Today, the number of confirmed exoplanets stands at 5,197 in 3,888 planetary systems, with another 8,992 candidates awaiting confirmation.
One 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 frequently than it observed super-Earths with radii about 1.4 times that of Earth and mini-Neptunes with radii about 2.5 times Earth’s.
The second mystery, known as “peas in a pod,” refers to neighboring planets of similar size that have been found in hundreds of planetary systems. Those include TRAPPIST-1 and Kepler-223, which also feature planetary orbits of near-musical harmony.
In a study led by the Cycles of Life-Essential Volatile Elements in Rocky Planets (CLEVER) project at Rice University, an international team of astrophysicists provide a new model that accounts for the interplay of forces acting on newborn planets that could explain these two mysteries.
In their research paper, which recently appeared in the Astrophysical Journal Letters, the team used a supercomputer to run a planetary migration model that simulated the first 50 million years of planetary system development.
“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 published this week in 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.”
In their model, protoplanetary disks of gas and dust also interact with migrating planets, pulling them closer to their parent stars and locking them in resonant orbital chains.
Within a few million years, the protoplanetary disk disappears, breaking the chains and causing orbital instabilities that cause two or more planets to collide.
While planetary migration models have been used to study planetary systems that retained orbital resonances, these findings represent a first for astronomers.
This work builds on previous work by Izidoro and the CLEVER Planets project. Last year, they used a migration model to calculate the maximum disruption to TRAPPIST -1’s seven-planet system.
In a paper that appeared on Nov. 21st, 2021, in Nature Astronomy, they used N-body simulation to show how this “peas in a pod” system could have retained its harmonious orbital structure despite collisions caused by planetary migration. This allowed them to place constraints on the upper limit of collisions and the mass of the objects involved.
Their results indicate that collisions in the TRAPPIST-1 system were comparable to the impact that created the Earth-Moon system.
“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. That means giant impacts, like the one that formed our moon, are probably a generic outcome of planet formation.” Said Izidoro.
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.
Sources
The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains
André Izidoro1,2, Hilke E. Schlichting3, Andrea Isella1, Rajdeep Dasgupta2, Christian Zimmermann4, and Bertram Bitsch4
Published 2022 November 2 • © 2022. The Author(s). Published by the American Astronomical Society.
