Rice University scientists and their collaborators are the first to use computer simulations to integrate a model of planet formation and evolution. It explains two puzzling observations of exoplanets orbiting distant stars. The scarcity of worlds is 1.8 times the size of Earth and the near-identical size of adjacent planets in hundreds of planetary systems.
The “radius valley” mystery refers to the scarcity of exoplanets with a radius of about 1.8 times that of Earth. The Kepler spacecraft of NASA observed planets of this size about 2-3 times less frequently than super-Earths with radii about 1.4 times that of Earth. The second mystery, “peas in a pod,” refers to neighbouring planets of comparable size discovered in hundreds of planetary systems. TRAPPIST-1 and Kepler-223, for example, have planetary orbits that are nearly musical in harmony.
“I believe we are the first to explain the radius valley using a model of planet formation. And dynamical evolution that self-consistently accounts for multiple constraints of observations,” said Andre Izidoro of Rice University, the study’s corresponding author. “We can also show that a planet-formation model incorporating giant impacts is consistent with exoplanets’ peas-in-a-pod feature.”
Izidoro, a Welch Postdoctoral Fellow at Rice’s NASA-funded CLEVER Planets project, and colleagues used a supercomputer. They simulated the first 50 million years of planetary system development using a planetary migration model. Protoplanetary discs of gas and dust that give birth to young planets interact with them in the model. These draw them closer to their parent stars and locking them in resonant orbital chains. The chains are broken within a few million years when the protoplanetary disc vanishes. It causes orbital instabilities that cause two or more planets to collide.
Planetary migration models have been used to investigate planetary systems with resonant orbital chains. Izidoro and his CLEVER Planets colleagues, used a migration model in 2021. They calculated the maximum amount of disruption TRAPPIST-1’s seven-planet system could have absorbed during bombardment while maintaining its harmonious orbital structure.
Izidoro collaborated on the new study with Rice researchers Rajdeep Dasgupta and Andrea Isella, 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.
“The migration of young planets towards their host stars causes overcrowding. These frequently results in cataclysmic collisions that deplete planets of their hydrogen-rich atmospheres,” Izidoro explained. “This implies that giant impacts, are most likely a generic result of planet formation.”
According to the research, planets come in two “flavours”: super-Earths, which are dry, rocky, and 50% larger than Earth. And mini-Neptunes, which are rich in water ice and about 2.5 times larger than Earth. New observations, according to Izidoro, appear to support the findings. It 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 NASA’s James Webb Space Telescope can test. They propose that a fraction of planets about twice the size of Earth will retain their primordial hydrogen-rich atmosphere.