Birthplaces of new planets: 71 blue squares with various yellowish rings and dots in them. A single white square is at the bottom right with a blue ring and black text in it.
View larger. | These are the 73 protoplanetary disks – or planet-forming disks – that ALMA observed in the Lupus region (2 of the images contain binary stars). New research shows that such disks – the birthplaces of new planets – come in a much wider range of sizes than previously thought. The size of Neptune’s orbit is shown at the bottom right for scale. AU means astronomical unit. One AU is the average distance from the sun to Earth. Image via Guerra-Alvarado et al./ Astronomie.nl.
Protoplanetary disks are huge rotating disks of gas and dust around newborn stars. They are the birthplaces of new planets.
Astronomers had thought almost all protoplanetary disks are big, about the size of our solar system or larger. But new observations show they come in a wide range of sizes.
About 2/3 of protoplanetary disks are much smaller than first thought. They tend to form around young red dwarf stars and also lack the gaps seen in larger disks, where giant planets form. This means our own solar system’s original disk was not typical of most stars.
What is a typical protoplanetary disk?
Planets are born in disks of dust and gas around newborn stars called protoplanetary disks. Many of those disks are immense, as large as our solar system or more. In fact, astronomers had thought that most, if not nearly all, such disks are this size. But a team of international astronomers said on March 26, 2025, that they’ve used the Atacama Large Millimeter/submillimeter Array (ALMA) to take a closer look at 73 protoplanetary disks in the Lupus star-forming region. And they found about 2/3 of those disks are much smaller than first thought. Unlike larger disks, the smaller ones don’t have gaps or rings. So what is a typical protoplanetary disk? Or is there no such thing?
The researchers’ new paper has been accepted for publication in Astronomy & Astrophysics. You can read a preprint version on arXiv submitted on March 25, 2025.
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The birthplaces of new planets
Planet-forming disks – or protoplanetary disks – are huge swirling disks of dust and gas around young stars. They are the birthplaces of new planets. Astronomers have imaged hundreds of them in recent years. How big are they? Until now, most of the disks that astronomers found have been enormous. If centered around our sun, they would extend past the orbit of Neptune.
Astronomers thought these were typical for protoplanetary disks. But that seems not to be the case after all.
Planet-forming disks come in many sizes, big and small
Osmar Manuel Guerra-Alvarado, Mariana Belen Sanchez and Nienke van der Marel at the Leiden Observatory in The Netherlands led the new study. Using ALMA in 2023 and 2024, they imaged all of the known 73 disks in Lupus, a star-forming region 400 light-years away in the constellation Lupus the Wolf. They imaged the disks with the highest possible resolution of 0.030 arcseconds. And they found something surprising.
Only about 1/3 of the disks were of the gargantuan variety. The rest – 2/3 of the 73 disks – were much smaller. They would only extend about as far as the orbit of Jupiter, if placed around our sun. That’s about 6 astronomical units (AU). The smallest one was a scant 0.6 AU in size. One AU is the average distance from the sun to Earth, 93 million miles (150 million km). Guerra-Alvarado said:
These results completely change our view of what a ‘typical’ protoplanetary disk looks like. 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.
Smiling man in t-shirt with his hands behind his back.
Lead author Osmar Manuel Guerra-Alvarado at Leiden University in The Netherlands. Image via Leiden University.
Small disks, small stars and super-Earths
Notably, most of the smaller disks were around small stars called red dwarfs. In fact, these low-mass stars are the most common type of star in our Milky Way galaxy. This suggests there should be many more small disks waiting to be discovered.
In addition, the researchers found these smaller disks should be ideal for super-Earth type exoplanets to form in. Those are rocky worlds larger and more massive than Earth, but smaller and less massive than Neptune. And, indeed, astronomers have found many super-Earths orbiting red dwarf stars. Sanchez 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.
The results also highlight a difference between our solar system and those that form from smaller protoplanetary disks. While our solar system has a wide range of planets, there is no super-Earth. Since the protoplanetary disk the solar system formed from was of the larger variety, there are now gas giant planets, but no super-Earths.
Large bright reddish disk of gas and dust around a bright white newborn star. The disk has darker concentric rings in it.
View larger. | Artist’s illustration of a large protoplanetary disk. The gaps are where new planets are forming. Image via ESO/ L. Calçada.
Missing gaps
The observations of the Lupus disks also provided another clue about planetary formation. Studies of full-grown stars have shown that most stars do not have giant gas planets. Smaller planets, like super-Earths and others, are more common. In fact, astronomers think super-Earths are the most common type of planet in our galaxy. This is also consistent with the new ALMA observations. Most of the smaller disks don’t have gaps in them. Those gaps are where new planets form. They gather surrounding material as they grow, clearing their orbit in the disk. This creates the gaps. As Van der Marel noted:
The discovery that the majority of the small disks 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 disk population directly to the exoplanet population.
Not so typical after all
Overall, the observations show that the “typical” protoplanetary disks are not so typical after all. They are both big and small, with gaps and without gaps. Van der Marel said:
The research shows that we’ve been wrong for a long time about how a typical disk looks. Clearly, we’ve been biased toward the brightest and largest disks. Now we finally have a full overview of disks of all sizes.
Bottom line: Researchers using the ALMA observatory have found that protoplanetary disks – the birthplaces of new planets – vary in size much more than previously thought.
Source: A high-resolution survey of protoplanetary disks in Lupus and the nature of compact disks
Via Astronomie.nl
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Paul Scott Anderson
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About the Author:
Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. He studied English, writing, art and computer/publication design in high school and college. He later started his blog The Meridiani Journal in 2005, which was later renamed Planetaria. He also later started the blog Fermi Paradoxica, about the search for life elsewhere in the universe. While interested in all aspects of space exploration, his primary passion is planetary science and SETI. In 2011, he started writing about space on a freelance basis with Universe Today. He has also written for SpaceFlight Insider and AmericaSpace and has also been published in The Mars Quarterly. He also did some supplementary writing for the iOS app Exoplanet. He has been writing for EarthSky since 2018, and also assists with proofing and social media.