New Model Explains Two Puzzling Planetary Mysteries

Exoplanet Types Illustration

An illustration of the variations among the more than 5,000 known exoplanets discovered since the 1990s. Credit: NASA/JPL-Caltech

A new model explains the rarity of planets with masses that fall between super-Earths and mini-Neptunes.

A new model that takes into account the various forces acting on newborn planets can explain two puzzling observations that have been observed among the more than 3,800 known planetary systems.

The first mystery, “radius valley”, refers to the unusual scarcity of exoplanets with a radius around 1.8 times that of Earth. According to observations made by NASA’s Kepler spacecraft, planets of this size are about 2-3 times less common than super-Earths (with radii around 1.4 times that of Earth) and mini-Neptunes (with radii around 2.5 times Earth’s). The second mystery, known as “peas in a pod,” refers to the presence of neighboring planets of similar size in hundreds of planetary systems, including TRAPPIST-1 and Kepler-223, which also have orbits with near-musical 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 Rice University’s André Izidoro, corresponding author of a study recently published in the journal Astrophysical Journal Letters. “We’re also able to show that a planet-formation model incorporating giant impacts is consistent with the peas-in-a-pod feature of exoplanets.”

Super Earth Gap Illustration

An illustration depicting the scarcity of exoplanets about 1.8 times the size of Earth that were observed by NASA’s Kepler spacecraft. Credit: A. Izidoro/Rice University

Izidoro, a Welch Postdoctoral Fellow at Rice’s NASA-funded CLEVER Planets project, and co-authors used a supercomputer to simulate the first 50 million years of the development of planetary systems 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 in resonant 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 slam into one another.

André Izidoro

André Izidoro is a Welch Postdoctoral Fellow at Rice University’s NASA-funded CLEVER Planets project. Credit: Jeff Fitlow/Rice University

Planetary migration models have been used to study planetary systems that have retained their resonant orbital chains. For example, Izidoro and CLEVER Planets colleagues used a migration model in 2021 to calculate the maximum amount of disruption TRAPPIST-1’s seven-planet system could have withstood during bombardment and still retained its harmonious orbital structure.

In the new study, Izidoro partnered with CLEVER Planets’ investigators 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.

“The migration of young planets towards their host stars creates overcrowding and frequently results in cataclysmic collisions that strip planets of their hydrogen-rich atmospheres,” Izidoro said. “That means giant impacts, like the one that formed our moon, are probably a generic outcome of planet formation.”

The research suggests planets come in two “flavors,” super-Earths that are dry, rocky, and 50% larger than Earth, and mini-Neptunes that are rich in water ice and about 2.5 times larger than Earth. Izidoro said new observations seem to support the results, which conflict with 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 instance, that a fraction of planets about twice Earth’s size will both retain their primordial hydrogen-rich atmosphere and be rich in water.

Reference: “The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains” by André Izidoro, Hilke E. Schlichting, Andrea Isella, Rajdeep Dasgupta, Christian Zimmermann and Bertram Bitsch, 2 November 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac990d

The study was funded by NASA, the Welch Foundation, and the European Research Council.

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