Protoplanetary discs are much smaller than previously thought, according to new research

Many protranetary discs on which new planets are formed are much smaller than thoughts. Using Atacama, we used scientists from Leiden Observatory (Netherlands) to examine 73 protranetary discs in the Lupus region. They discovered that many young stars host modest discs of gas and dust. Accepted for publication in Astronomy and Astrophysics, this study establishes an important link between observed protranetardiscs and deplanets.

Over the past decade, astronomers have used powerful radio telescopes on Earth, like Alma, to image hundreds of protranetary discs around young stars. Compared to the size of our own solar system, many of these discs are far beyond the orbit of our outermost planet, Neptune. Furthermore, most discs show gaps where giant planets are thought to form. PhD research candidates Osmar M. Guerra-Alvarado, Postdoc Mariana B. Sanchez, and Assistant Professor Nienke Van Der Marel of the Leiden Observatory, have shown that these discs may not be typical.

Using Alma, the researchers imaged all known protranetary discs around the young stars of Lupus, a star-forming region located about 400 light years from Earth in the Southern Constellation Lupus. The study revealed that two-thirds of the 73 discs are small and astronomical units with an average radius of six. This is about Jupiter’s orbit. The only smallest disks found were 0.6 astronomical units with a radius smaller than Earth’s orbit.

“These results completely change our views on what a ‘typical’ protranetary disc looks like,” says Guerra Alvarado. “Only the brightest disks that are easiest to observe show a large gap, but compact disks without such substructures are actually much more common.”

The best conditions for the super-earth

Small discs were found mainly around low-mass stars, with masses of 10-50% of the solar mass. This is the most common type of star found in the universe.

“Observations show that these compact discs may have the best conditions for the formation of what is called super-earth. Because most of the dust is close to stars, super-earth is commonly found,” says Sanchez. She is a postdoc at Leiden Observatory and a contributor to this research. Super Earth is a rocky planet like Earth, but it has up to ten times as much chunks as our planet. This could also explain why super earths are often seen around low-mass stars.

Furthermore, in this study, our solar system produces large gas planets like Jupiter and Saturn, but there was no super-Earth. Super-Earth is considered to be the most common planet type in the universe.

Missing Link

This study establishes a “missing link” between protozoan observations and deplanetary observations. “The discovery that most of the small discs do not show gaps means that most of the stars do not host a giant planet,” says van der Marel. “This is consistent with what is seen in exoplanet populations around grown stars. These observations link the disk population directly to the Explanet population.”

Alma’s previous high-resolution observations focused primarily on bright discs, which often became much larger. For small discs, only brightness was measured, and no size was measured. High-resolution observations could be more complicated, and it was not clear whether Alma could image a relatively faint disc.

For their research, scientists used Alma Observations, taken in 2023 and 2024, with the highest possible resolution of 0.030 arc seconds. We also created the first full high-resolution disc survey of the entire star-forming region using archival data.

Van der Marel said, “This study shows that we’ve been wrong for a long time about what a typical disk looks like. Clearly, we’re biased towards the brightest and largest disks. Now there’s a complete overview of all sizes of disks.”