Supercomputer simulations provided an explanation for why many exoplanets are either super-Earths or mini-Neptunes, with only a few planets in between.
outer planets They can come in a variety of sizes and masses. If you were to plot on a graph the number of planets that astronomers have discovered of each size, you would find two peaks: one at 1.4 times a landAnd another radius of 2.4 times the radius of the Earth. Between them is a cliff or valley, the radius of which is about 1.8 times that of Earth, which indicates the relative rarity of planets of this size.
This “Valley Radius” doesn’t happen by accident; Something happens that means planets more than 1.8 times the size of Earth are found two or three times as infrequently. New supercomputer simulations, by a team led by Andre Isidoro, a planetary scientist at Rice University in Texas, examine the first 50 million years of existence of a model planetary system to evaluate two major hypotheses to explain the gap in planetary sizes.
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One hypothesis is that differences in composition between rocky superplanets and miniature Neptunes rich in water and hydrogen preferentially lead to the formation of planets of certain sizes. The other hypothesis is that the super-Earth begins its life as a small but Neptune lose its thick atmosphere when they migrate near them star due to gravitational interactions.
The new simulations support the migration model, and also explain why we so often find strings of exoplanets of similar size in what scientists call near-resonance orbits. Resonance occurs when the orbital periods of the planets are in multiples of each other; For example, an outer planet may orbit once for every two orbits of its inner planet. New supercomputer simulations confirm that the internal migration of planets within the massive disk of dust and gas of a young star system is causing resonant strings of worlds, like “peas in a pod.”
However, astronomers know that Protoplanetary disk That allows this migration does not last forever. As the young star begins to generate more energy, its radiative winds blow the disk away; As the disk dissipates, the planets become unstable, causing collisions between worlds and smaller protoplanets.
“The migration of young planets toward their host stars creates overcrowding and often leads to catastrophic collisions that strip planets of their hydrogen-rich atmospheres,” Isidoro said. statement. “It means giant effects, like the one that formed our moonprobably a general consequence of planetary formation.”
The simulations found that planetary migration, subsequent orbital perturbation, and loss of the planets’ thick atmospheres all conspire to preferentially create two groups of planets: rocky, dry super-Earths, and young Neptunes that haven’t migrated as far inland and are able to retain their thick atmospheres of hydrogen and water.
“I believe we are the first to explain the Valley of Radius using a model of planet formation and dynamical evolution that consistently accounts for multiple observational constraints,” said Isidoro. “We are also able to show that a model of planet formation that includes giant effects is consistent with the pea 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 sizes in close orbits that are in tune with each other. The new results indicate that we should expect to find more multi-planet systems with planets of similar size in resonant orbits in the future.
The results were published on November 2 in Astrophysical Journal Letters.
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