Southwest Research Institute Unveils Model for Compact Exoplanet Systems

In a groundbreaking study published on June 9, 2025, researchers from the Southwest Research Institute (SwRI) have proposed a novel model explaining the formation of compact exoplanetary systems, characterized by multiple planets orbiting closely to their host stars. This research challenges traditional views that planetary formation occurs only after the stellar formation phase has concluded. The findings, detailed in the journal Nature Communications, reveal that these compact systems, which include examples like the TRAPPIST-1 system, may develop during the final stages of the star's formation due to ongoing interactions with gas and dust in a circumstellar disk.

Dr. Raluca Rufu, a Sagan Fellow at SwRI and lead author of the study, emphasized the significance of the research, stating, "Compact systems are one of the great mysteries of exoplanet science. They contain multiple rocky planets of similar size, like peas-in-a-pod, and a common mass ratio that is very different than that of our solar system's planets." The research highlights that in contrast to our solar system—where no planets lie closer to the Sun than Mercury—compact systems demonstrate a consistent mass ratio across hundreds of known systems.

The SwRI team utilized advanced simulations to illustrate how planets can begin forming while still in the infall phase of stellar creation. This new model suggests that as material from a collapsing molecular cloud infalls toward a forming star, it creates a circumstellar disk that can support the early stages of planet formation. As these planets grow, they interact with the remaining gas in the disk, causing them to spiral inward, resulting in the unique compact configurations observed today.

Dr. Robin Canup, also from SwRI, noted, "The common mass ratio seen in compact exoplanetary systems is similar to that of the satellite systems of our gas planets, suggesting a potentially similar formation process. This finding opens up new avenues for understanding how various planetary systems develop across different scales."

The implications of this research extend beyond theoretical modeling. Dr. Rufu pointed out that this early formation mechanism aligns with prior observations made by the Atacama Large Millimeter Array (ALMA), which detected disks around young stars. The study presents a compelling case for a re-evaluation of our assumptions regarding planet formation timelines.

As scientists continue to explore the diversity of planetary systems in our universe, this new model could reshape our understanding of how planets like Earth might form and evolve in relation to their parent stars. The findings not only provide insights into the nature of compact exoplanetary systems but also encourage further observational efforts to study these intriguing celestial formations.

To access the full study titled "Origin of compact exoplanetary systems during disk infall," readers may refer to the publication available at https://doi.org/10.1038/s41467-025-60017-8.