The dense rings of dust offer more time to young planetesimals to grow and become fully-fledged planets.

The first large-scale, high-resolution survey Protoplanetary Disks by making use of the Atacama Large Millimeter/submillimeter Array (ALMA) yielded around 4,000 exoplanets. Some popular ones among them are massive rocky worlds, hot Jupiters, and icy dwarf planets. The Disk Substructures at High Angular Resolution Project (DSHARP) of ALMA provided detailed images of 20 protoplanetary disks to enhance the knowledge of astronomers about the various features and the speed at which they can emerge. Although this discovery has given us some important insights, we still need to know a lot more about the precise cosmic recipes that result in the birth of planets.

The researching team mentioned that the most critical result of these observations is that large planets, similar to Saturn and Neptune in size and composition, form much quicker than predicted in the current theory. Similarly, these massive bodies are formed at incredible distances from their host stars (outer reaches of the solar system). This finding helps us to understand the survival and growth of rocky, Earth-sized exoplanets, despite their self-destructive youth. Sean Andrews, a Leader of the Survey Campaign who works as an Astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), briefed the primary goal of their research by saying,

“The goal of this months-long observing campaign was to search for structural commonalities and differences in protoplanetary disks. ALMA’s remarkably sharp vision has revealed previously unseen structures and unexpectedly complex patterns. We are seeing distinct details around a wide assortment of young stars of various masses. The most compelling interpretation of these highly diverse, small-scale features is that there are unseen planets interacting with the disk material.”

Contrary to popular beliefs, the observations of ALMA revealed well-defined structures (gaps and rings) in young protoplanetary disks, some of which were only a million years old. Having said that, scientists were initially reluctant to ascribe these features to the actions of planets as planet formation is a gradual process. The conventional method begins with the accumulation of gas and dust inside a protoplanetary disk. The grains of icy dust combine together to form larger rocks until the emergence of huge bodies like planets and asteroids. This process can potentially take millions of years to complete which means that these features will be more prevalent in older systems. However, physical evidence clarified that it is not true all the time. Jane Huang, a member of the team, referred to that and said,

“It was surprising to see possible signatures of planet formation in the very first high-resolution images of young disks. It was important to find out whether these were anomalies or if those signatures were common in disks.”

Prior to the DSHARP survey, there were not enough samples to determine the most likely reasons for this unexpected behavior as researchers were only observing atypical systems. The Large Program of ALMA offered an ideal solution to this problem by analyzing the relatively small-scale distribution of dust particles around 20 protoplanetary disks. The fact that these dust particles naturally glow in the light wavelengths produced by ALMA ensured that a precise density distribution of solid particles was mapped around young stars. Hence, the astronomers succeeded in imaging an entire population of nearby protoplanetary disks and their AU-scale features.

A lot of substructures including narrow rings and concentric gaps were common to almost all the protoplanetary disks. In addition to that, arc-like features and large-scale spiral patterns were also found in some of these disks. These features offered a reasonable explanation for the formation and growth of rocky, Earth-sized planets. The dense rings of dust protect the small dusty bodies (about 1 cm in diameter) from falling in on their host star. They create certain zones of higher densities to provide more time to planetesimals to grow. Laura Pérez, an Astronomer from the University of Chile, praised the abilities of ALMA in the following words:

“When ALMA truly revealed its capabilities with its iconic image of HL Tau, we had to wonder if that was an outlier since the disk was comparatively massive and young. 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.”