Recent research suggests that planets start to form in a solar system when the star itself is very young, and the disk is not yet fully formed, but a survey, co-authored by a University of Michigan researcher, refines the timeline of the formation of planetary systems.

An international research team used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe disks around 19 protostars with a very high resolution to search for the earliest signs of planet formation. Contrary to its expectations, the team found that the disks around protostars may not be fully ready for planet formation.

“Many of the mature protoplanetary disks show rings and gaps, which could be sign posts of forming planets. With this program, we searched for the first time for such signatures in a set of 19 of the youngest known disks,” said Merel van ‘t Hoff, U-M postdoctoral researcher and first author on one of the papers in a series published in The Astrophysical Journal. “To our surprise, we only found rings and gaps in a few of the young disks, telling us that planets do not start to form right away, as we were thinking.”

This survey was motivated by the recent findings that planet formation may be well underway in the more evolved proto-planetary disks, but until now there had been no systematic study to search for signs of planet formation in younger protostellar systems.

“These previous results motivated us to examine even younger disks around protostars to answer the question, at what stage of star formation do planet forms,” said Nagayoshi Ohashi at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), who led the team.

Planets form in a disk around a newborn star. These ‘proto-planetary’ disks only last a few million years, meaning that a forming planetary system only has this amount of time to finish its formation. However, it is still not clear just how rapidly planet formation begins within these disks.

Recent ALMA observations have revealed that many mature proto-planetary disks have substructures such as gaps and rings, indicating that planets are already forming from the disk.

Van ‘t Hoff analyzed the observations for one of the young disks in the sample, L1527. She focused on the emission from different molecules, which tells researchers the composition of planetary building blocks.

“With the unprecedented high resolution of these observations, I could see evidence for an enhancement in temperature where material falls onto the disk,” van ‘t Hoff said. “This tells us that the material in the cloud where the star formed may undergo chemical changes on its way to newly formed planets.”

The team observed young disks around 19 protostars located within about 650 light-years from Earth. This is the first systematic study to investigate the detailed structure of young disks around a large sample of protostars with high angular resolution. The observations clearly show that the young disks around protostars are different from more evolved proto-planetary disks, suggesting that planets have not yet started forming in young disks.

Among the 19 protostars, rings and gaps, which are signs of planet formation, were observed only in a few disks. Moreover, the ring structures are less distinct than those seen in the proto-planetary disks.

The researchers did not expect to see such clear differences between disks around protostars and more evolved disks, according to John Tobin, a co-principal investigator of the program at the National Radio Astronomical Observatory.

“Our results suggest that disks around protostars are not fully ready for planet formation,” Tobin said. “We believe that the actual formation of the planetary system progresses rapidly in the 100,000 years to 1,000,000 years after star formation begins.”