Is our solar system similar to other solar systems? What do other systems look like? We know from exoplanet research that many other systems have hot Jupiters, massive gas giants orbiting extremely close to their stars. Is that normal and is our solar system the outlier?
One way to answer these questions is to study the planet-forming disks around young stars to see how they evolve.
But studying a large sample of these systems is the only way to get an answer.
So that’s what a group of astronomers did when they examined 873 protoplanetary disks.
Mass is the critical component in a new study of planet-forming disks. The mass of the disk determines how much matter is available to form planets.
By measuring the masses of the disks around young stars, astronomers can limit the total mass of planets that form there and get one step closer to understanding the architecture of the solar system.
The new study is “Overview of Orion disks with ALMA (SODA): I. Cloud-level demographics of 873 protoplanetary disks.“It has been published in the magazine Astronomy and Astrophysicsand the lead author is Sierk van Terwisga, a scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany.
“Until now, we weren’t sure which properties dominate the evolution of planet-forming disks around young stars,” van Terwisga said in a press release.
“Our new results now indicate that in environments without any relevant external influence, the observed disk mass available to form new planets depends only on the age of the star disk system.”
The mass of dust doesn’t just tell astronomers the masses of planets that could form from a disk. Depending on the age of the disk, it could also tell astronomers which planets have already formed.
But other factors also affect disk mass, and those factors vary from disk to disk. Things like stellar winds and irradiation from nearby stars outside the disk can also affect its mass.
So how could the researchers isolate those effects in such a large sample?
They focused on a well-known region of protoplanetary disks, the Orion A cloudwhich is part of the Orion Molecular Cloud Complex (OMCC).
Located about 1,350 light-years away, the OMCC is home to the well-studied Orion Nebula, a feature even backyard astronomers can spot.
Above: This image shows the giant star-forming cloud Orion A, as observed by the SPIRE (Spectral and Photometric Imaging Receiver) instrument aboard the Herschel Space Telescope. It follows the large-scale spread of cold dust. Orion A is located about 1,350 light-years away and consists of discrete star-forming regions as indicated by their labels. The locations of planet-forming disks (+) observed by ALMA are indicated, while disks with dust masses greater than 100 Earth masses are shown as blue dots.
Álvaro Hacar is a co-author of the study and a scientist at the University of Vienna, Austria. “Orion A provided us with an unprecedented sample size of more than 870 disks around young stars,” Hacar said† “It was crucial to be able to look for small variations in disk mass depending on age and even on the local environments in the cloud.”
This is a good example because all drives belong to the same cloud. That means their chemistry is uniform and they all have the same history.
The nearby Orion Nebular Cluster (ONC) is home to some massive stars that could affect other disks, so the team rejected all disks in Orion A closer than 13 light-years to the ONC.
Measuring the mass of all these disks was tricky. The team used the Atacama Large Millimeter/Submillimeter Array (ALMA) to observe the dust. ALMA can be tuned to different wavelengths, so the team observed the young disks at a wavelength of 1.2 mm.
At that wavelength, the dust is bright, but the star is faint, helping to eliminate the effect of the star in each disk. Because observations at 1.2 millimeters render the observations insensitive to objects larger than a few millimeters — for example, planets that have already formed — the team’s measurements only measured dust that is available to form new planets.
Measuring dust without interference from stars was one hurdle, but the researchers faced another: data.
A detailed survey of nearly 900 protoplanetary disks yields a lot of data, and all that data has to be processed before it has any collective meaning. If the team had relied on existing methods, it would have taken about six months to process all that data.
Instead, they developed their own method of processing the data using parallel processing† What would have taken months took less than a day. “Our new approach improved processing speed by a factor of 900,” co-author Raymond Oonk said†
When they processed the data, the researchers found that most of the disks contained only 2.2 Earth masses of dust. Only 20 of the nearly 900 disks contain enough dust for 100 or more Earths.
“To find variations, we dissected the Orion A cloud and analyzed these regions separately. Thanks to the hundreds of disks, the subsamples were still large enough to provide statistically meaningful results,” from Terwisga explained†
The researchers found some variability in disk dust mass in different regions of Orion A, but the variations were minimal. According to the authors, the age effect may explain the variations. As disks age, disk mass decreases and clusters of disks of the same age have the same mass distribution.
“We must emphasize that the differences between these clusters, which are far from each other in the sky, are small and not very significant relative to each other and the field, even in the most extreme cases,” the authors said. write in their newspaper†
Above: This figure shows the six low-mass, low-density clusters of YSOs in the study. Despite their wide distribution in Orion A, the disks show the same correlation between mass and age.
It is expected that as disks age, their dust mass decreases. Planetary formation accounts for most of that decline: What was once dust becomes planets.
But other effects also contribute to dust loss. Dust can migrate to the disk center and irradiation of the host star can vaporize the dust.
But this study reinforces the correlation between age and dust loss.
Could the results of this study be applicable to other young stellar disk populations? The authors compared their results from Orion A to several neighboring star-forming regions with young disks.
Most, but not all, fit the age-related mass loss seen in Orion A.
“Overall, we think our study proves that all populations of planet-forming disks at least within the next 1,000 light-years show the same mass distribution at some age. And they appear in more or less the same way,” van Terwisga said†
The researchers have more work they want to do. They will investigate what effect smaller stars can have on a smaller scale of a few light years.
In this study, they avoided the effect that massive stars in the ONC can have on neighboring disks. But smaller background stars could still affect the disks, explaining some of the small variations in the age-mass correlation.
The age of the star and its disk, the chemical properties and dynamics of the parent cloud all combine with mass to paint a clearer picture of the solar system emerging from the disk. Astronomers are unable to collect data and predict what type of planets may form in a particular solar system in this way.
But it is noteworthy that the correlation between disk age and disk mass is strong, even across large structures such as Orion A.
“The remarkably homogeneous properties of disk samples of the same age are a surprising finding,” the authors said to concludeand their results confirm what previous studies and surveys hinted at.
“Now, however, we can show that this applies to a larger number of YSOs and YSO clusters, which form in well-separated parts of the same giant cloud. For the first time, the unprecedented size of the SODA (Survey of Orion Disks with Alma ) disk sample allows us to zoom in on the effects of age gradients and clustering in a single star-forming region.”
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