A new model that takes into account the different forces acting on newborn planets could explain two surprising observations among the more than 3,800 known planetary systems.

The first mystery, the “radius valley”, belongs to the unusual scarcity of exoplanets with a radius of about 1.8 times the Earth. According to observations by NASA’s Kepler spacecraft, planets of this size are about 2 times larger than super-Earths (about 1.4 times the radius of Earth) and mini-Neptunes (about 2.5 times the radius of Earth). -3 times less common.

The second conundrum, known as the “pea in a coke”, concerns the existence of similarly sized nearby planets in hundreds of planetary systems with almost musically harmonious orbits, including TRAPPIST-1 and Kepler-223.

“I believe we are the first to explain the radius valley with a model of planet formation and dynamic evolution that consistently explains multiple observational constraints,” said Andre Isidoro of Rice University, author of the study, which was recently published in the journal Science. Astrophysics. magazine letters. “We can also show that a pattern of planet formation involving giant collisions is consistent with the shell pea feature of exoplanets.”

Isidoro, a postdoctoral researcher on the NASA-funded Rice CLEVER Planets project, and co-authors used a supercomputer to simulate the first 50 million years of the evolution of planetary systems using a planetary transit model. In the model, the protoplanetary disks of gas and dust that gave birth to the young planets also interact with them, bringing them closer to their parent stars and locking them into resonant orbital chains. The chains break in a few million years when the disappearance of the protoplanetary disk causes orbital instabilities that cause two or more planets to crash into each other.

Planetary migration models have been used to study planetary systems that maintain resonant orbital chains. For example, Isidoro and his CLEVER Planets colleagues used a migration model in 2021 to calculate the maximum number of interruptions the seven-planet TRAPPIST-1 system can withstand during a bombardment and maintain its cohesive orbital structure.

In the new study, Isidoro collaborated with CLEVER Planets researchers Rajip Dasgupta and Andrea Isella, both Rice, Hilke Schlichting of UCLA, and Christian Zimmermann and Bertram Beach of the Max Planck Institute for Astronomy in Heidelberg, Germany.

“The migration of young planets to their host stars creates overcrowding and often leads to devastating collisions that destroy the planets’ hydrogen-rich atmospheres,” Isidoro said. “This means that giant collisions such as those that formed our Moon were likely a common result of planet formation.”

The study shows that planets have two “flavors”: super-Earths that are dry, rocky, and 50% larger than Earth, and mini-Neptunes, rich in water ice and about 2.5 times larger than Earth. Isidoro said the new observations support results that contradict the traditional view that both super-Earth and mini-Neptune are completely dry and rocky worlds.

Based on their findings, the researchers made predictions that could be tested using NASA’s James Webb Space Telescope. For example, they suggest that some of the planets roughly twice the size of Earth will retain their original hydrogen-rich atmospheres and become water-rich.