A groundbreaking study by Rice University researchers Sho Shibata and Andre Izidoro introduces a compelling new model for the formation of super-Earths and mini-Neptunes—planets ranging from one to four times Earth’s size and among the most prevalent in our galaxy. Leveraging advanced simulations, the researchers suggest that these planets originate from distinct rings of planetesimals, offering fresh perspectives on planetary evolution beyond our solar system.
For decades, scientists have debated the formation of super-Earths and mini-Neptunes. While traditional models propose that planetesimals—the fundamental building blocks of planets—form across vast regions of a young star’s disk, Shibata and Izidoro offer a different perspective. Their research suggests that these materials coalesce within narrow, well-defined rings at specific locations in the disk, indicating a more structured and organized process of planet formation than previously thought.
“This paper is particularly significant as it models the formation of super-Earths and mini-Neptunes, which are believed to be the most common types of planets in the galaxy,” said Shibata, a postdoctoral fellow of Earth, environmental and planetary sciences. “One of our key findings is that the formation pathways of the solar system and exoplanetary systems may share fundamental similarities.”
Utilizing advanced N-body simulations—computer models that examine gravitational interactions between celestial bodies—Shibata and Izidoro investigated planet formation in two distinct regions: one within 1.5 astronomical units (AU) of the host star and another beyond 5 AU, near the water snowline. By tracking the collisions, growth, and migration of planetesimals over millions of years, their simulations revealed a key distinction: super-Earths primarily emerge through planetesimal accretion in the inner disk, while mini-Neptunes develop beyond the snowline, predominantly via pebble accretion.
“Our results suggest that super-Earths and mini-Neptunes do not form from a continuous distribution of solid material but rather from rings that concentrate most of the mass in solids,” said Izidoro, an assistant professor of Earth, environmental and planetary sciences. ” Related research at Rice has explored aspects of this idea, but this new paper brings these concepts together into a single, coherent picture.”
The researchers’ model successfully reproduces key features of exoplanetary systems, including the “radius valley”—a distinct scarcity of planets around 1.8 times Earth’s size. Instead, exoplanets tend to cluster into two size groups: approximately 1.4 and 2.4 times Earth’s radius. Their findings explain this gap by suggesting that planets smaller than 1.8 Earth radii are predominantly rocky super-Earths, while larger ones are water-rich mini-Neptunes, closely matching real-world observations.
The study also sheds light on the remarkable size uniformity seen in multiplanet systems. Many exoplanetary systems follow a “peas-in-a-pod” pattern, where planets within the same system are strikingly similar in size. The ring model naturally accounts for this uniformity by regulating planetary formation and growth within specific rings.
Additionally, Shibata and Izidoro’s simulations align with observed planetary orbit distributions, reinforcing the idea that planets emerge from well-defined regions rather than forming randomly across the disk.
Beyond explaining existing observations, the model offers a predictive framework for planetary formation and even suggests the possibility of other Earth-like planets. While rare, Izidoro notes that rocky planets in the habitable zone could still form through late-stage giant impacts—similar to the process that led to Earth’s formation and the creation of its moon.
“We can push our model further and use it to make predictions about the types of planets expected at Earth-sun equivalent distances—regions currently beyond the reach of observations,” Izidoro said.
“Based on our predictions, up to about 1% of super-Earth and mini-Neptune systems could host Earth-like planets within the habitable zone of their stars. While this fraction is relatively low given how common super-Earths and mini-Neptunes are, it implies an occurrence rate of approximately one Earth-like planet per 300 sun-like stars.”
Looking ahead, these findings could significantly impact future exoplanet research, providing a refined framework for understanding planetary formation.
“These predictions will be tested with future telescopes, providing crucial insights into planetary formation and habitability,” Shibata said. “If future observations confirm our predictions, it could completely change our understanding of how planets form—not just in our galaxy but throughout the universe.”
The findings were recently published in The Astrophysical Journal Letters.
