Planet-forming discs are much smaller than previously thought

The research team used the Atacama Large Millimeter/submillimeter Array (ALMA) to look at 73 protoplanetary discs in the Lupus region. They found that many young stars host modest discs of gas and dust, some as small as 1.2 astronomical units (an astronomical unit is the average distance from Earth to the sun).

In the past decade astronomers have imaged hundreds of protoplanetary discs around young stars using powerful radio telescopes on Earth, like ALMA. When compared to the size of our own solar system, many of these discs extend far beyond the orbit of Neptune, our outermost planet. Furthermore, most of the discs show gaps where giant planets are thought to be formed.

The new study, led by researchers at the Leiden Observatory in the Netherlands and posted on a preprint server (and not yet peer reviewed), shows that these discs might not be typical.

Using ALMA, a radio telescope consisting of 66 antennas spread across the Chilean Andes, the researchers imaged all known protoplanetary discs 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 discs are small, with an average radius of six astronomical units - about the orbit of Jupiter. The smallest disc found was only 1.2 astronomical units in diameter.

Lead author Osmar Guerra-Alvarado, a PhD candidate at Leiden Observatory, said: “These results completely change our view of what a ‘typical’ protoplanetary disc looks like.

“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.”

Co-author Dr Paola Pinilla, based at the Mullard Space Science Laboratory at UCL, said: “It is astonishing to discover that protoplanetary discs can be so small and that they are so common. As the large and bright discs are the easiest to observe, our previous view of the birth-site of planets was biased. 

“Thanks to the incredible capabilities of ALMA, we are finally able to characterise the small and faint discs around stars that are only 10% to 50% the mass of our sun. These stars are the most common in our galaxy, so we are finally revealing and understanding the most common conditions for planet formation.”

Optimal conditions for super-Earths
The small discs 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.

Co-author Dr Mariana B. Sanchez, from Leiden Observatory, said: “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.” 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 disc 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 discs and observations of exoplanets. Dr Nienke van der Marel, from Leiden Observatory, said: “The discovery that the majority of the small discs do not show gaps, implies that the majority of stars do not host giant planets”.

This is consistent with what we see in exoplanet populations around full-grown stars. These observations link the disc population directly to the exoplanet population.”

Previous high-resolution observations of ALMA mainly focused on bright discs which are often much larger. For the smaller discs 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 discs.

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

Links

• The paper
• Dr Paola Pinilla’s academic profile
• Mullard Space Science Laboratory at UCL
• UCL Mathematical & Physical Scientists

Image

• Top: Impression of a young star surrounded by a protoplanetary disc in which planets can form. Credit: ESO/L. Calçada
• Embedded: Images of 73 protoplanetary discs in the Lupus star forming region (two of the images contain binary stars). Only a fraction of the discs extend beyond the orbit of Neptune, when compared to our own Solar System. Most of the observed discs are small and show no structures like gaps and rings. Credit: Based on Guerra-Alvarado et al., Figure 1

Media contact

Mark Greaves

m.greaves [at] ucl.ac.uk

+44 (0)20 3108 9485