For decades, astronomers have peered into the depths of space, searching for answers to one of the most fundamental questions: how do planets form? Until recently, the prevailing view was that protoplanetary disks—the swirling clouds of gas and dust that birth planets—were often large, extending far beyond the orbit of Neptune. However, new research from Leiden Observatory, using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), is rewriting that narrative. Their findings reveal that many protoplanetary disks are significantly smaller than previously believed, fundamentally changing our understanding of planetary formation.
Revealing the Smallest Planet-Forming Disks
A team of scientists from Leiden Observatory in the Netherlands, led by Ph.D. candidate Osmar M. Guerra-Alvarado, postdoctoral researcher Mariana B. Sanchez, and assistant professor Nienke van der Marel, has conducted the most comprehensive high-resolution survey of protoplanetary disks in the Lupus star-forming region. Located about 400 light-years from Earth in the southern constellation Lupus, this region is a cradle of young stars and planetary systems in the making.
By imaging 73 protoplanetary disks with ALMA, the researchers discovered a striking fact: two-thirds of the disks were remarkably small, with an average radius of just six astronomical units (AU)—approximately the distance from the Sun to Jupiter. The smallest disk observed measured a mere 0.6 AU in radius, smaller than Earth’s orbit.
“These results completely change our view of what a ‘typical’ protoplanetary disk looks like,” Guerra-Alvarado states. “Only the brightest disks, which are the easiest to observe, show large-scale gaps where giant planets are thought to form. In contrast, compact disks without such substructures are actually far more common.”
This finding is a major revelation, as previous high-resolution ALMA observations had focused on bright, large disks, creating a bias in our understanding of planet formation. By targeting an entire star-forming region rather than only the brightest disks, the Leiden team has provided a much-needed reality check on what is truly typical in the universe.
Small Disks, Big Implications for Planetary Formation
The small disks identified in the study were primarily found around low-mass stars, those with only 10% to 50% of the mass of our Sun. These stars are the most common in the universe, making their planet-forming disks particularly significant.
“The observations also show that these compact disks could have optimal conditions for the formation of so-called super-Earths,” explains Sanchez. “Most of the dust is concentrated close to the star, where super-Earths are typically found.”
Super-Earths are rocky planets like our own but with masses up to ten times greater. These planets have been found orbiting many exoplanetary systems, particularly around low-mass stars, but their formation had remained a puzzle. The new study suggests that small protoplanetary disks provide the perfect environment for their creation.
This could explain why super-Earths are among the most common types of planets in the galaxy while gas giants like Jupiter and Saturn are relatively rare. Our own solar system, which contains large gas giants but no super-Earths, may have originated from a much larger protoplanetary disk than is typical.
A Missing Link Between Disks and Exoplanets
One of the most profound implications of this research is its potential to connect protoplanetary disks with the observed population of exoplanets. Until now, there has been a disconnect between what astronomers observed in young planetary disks and the diversity of planets found around mature stars.
“The discovery that most small disks do not show gaps suggests that the majority of stars do not host giant planets,” says Van der Marel. “This is consistent with exoplanet surveys, which show that most stars are surrounded by super-Earths and smaller planets rather than massive gas giants.”
This “missing link” between protoplanetary disks and exoplanetary populations brings astronomers closer to understanding how planets form and evolve. If giant planets like Jupiter require large disks with visible gaps, then their rarity in the universe makes sense. Meanwhile, the prevalence of compact disks around low-mass stars aligns with the widespread presence of super-Earths.
A Revolution in Planet-Formation Theory
For years, astronomers have assumed that large disks with dramatic gaps were the standard blueprint for planetary formation. This assumption shaped theoretical models and simulations. But the new findings challenge that view and call for a reassessment of planet-formation theories.
The study, which is set to be published in Astronomy & Astrophysics, utilized ALMA’s highest-resolution imaging capabilities, reaching an astonishing precision of 0.030 arcseconds. The researchers also incorporated archival data to ensure a comprehensive analysis of the entire Lupus region.
“Our research shows that we’ve been wrong for a long time about what a typical disk looks like,” Van der Marel concludes. “Clearly, we’ve been biased towards the brightest and largest disks. Now, we finally have a full overview of disks of all sizes.”
This shift in perspective has profound consequences for future planetary studies. Astronomers will need to rethink how planets emerge from their dusty nurseries and consider that smaller disks may be the true norm rather than an exception.
The Future of Protoplanetary Disk Research
The discovery of compact protoplanetary disks opens up exciting new avenues for further research. Scientists now have a clearer target when searching for the birthplaces of super-Earths and other common exoplanets. Future observations, possibly with next-generation telescopes such as the Extremely Large Telescope (ELT) or the James Webb Space Telescope (JWST), could provide even greater insight into the early stages of planet formation.
Additionally, understanding the frequency of different disk sizes and structures will help refine models of planetary system evolution. If most planetary systems begin with small, dust-rich disks, this could reshape how we predict the types of planets likely to form around different kinds of stars.
Conclusion: A New Cosmic Perspective
The universe is a place of constant discovery, where every new observation has the potential to reshape our understanding of the cosmos. The revelation that many protoplanetary disks are much smaller than previously thought challenges long-held assumptions and offers a more accurate picture of how planets form.
By studying an entire star-forming region rather than focusing solely on bright, prominent disks, the Leiden team has provided a more representative view of planetary nurseries across the galaxy. Their findings suggest that our own solar system—with its large, structured disk and giant planets—may be the exception rather than the rule.
This research is not just about protoplanetary disks; it is about understanding the origins of worlds, including our own. As astronomers continue to refine their models and probe deeper into the mechanisms of planetary birth, we may soon uncover even more surprises about the vast and diverse universe we call home.
Reference: O.M. Guerra-Alvarado et al, A high-resolution survey of protoplanetary discs in Lupus and the nature of compact discs, Astronomy & Astrophysics (2025). Preprint (pdf): www.astronomie.nl/upload/files … arado-et-al-2025.pdf