Many protoplanetary disks in which new planets are formed are much smaller than thought. Using the Atacama Large Millimeter/submillimeter Array (ALMA) scientists of the Leiden Observatory (the Netherlands) looked at 73 protoplanetary disks in the Lupus region. They found that many young stars host modest disks of gas and dust, some as small as 1.2 astronomical units. The research, accepted for publication in Astronomy & Astrophysics, establishes an important link between observed protoplanetary disks and exoplanets.

In the past decade, astronomers have imaged hundreds of protoplanetary disks around young stars using powerful radio telescopes on Earth, like ALMA. When compared to the size of our own solar system, many of these disks extend far beyond the orbit of Neptune, our outermost planet. Furthermore, most of the disks show gaps where giant planets are thought to be formed. Research of Ph.D. candidate Osmar M. Guerra-Alvarado, postdoc Mariana B. Sanchez and assistant professor Nienke van der Marel of the Leiden Observatory now show that these disks might not be typical.

Using ALMA, the researchers imaged all known protoplanetary disks around young stars in Lupus, a star-forming region located about 400 light years from Earth in the southern constellation Lupus. The survey reveals that two-thirds of the 73 disks are small, with an average radius of six astronomical units. This is about the orbit of Jupiter. The smallest disk found was only 0.6 astronomical units in radius, smaller than the orbit of Earth.

“These results completely change our view of what a ‘typical’ protoplanetary disk looks like,” Guerra-Alvarado says. “Only the brightest disks which are the easiest to observe show large-scale gaps, whereas compact disks without such substructures are actually much more common.”

Optimal conditions for super-Earths

The small disks were primarily found around low-mass stars, with a mass between 10 and 50% of the mass of our sun. This is the most common type of star found in the universe.

“The observations also show that these compact disks 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 says. She is a postdoc at Leiden Observatory and a contributor to this research. Super-Earths are rocky planets like Earth but with masses up to ten times that of our planet. This could also explain why super-Earths are often found around low-mass stars.

Furthermore, the research suggests that our solar system was formed from a large protoplanetary disk that produced large gas planets like Jupiter and Saturn, but no super-Earth. Super-Earths are thought to be the most common planet types in the universe.

The missing link

The research establishes a “missing link” between observations of protoplanetary disks and observations of exoplanets. “The discovery that the majority of the small disks do not show gaps, implies that the majority of stars do not host giant planets,” Van der Marel says. “This is consistent with what we see in exoplanet populations around full-grown stars. These observations link the disk population directly to the exoplanet population.”

Previous high-resolution observations of ALMA mainly focused on bright disks which are often much larger. For the smaller disks only the brightness was measured and not the size. High-resolution observations can be more complicated to take, and it was not clear if ALMA would be able to image the relatively faint disks.

For their research, the scientists used ALMA observations, taken in 2023 and 2024, with the highest possible resolution of 0.030 arcseconds. They also used archival data to create a complete high-resolution disk survey of an entire star-forming region for the first time.

Van der Marel says, “The research shows that we’ve been wrong for a long time about what a typical disk looks like. Clearly, we’ve been biased towards the brightest and largest disks. Now we finally have a full overview of disks of all sizes.”