Protoplanetary discs have a lot of explaining to do

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Phenomena in space have a way of spawning unsolved mysteries.

Ever since advanced technologies made it possible to unearth thousands of previously undiscovered planets, the one case that has mystified scientists has to do with their birth. Most exoplanets in the Milky Way are super-Earths (with a mass somewhere between that of Earth and Neptune), but there is a problem with that. Observations determined their star systems were technically supposed to produce leviathans like Jupiter—which is 2.5 times more massive than all the other planets in our solar system combined. Something wasn’t adding up.

New research is finally shedding light on why scientists' expectations weren’t exactly aligned with what was emerging from the protoplanetary discs in which embryonic planets develop. There is a mystery inside this mystery; despite a universe of theories, no one really knows how planets form. Postdoctoral fellow Ruobing Dong at the University of Arizona’s Steward Observatory recently led a study, published in the Astrophysical Journal, investigating why the multiple gaps in protoplanetary discs make them seem like they are incubators for Jupiter-mass spheres, when they instead produce the types of planets most common in our galaxy.

"We propose a scenario that was previously deemed impossible: how a super-Earth can carve out multiple gaps in disks," said Dong. "For the first time, we can reconcile the mysterious disk features we observe and the population of planets most commonly found in our galaxy."

A computer-generated simulation of a protoplanetary disc with one super-Earth orbiting the central star. 

Protoplanetary discs are formed from immense clouds of interstellar gas and dust that condense under the force of gravity until they collapse into a disc that swirls around a young (meaning several million years old) star. Star systems are born when debris in the disc eventually clumps together over a vast stretch of time to form asteroids and finally planets. While we know that the stardust in these discs eventually dissipates, how is yet another enigma. It is thought that most accretes onto the star while planets take shape from anything that isn’t swept away by stellar radiation.

As if all the questions that arise from protoplanetary discs along with new star systems aren’t enough, the discs are almost impossible to dissect by telescope. Features such as rings and gaps caused by planets orbiting protostars have been the only way for researchers to get an idea of what is brewing inside. Explanations for more intricate details recently observed by the Atacama Large Millimeter/submillimeter Array (ALMA) have eluded scientists. Dong’s solution was to create disc simulations that mirrored ALMA observations and could have parameters such as dust and viscosity adjusted. What emerged from the simulation was a super-Earth, which was enough to create all those rings and gaps—no Jupiter clone required.

More research is on the horizon, but protoplanetary discs have just gotten a little less nebulous.