New SwRI Model Explains Exoplanetary Systems With Compact Orbits
Star and planet formation has largely been considered separate, sequential processes. But in a new study, scientists at Southwest Research Institute (SwRI) have modeled a different scenario where planets start developing early — during the final stages of stellar formation — rather than after this phase ends, as previously assumed.
Among the many thousands of known exoplanets there is a large population of compact systems that each have multiple planets orbiting very close to their central star. This contrasts with our solar system, which lacks planets orbiting closer than Mercury. Interestingly, in compact systems, the total mass of the planets in each system relative to the host star’s mass is remarkably consistent across hundreds of systems. The cause of this common mass ratio remains a mystery.
Dr. Raluca Rufu and Dr. Robin Canup of SwRI’s Solar System Science and Exploration Division in Boulder, Colorado, used advanced simulations that show surviving early-formed planets match multiple observed features of compact systems, including both tight planetary orbits and a common mass ratio. Early planet growth also is consistent with prior observations of disks around young stars made by the Atacama Large Millimeter Array (ALMA) telescope.
“Compact systems are one of the great mysteries of exoplanet science,” said Rufu, a Sagan Fellow and lead author of a Nature Communications describing this research. “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.”
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Southwest Research Institute scientists propose a new model for the formation of compact exoplanetary systems that contain multiple rocky planets in tight orbits around their star. In this model, planets begin to form in regions of a disk around a young star that are fed by an ongoing infall of gas and small grains. Growing planets collect rocky material while gradually spiraling inward through interactions with surrounding gas. As a planet gains mass, its inward migration accelerates. This process yields a compact planetary system with a planets-to-star mass ratio consistent with observed compact exoplanetary systems.
“Intriguingly, the common mass ratio seen in compact exoplanetary systems is similar to that of the satellite systems of our gas planets. These moons are thought to have developed as gas planets finalized their formation. This seems a powerful clue that compact systems may reflect a similar underlying process,” said Canup.
A star forms as a molecular cloud of gas and dust collapses due to its own gravity. As material from the cloud infalls towards the central star, it is first deposited into a circumstellar disk orbiting the star. After infall ends, the disk persists for a few million years before its gas disperses. Planets form within the disk, starting with collisions and accumulation among dust grains and ending with the gravitational assembly of planets.
“Conventionally, it has been assumed that planetary assembly started after stellar infall ended. However, recent ALMA observations provide strong evidence that planetary accretion, or formation, may begin earlier,” said Rufu. “We propose that compact systems are surviving remnants of planet accretion that occurred during the final phases of infall.”
The new numerical simulations show that during infall, growing planets collect rocky material while their orbits gradually spiral inward through interactions with surrounding disk gas. As a planet gains mass, its inward orbit migration accelerates, so that planets above a critical mass fall into the star and are consumed. This balance between planetary growth and loss tends to produce similarly sized planets with characteristic masses determined by infall and disk conditions.
“We find that planets that accrete during infall can survive until the gas disk disperses and orbital migration ends,” said Canup. “Importantly, across a broad range of conditions, the mass of surviving systems is proportional to the mass of the host star, providing the first explanation for the similar mass ratios of observed multi-planet compact systems.”
The envisioned process is similar to the way moons may form around giant planets like Jupiter. Moons grow within a disk surrounding the planet that is fed by infalling gas and dust material from the circumstellar disk. A key difference lies in the timing: moon-forming disks disperse quickly once infall stops, while planet-forming disks around stars can last up to several million years. This subtle difference yields somewhat lower mass ratios for compact planetary systems than for gas planet satellite systems.
“It’s exciting to see that the process of early assembly in young disks may work in a similar way across very different scales,” the team notes.
To read the Nature article titled “Origin of compact exoplanetary systems during disk infall,” see: https://doi.org/10.1038/s41467-025-60017-8.
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