Plausible Explanation for the Pileups of Giant Planets

Some zones encircling baby stars are far more popular than others, drawing crowds of giant

planets while the other potential paths for orbits remain empty. Now computer simulations have revealed a plausible explanation of this phenomenon. Rather than occupying orbits at regular distances from a star, giant gas planets similar to Jupiter and Saturn appear to prefer to occupy certain regions in mature solar systems while staying clear of others. As reported study lead author Richard Alexander, an astrophysicist at the University of Leicester in England their models offer a plausible explanation for the pileups of giant planets observed recently detected in exoplanet surveys. The radiation in question carves gaps in the protoplanetary disks of gas and dust that swirl around young stars and provide the raw materials for worlds. This process, called photo-evaporation, is the result of ultraviolet light and other high-energy photons from the star heating the disk material. The disk material closest to the star gets very hot but is held in place by the strong gravity of the star. As such, any giant planets that migrate there from outer portions of the disk, often called hot Jupiters, will stay, perhaps eventually getting all their gas stripped off. Farther out, where the star’s gravity is much weaker, the heated disk matter evaporates into space, forming the gaps. These gaps then essentially act as barricades that keep any more planets from spiraling in ward. The precise locations of those gaps depend on the mass of the planets, but they generally pop up in a zone between 1 and 2 astronomical units from a star like the sun. According to Ilaria Pascucci, an assistant professor at the University of Arizona’s Lunar and Planetary Laboratory supercomputer models of the effects of photo-evaporation on protoplanetary disks around young stars revealed that the final distribution of planets does not vary smoothly with distance from the star, but instead has clear ‘deserts’, deficits of planets, and ‘pileups’ of planets at particular locations. The experiments considered young solar systems with various combinations of giant planets at different locations and different stages in time, since researchers do not yet know exactly where and when planets form around baby stars. They found, just as observations of real alien stellar systems have shown, that giant planets migrate inward, dragged by protoplanetary material falling toward the star. However, once a giant planet encounters a gap cleared by photo-evaporation, it stays put, settling into a stable orbit around its star. Future research also could model the effects of photo-evaporation on lower-mass planets and multiple-planet systems.

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