe stars with planets being born around them.
Recently NASA’s Kepler space telescope have revealed thousands of planets orbiting other stars: While the majority of these exoplanets fall into a category called super-Earths—bodies with a mass somewhere between Earth and Neptune—most of the features observed in nascent planetary systems were thought to require much more massive planets, rivaling or dwarfing Jupiter, the gas giant in our solar system.
In other words, the observed features of many planetary systems in their early stages of formation did not seem to match the type of exoplanets that make up the bulk of the planetary population in our galaxy. How exactly planets form is still an open question with a number of outstanding problems.
Kepler has found thousands of planets, but those are all very old, orbiting around stars a few billion years old, like our sun. They seems senior citizens of our galaxy, but we don’t know how they were born.
To find answers, astronomers turn to the places where new planets are currently forming: protoplanetary disks—in a sense. Although protoplanetary disks have been observed in relative proximity to the Earth, it is still extremely difficult to make out any planets that may be forming within. Rather, researchers have relied on features such as gaps and rings to infer the presence of planets.
“Among the explanations for these rings and gaps, those involving planets certainly are the most exciting and drawing the most attention,” says co-author Shengtai Li, a research scientist at Los Alamos National Laboratory in Los Alamos, New Mexico. “As the planet orbits around the star, the argument goes, it may clear a path along its orbit, resulting in the gap we see.”
Except that reality is a bit more complicated, as evidenced by two of the most prominent observations of protoplanetary disks, which were made with ALMA. The images of HL Tau and TW Hydra, obtained in 2014 and 2016, respectively, have revealed the finest details so far in any protoplanetary disk, and they show some features that are difficult, if not impossible, to explain with current models of planetary formation, Dong says.
“Among the gaps in HL Tau and TW Hya revealed by ALMA, two pairs of them are extremely narrow and very close to each other,” he explains. “In conventional theory, it is difficult for a planet to open such gaps in a disk. They can never be this narrow and this close to each other for reasons of the physics involved.”
In the case of HL Tau and TW Hya, one would have to invoke two planets whose orbits hug each other very closely—a scenario that would not be stable over time and therefore is unlikely.
While previous models could explain large, single gaps believed to be indicative of planets clearing debris and dust in their path, they failed to account for the more intricate features revealed by the ALMA observations.
The model created by Dong and his co-authors results in what the team calls synthetic observations—simulations that look exactly like what ALMA would see on the sky. Dong’s team accomplished this by tweaking the parameters going into the simulation of the evolving protoplanetary disk, such as assuming a low viscosity and adding the dust to the mix. Most previous simulations were based on higher disk viscosity and accounted only for the disk’s gaseous component.
“The viscosity in protoplanetary disks may be driven by turbulence and other physical effects,” Li says. “It’s a somewhat mysterious quantity—we know it’s there, but we don’t know its origin or how large its value is, so we think our assumptions are reasonable, considering that they result in the pattern that has actually been observed on the sky.”
Even more important, the synthetic observations emerged from the simulations without the necessity to invoke gas giants the size of Jupiter or larger.
“One super-Earth turned out to be sufficient to create the multiple rings and multiple, narrow gaps we see in the actual observations,” Dong says.
As future research uncovers more of the inner workings of protoplanetary disks, Dong and his team will refine their simulations with new data. For now, their synthetic observations offer an intriguing scenario that provides a missing link between the features observed in many planetary infants and their grown-up counterparts.
Image is an artist’s impression of a young star surrounded by a protoplanetary disk.