Catastrophic collisions may explain ‘forbidden’ exoplanets

Just 30 years ago, scientists weren’t sure if there were any planets outside our solar system. Now they have discovered more than 5,000 of them. But when astronomers calculated the sizes of these exoplanets, a strange gap appeared. There are a lot of “super-Earths” out there – rocky orbs about 1.4 times larger than Earth. And there are many small “Neptunes” that are about 2.4 times the width of Earth. But very few planets lie in between; It’s about the size of most worlds with one of two cookie cutters. A new model has been published Astrophysical Journal Letters Provides a new answer to why this is so: It’s all about collisions.

Previous hypotheses about a planetary “radius gap” have suggested that higher temperatures shrink certain planets, says Rice astrophysicist Andre Isidoro, lead author of the new study. He says planets tend to move closer to their host stars over time. This causes relatively light planets to evaporate faster as they heat away from their outer gases, the thinking goes, while heavier planets have enough gravity to hold those gases and maintain their size.

Izidoro’s work challenges this heat-based explanation, suggesting that the gap resulted from planetary collisions instead. His team ran computer simulations based on theories about how planetary systems are likely to evolve: Planets that form near stars are typically rocky, while distant planets are generally very rich in water or ice — and most in both categories start at the larger planets. , the size range of miniature Neptune, says Isidoro.

As planetary systems age and young planets drift toward their stars, the planets’ orbits become unstable, and they often collide. When rocky planets smash together, Isidoro says, they have more mass combined. But they also lose gas layers, so their combined radius tends to decrease; The two form one denser planet. Isidoro adds that when two water-rich planets collide, “they don’t change in size much because the water is less dense, so they stay over the valley of the radius” even after the outer gases have disappeared. A collision of a rocky planet with a water-rich planet usually results in a larger, water-rich planet – again over the radius gap.

Credit: Amanda Montañez; Source: “The California-Kepler Survey. VII. Precise Planet Radius, Leveraging Gaia DR2, Revealing the Stellar Mass Dependence of the Planet’s Gap Radius,” by Benjamin J. Fulton and Eric A. Petegora, in The Astronomical Journal, Vol. 156; November 14, 2018 (data)

For this collision model to prove valid, the planets must not lose mass to heating as much as previously thought, says James Owen, an astrophysicist at Imperial College London who was not involved in the new study. But on the other hand, Owen notes, “If you believe in mass-loss models, you should point out that collisions between planetary bodies … are much less frequent than we think.”

To test both hypotheses, Owen says that high-resolution space telescopes in the future could observe Neptune’s young makeup. If exoplanets in this size range had a lot of hydrogen and helium, this would favor a mass-loss picture; A high percentage of water and ice would support the impact explanation.

However, “there is no way to answer our questions entirely through observational means,” says study co-author Helke Schlichting, an astrophysicist at the University of California, Los Angeles. Planet formation over millions of years cannot be observed in real time. “I think you need to model the research to really understand what the data is telling us,” says Schlichting, and such insights “may revolutionize our thinking about the formation of our solar system.”

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