A new model may explain two puzzling observations that have appeared repeatedly among the more than 3,800 planetary systems recorded to date.
A puzzle known as the “radius valley” refers to the rarity of exoplanets with a radius about 1.8 times that of Earth. NASA’s Kepler spacecraft observed planets of this size about 2-3 times less often than it observed super-Earths with radii about 1.4 times that of Earth and mini-Neptunes with radii about 2.5 times that Earth.
The second mystery, known as “peas in a pod,” refers to similar-sized neighboring planets that have been found in hundreds of planetary systems. These include TRAPPIST-1 and Kepler-223, which also have planetary orbits of almost musical harmony.
“I believe we are the first to explain the radius valley using a model of planet formation and dynamical evolution that consistently accounts for multiple observational constraints,” says André Izidoro of Rice University, corresponding author of a study in Astrophysical Journal Letters.
“We are also able to show that a model of planet formation that incorporates giant impacts is consistent with the pea-in-a-pod feature of exoplanets.”
Izidoro, a postdoctoral fellow in Rice University’s NASA-funded CLEVER Planets program, and his colleagues used a supercomputer to simulate the first 50 million years of planetary system development using a planetary migration model.
In the model, protoplanetary disks of gas and dust that give rise to young planets also interact with them, pulling them closer to their parent stars and locking them into coordinated orbital chains. The chains are broken within a few million years, when the disappearance of the protoplanetary disk causes orbital instabilities that lead two or more planets to collide with each other.
Planetary migration models have been used to study planetary systems that have retained their coordinated orbital chains. For example, Izidoro and CLEVER Planets colleagues used a migration model in 2021 to calculate the maximum amount of disruption that TRAPPIST-1’s seven-planet system could withstand during bombardment and maintain its harmonic orbital structure.
“The migration of young planets toward their host stars creates crowding and often leads to cataclysmic collisions that strip the planets of their hydrogen-rich atmospheres,” says Izidoro. “This means that giant impacts, like the one that formed our moon, are probably a general result of planet formation.”
The research suggests that the planets come in two “flavors”, super-Neptunes that are dry, rocky and 50% larger than Earth, and mini-Neptunes that are rich in water ice and about 2.5 times the size of Earth . Izidoro says the new observations seem to support the results, which contradict the traditional view that both super-Earths and mini-Neptunes are exclusively dry and rocky worlds.
Based on their findings, the researchers made predictions that can be tested by NASA’s James Webb Space Telescope. They suggest, for example, that a fraction of planets roughly twice the size of Earth will retain their primordial hydrogen-rich atmospheres and be rich in water.
In the new study, Izidoro collaborated with CLEVER Planets researchers Rajdeep Dasgupta and Andrea Isella, both of Rice, Hilke Schlichting of the University of California, Los Angeles, and Christian Zimmermann and Bertram Bitsch of the Max Planck Institute for Astronomy in Heidelberg, Germany . .
Research funding came from NASA, the Welch Foundation and the European Research Council.
Source: Rice University
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