Supercomputer simulations have provided an explanation for 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 graph how many planets of each size astronomers have discovered, you would find two peaks: one at 1.4x Earth‘s radius and another at 2.4 times the Earth’s radius. In between is a depression or valley about 1.8 times Earth’s radius, indicating the relative scarcity of planets this size.
This “radius valley” does not arise by chance; something is happening that results in planets 1.8 times the size of Earth being found two to three times less often. The new supercomputer simulations by a team led by André Izidoro, a planetary scientist at Rice University in Texas, modeled the first 50 million years of existence of a typical planetary system to evaluate two leading hypotheses explaining the planetary size gap.
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One hypothesis is that the compositional differences between rocky super-Earths and hydrogen- and water-rich mini-Neptunes favor the formation of planets of specific sizes. The other hypothesis is that super-Earths begin life as mini-Neptunes, however lose their thick atmosphere when they wander closer to theirs star due to gravitational interactions.
The new simulations support the migration model and also explain why we often find chains of similar-sized exoplanets in what scientists call near-resonant orbits. Resonance occurs when planetary periods fall in multiples of each other; For example, an outer planet could orbit once every two orbits of a planet within it. The new supercomputer simulations confirm that the influx of planets within 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 disk this allows this migration not to last forever. As the young star begins to generate more energy, its radiative wind blows the disk away; As the disk disintegrates, the planets become destabilized, leading to collisions between worlds and smaller protoplanets.
“Migration of young planets toward their host stars causes overcrowding and often results in catastrophic collisions that deprive the planets of their hydrogen-rich atmospheres,” Izidoro said in a expression. “That means huge impacts like this one formed our moonare likely a generic result of planet formation.”
The simulations found that the migration of planets, subsequent orbital destabilization, and the loss of dense planetary atmospheres all combine to create preferentially two populations of planets: the super-Earths, which are rocky and arid, and the mini-Neptunes, which are not thus migrated far inwards and are able to preserve their dense atmospheres of hydrogen and water.
“I believe we are the first to explain Radius Valley with a model of planet formation and dynamic evolution that self-consistently accounts for several observational limitations,” Izidoro said. “We are also able to show that a model of planet formation involving giant impacts is consistent with the pea-in-a-pod feature of exoplanets.”
This “peas-in-a-pod” property is commonly found in planetary systems such as TRAPPIST-1, which is home to seven rocky worlds of similar size in tight, resonant orbits. The new results suggest that we should expect to find many more multiplanet systems with similarly sized planets in resonant orbits in the future.
The results were published on November 2nd The Letters of the Astrophysical Journal.
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