When it comes to the planets produced in protoplanetary disks, size matters, but not the way you might think. That’s the conclusion a group of astronomers found when they aimed the Atacama Large Millimeter Array in Chile at hundreds of these disks around young stars in the southern constellation Lupus. They used the observatory in 2023 and 2024 to focus on the disks and supplemented that with archival data.
The team, led by Osmar Guerra Alvarado of the Leiden Observatory in the Netherlands, looked at 73 protoplanetary disks and found that about two-thirds of them are small. They extend out beyond their star at an average of six astronomical units (AU), or about the distance of Jupiter from the Sun. The smallest disk stretched out to only about the orbit of Earth. The smallest disks orbit around low-mass stars, those much less massive than the Sun. They’re also the most common type of star.
Most known disks that have been studied before this have gaps where giant planets are likely to form. Compared to the larger number of small protoplanetary disks, the larger ones might not be typical of all such planet-forming regions. “The research shows that we’ve been wrong for a long time about how a typical disk looks like, said team member Nienke van der Marel. “Clearly, we’ve been biased towards the brightest and largest disks. Now we finally have a full overview of disks of all sizes.”
What it Means for Disk and Planet Sizes
“These results completely change our view of what a ‘typical’ protoplanetary disc looks like”, Guerra-Alvarado said. “Only the brightest discs which are the easiest to observe show large-scale gaps, whereas compact discs without such substructures are actually much more common.”
Based on their research, team member Mariana Sanchez, a post-doctoral researcher at Leiden, suggested that the disks around the lower-mass stars can surprise us with their planets. “The observations also show that these compact discs could have optimal conditions for the formation of so-called super-Earths, as most of the dust is close to the star, where super-Earths are typically found”, Sanchez said. Super-Earths are similar to Earth in many ways, but can be up to ten times more massive than our home world. This could explain why low-mass stars seem to have so many super-Earths and why they’re the most common type of planet in the Universe.
In contrast, stars with larger disks seem to produce more Jupiter- and Saturn-sized worlds, but no super-Earths. So, what does that say about our Solar System? The team’s survey seems to imply that our collection of planets was born in a large protoplanetary disk. That’s because we have Jupiter and Saturn and we only have a “regular-sized” Earth. “This research provides a fascinating link between the sizes of observed planets and the sizes of observed protoplanetary disks, said van der Marel, also of Leiden Observatory.
Impression of a young star surrounded by a protoplanetary disc in which planets can form. Credit: ESO/L. Calçada
Protoplanetary Processes
Young stars form in molecular clouds of hydrogen and dust. As a protostar forms, a disk of gas and dust forms around it. When conditions are right, say about a hundred thousand years of swirling and coalescing, the star is “born”. The cloud flattens out to create the disk. Eventually, materials inside the cloud coalesce to form the seeds of planets. Those combine to create protoplanets and eventually planets.
Hubble Space Telescope was the first to observe these disks in 1990. It found them in the Orion Nebula, and that discovery sent astronomers rushing to observe other young stars and their disks. The most detailed disk observations come from radio telescopes can detect more structure in the disks. In particular, they can spot the “breaks” in the disk where the giant planets sweep up material as they form.
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Protoplanetary disks (often referred to as “proplyds”) as seen insets in the Orion Nebula. Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA), the Hubble Space Telescope Orion Treasury Project Team and L. Ricci (ESO)
Astronomers want to study these planetary construction zones to understand the many different types of planets we see around other stars. It may well turn out that our own Solar System isn’t as “typical” as we think. That’s because extrasolar planets seem to come in a more extensive range of sizes and orbits than what we see here at home. In addition to the super-Earths, there are hot Jupiters, super-Jupiters, and mini-Neptunes, in addition to more exotic suggestions such as ocean planets and lava planets. Each type of planet has a direct link to the conditions that existed in the protoplanetary disk where it formed. Finding the missing link between disk size and the types of planets a disk creates is a big step forward in understanding the overall process of planetary formation.
For More Information
Protoplanetary Disks are Much Smaller Than Previously Thought
A High-resolution Survey of Protoplanetary Disks in Lupus and the Nature of Compact Disks