Astronomers see an Accretion Disk Where Planets are About to Form

Planet formation is notoriously difficult to study.  Not only does the process take millions of years, making it impossible to observe in real time, there are myriad factors that play into it, making it difficult to distinguish cause and effect.  What we do know is that planets form from features known as protoplanetary disks, which are made up of gas and dust surrounding young stars.  And now a team using ALMA have found a star system that has a protoplanetary disk and enough variability to help them nail down some details of how exactly the process of planet formation works.

The research is described in two new papers in The Astrophysical Journal.  They describe the star system Elias 2-27, which is located about 400 light years from Earth in Ophiuchus, the Serpent Bearer.  It has attracted the attention of astronomers for the last 5 years, first being studied in 2016 when it revealed a pinwheel of dust surrounding the star.

Visualization from NASA of planets forming in a protoplanetary disk.
Credit – NASA

Usually protoplanetary disks don’t take the shape of a pinwheel, which is more commonly found in galactic formations such as the Pinwheel Galaxy.  Researchers speculated that the two pinwheel arms visible around the star were caused by gravitational instabilities, which could also contribute to planetary formation processes.  But they needed further data to prove their idea.

That is where the new papers come in.  Data that was collected over the last 5 years proved the existence of gravitational instabilities, but also found a few things that weren’t caught in the first round of data.  It appears there may have been more material accreting to the disk itself, causing more gravitational chaos. More surprisingly, some parts of the protoplanetary disk were much taller than others.

This type of “vertical asymmetry” had never been observed before in a protoplanetary disk, and allowed the researchers to take a step forward in one of the computational hurdles that block the path to fully understanding planetary formation.  Computational members of the team had predicted that gravitational instabilities might cause the huge pillars of matter that appear to tower over the disk.  Those towers also open up the possibility of calculating the actual quantity of material present in the disk itself – a measurement that has eluded planetary scientists so far.  

“Knowing the amount of mass present in planet-forming disks allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form.” said Venedetta Veronesi, lead author of one of the papers and a graduate student at the University of Milan.  

Map of Elias 2-27 system showing why causes of gravitational instabilities are important in causing planetary formation.
Map of Elias 2-27 system showing why causes of gravitational instabilities are important in causing planetary formation.
Credit – ALMA (ESO / NAOJ / NRAO) / T. Penque-Carreño (Universidad de Chile), B. Saxton (NRAO)

Even with the possibility of finally being able to calculate a protoplanetary disk’s size, there is still a lot of work to be done to fully flesh out the entire planetary formation process.  Luckily, there are plenty more star systems out there to study, and some of them undoubtedly have planets at every stage of that formation process.  With tools like ALMA, scientists will continue searching for them, and help draw an even more complete picture of where planets come from.

Learn More:
NRAO – Study of Young Chaotic Star System Reveals Planet Formation Secrets
UT – How Was the Solar System Formed? – The Nebular Hypothesis
UT – Planets Form in Just a Few Hundred Thousand Years

Lead Image:
Different images of the Elias 2-27 star system showing dust (blue) and various gases (red & yellow).
Credit – ALMA (ESO / NAOJ / NRAO) / T. Paneque-Carreño (Universidad de Chile), B. Saxton (NRAO)