Supercomputer simulations have provided an explanation as to why so many exoplanets are either super-Earths or mini-Neptunes, with few planets in between.
Exoplanets can come in a variety of sizes and masses. If you were to plot on a graph the number of planets of each size discovered by astronomers, you would find two peaks: one at 1.4 times Earthradius of , and another at 2.4 times the radius of the Earth. Between them is a trough or valley, about 1.8 times the radius of Earth, signifying the relative rarity of planets of this size.
This “ray valley” does not happen by chance; something is happening that means planets that are 1.8 times the size of Earth are found two or three times more rarely. The new supercomputer simulations, from a team led by Andre Izidoro, a planetary scientist at Rice University in Texas, modeled the first 50 million years of a typical planetary system’s existence to assess two main hypotheses. to explain the discrepancy in the size of the planets.
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One hypothesis is that compositional differences between rocky super-Earths and hydrogen- and water-rich mini-Neptunes preferentially lead to the formation of planets of certain sizes. The other hypothesis is that super-Earths begin life as mini-Neptunes but lose their thick atmospheres as they migrate closer to their star through gravitational interactions.
The new simulations support the migration model and also explain why we frequently find strings of similarly sized exoplanets in what scientists call near-resonant orbits. Resonance occurs when the planets’ orbital periods fall in multiples of each other; for example, an outer planet might orbit once for every two orbits of an inner planet. The new supercomputer simulations confirm that the inward migration of planets in the vast disk of dust and gas of a young star system causes resonant chains of worlds, like “peas in a pod”.
However, astronomers know that the protoplanetary disc that allows this migration does not last forever. As the young star begins to generate more energy, its radiation wind carries the disk away; as the disk dissipates, the planets destabilize, causing collisions between worlds and smaller protoplanets.
“The migration of young planets to their host stars creates overpopulation and frequently results in cataclysmic collisions that strip planets of their hydrogen-rich atmospheres,” Izidoro said in a statement. statement. “That means giant impacts, like the one that formed our moonare probably a generic result of planet formation.”
The simulations showed that the migration of planets, the ensuing orbital destabilization, and the loss of thick planetary atmospheres all conspire to preferentially create two populations of planets: super-Earths which are rocky and dry, and mini-Neptunes. who have not migrated as quickly. far inward and are able to retain their thick hydrogen and water atmospheres.
“I believe we are the first to explain the valley of the ray using a model of planet formation and dynamic evolution that consistently accounts for multiple constraints from observations,” Izidoro said. “We are also able to show that a model of planet formation incorporating giant impacts is consistent with the pea-in-a-pod feature of exoplanets.”
This “pea in a pod” feature is commonly found in planetary systems such as TRAPPIST-1, which is home to seven rocky worlds of similar sizes in close and resonant orbits with each other. The new findings suggest that we should expect to find many more multiplanetary systems with similarly sized planets in resonant orbits in the future.
The results were published on November 2 in Letters from the Astrophysical Journal.
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