Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have conducted one of the deepest surveys ever of protoplanetary disks — quasi-steady circumstellar disks of gas and dust, from which planets may eventually form or be in the process of forming.
ALMA’s high-resolution images of nearby protoplanetary disks. Image credit: ALMA / ESO / NAOJ / NRAO / S. Andrews et al / AUI / NSF / S. Dagnello.
The leading models for planet formation hold that planets are born by the gradual accumulation of dust and gas inside a protoplanetary disk, beginning with grains of dust that coalesce to form larger and larger rocks, until asteroids, planetesimals, and planets emerge.
This hierarchical process should take many millions of years to unfold, suggesting that its impact on protoplanetary disks would be most prevalent in older, more mature systems. Mounting evidence, however, indicates that is not always the case.
ALMA’s earlier observations of young protoplanetary disks revealed striking and surprising structures, including prominent rings and gaps, which appear to be the hallmarks of planets.
Astronomers were initially cautious to ascribe these features to the actions of planets since other natural process could be at play.
“It was surprising to see possible signatures of planet formation in the very first high-resolution images of young disks,” said Jane Huang, a graduate student at the Harvard-Smithsonian Center for Astrophysics.
“It was important to find out whether these were anomalies or if those signatures were common in disks.”
Since the sample set was so small, however, it was impossible to draw any overarching conclusions. It could have been that astronomers were observing atypical systems.
More observations on a variety of protoplanetary disks were needed to determine the most likely cause of the features we were seeing.
The Disk Substructures at High Angular Resolution Project (DSHARP) survey was designed to do precisely that by studying the relatively small-scale distribution of dust particles around 20 nearby protoplanetary disks.
These dust particles naturally glow in millimeter-wavelength light, enabling ALMA to precisely map the density distribution of small, solid particles around young stars.
Depending on the star’s distance from Earth, ALMA was able to distinguish features as small as a few AU (astronomical units).
Using these observations, the astronomers were able to image an entire population of nearby protoplanetary disks and study their AU-scale features.
They found that many substructures — concentric gaps, narrow rings — are common to nearly all the disks, while large-scale spiral patterns and arc-like features are also present in some of the cases.
Also, the disks and gaps are present at a wide range of distances from their host stars, from a few AU to more than 100 AU, which is more than three times the distance of Neptune from our Sun.
These features, which could be the imprint of large planets, may explain how rocky Earth-like planets are able to form and grow.
“For decades, astronomers have puzzled over a major hurdle in planet-formation theory: once planetesimals grow to a certain size (about 1 km is diameter), the dynamics of a smooth protoplanetary disk would induce them to fall in on their host star, never acquiring the mass necessary to form planets like Mars, Venus, and Earth,” the scientists said.
“The dense rings of dust we now see with ALMA would produce a safe haven for rocky worlds to fully mature.”
“Their higher densities and the concentration of dust particles would create perturbations in the disk, forming zones where planetesimals would have more time to grow into fully fledged planets.”
“The ultimate question is whether we can see the formation of a planetary system like the Solar System,” said Dr. Andrea Isella, an astronomer at Rice University.
“Can we see the formation of an Earth-like planet? We know how to get there: ALMA was the first step, and the next step is either to expand ALMA or build something like the Very Large Array, but 10 times bigger.”
The results of the DSHARP survey will appear in a series of ten papers in the Astrophysical Journal Letters.
