ALMA finds well-developed protoplanetary discs around young suns

Twenty nearby protoplanetary discs, as imaged by the Atacama Large Millimeter/submillimeter Array, all show rings and gaps that indicate the formation of planets on shorter-than-expected time scales. Image: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

The Atacama Large Millimeter/submillimeter Array has captured stunning views of 20 nearby protoplanetary discs suggesting that large planets similar to Saturn and Neptune form much faster than current theory allows and that they tend to form in the outer reaches of their solar systems.

The observations, part of a major ALMA initiative known as the Disk Substructures at High Angular Resolution Project, or DSHARP, may also may help explain how smaller rocky planets manage to survive in the chaos of young systems.

“The goal of this months-long observing campaign was to search for structural commonalities and differences in protoplanetary discs. ALMA’s remarkably sharp vision has revealed previously unseen structures and unexpectedly complex patterns,” said Sean Andrews, an astronomer at the Harvard-Smithsonian Center for Astrophysics and one of the leaders of the DSHARP campaign.

“We are seeing distinct details around a wide assortment of young stars of various masses,” he said. “The most compelling interpretation of these highly diverse, small-scale features is that there are unseen planets interacting with the disk material.”

Leading models of planet formation hold that the gradual accumulation of gas and dust inside a protoplanetary disc eventually results in the formation of a planet in a hierarchical process expected to take many millions of years to complete. That would suggest such planets would be found primarily in older, more mature solar systems.

Four of the 20 disks observed by ALMA: AS 209’s disk is about one million years old, with tightly packed inner rings and two at extreme distances; HD 143006 is about five million years old, featuring wide gaps and a dense arc-like region at lower left that could indicate a location where comets or other cold bodies are forming; IM Lup features spiral arms that may indicate instabilities in the disk or perturbations due to an unseen planet; AS 205 is a binary system in which both components sport ringed discs. Image: ALMA (ESO/NAOJ/NRAO) S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

But ALMA’s observations of young protoplanetary discs, some just a million years old, reveal well-defined rings and gaps, the calling cards of planets well along in the process of forming.

“It was surprising to see possible signatures of planet formation in the very first high-resolution images of young discs,” said Jane Huang, a graduate student at CfA and a member of the research team. “It was important to find out whether these were anomalies or if those signatures were common in disks.”

The DSHARP project was designed to do just that by focusing on the distribution of dust in 20 nearby protoplanetary discs. The researchers found that concentric gaps and rings are common to nearly all the discs, at distances ranging from a few astronomical units to more than 100, more than three times the distance between Neptune and the Sun.

The dense rings of dust seen by ALMA could create zones in a protoplanetary disc where smaller, rocky planets could successfully grow. ALMA recently captured an iconic image of a protoplanetary disc that showed well-developed rings and gaps around the young star HL Tau.

“We had to wonder if that was an outlier since the disc was comparatively massive and young,” said Laura Perez with the University of Chile and a member of the research team. “These latest observations show that, though striking, HL Tau is far from unusual and may actually represent the normal evolution of planets around young stars.”