Turns out there may be a ninth planet* after all.
No, it’s not Pluto, but lurking in the dark outer reaches of our solar system, twenty times farther from the Sun than Neptune, is what NASA claims is a large ninth planet. On October 4, 2017, NASA issued a press release claiming that a massive, invisible planet best explains gravitational and orbital anomalies in the outer solar system. Planets don’t emit their own light, and because Planet Nine is so far away, it’s too dark to view directly. “There are now five different lines of observational evidence pointing to the existence of Planet Nine,” said Caltech planetary astrophysicist Konstantin Batygin, who along with Caltech astronomer Mike Brown, co-authored a 2016 study of Kuiper Belt objects. The Kuiper Belt contains trillions of leftover objects from the formation of our solar system, such as comets and dwarf planets like Pluto and Sedna (which are also known as “trans-Neptunian” objects). It’s shaped like a disc and lies beyond the orbit of Neptune.
Planet Nine is also responsible for the precession, or tilt, of our solar system’s axis. NASA estimates that Planet Nine is about 10 times as massive as Earth, making it a rocky “super-Earth” (more on super-Earths later). It has a wide elliptical orbit that takes it as far as 100 billion miles from the Sun.
* Author’s note: Pluto is not the ninth planet. Pluto is five times smaller than Earth’s Moon, and was downgraded to dwarf planet status in 2006 by astronomer Mike Brown. It is one of potentially hundreds of thousands of dwarf planets in the Kuiper Belt.
Gravitational and orbital anomalies
The existence of Planet Nine best explains various gravitational and orbital anomalies of objects in the Kuiper Belt. In a survey of the six most distant known objects in our solar system with orbits exclusively beyond Neptune, Batygin and Brown found that the distant icy bodies “all have elliptical orbits pointing in the same direction [relative to the plane of the planets].” Gravity from hypothetical Planet Nine explains why these objects have a relative tilt of 30 degrees downward, relative to the planets. They simply mirror Planet Nine’s orbital path, due to the pull of its gravity. Planet Nine’s supposed gravity, according to Batygin, should also result in objects with orbital tilts of up to 90 degrees. Sure enough, five known Kuiper Belt objects exert this behavior as well. In addition to their extreme tilt, these Kuiper Belt objects orbit the Sun in the opposite direction as the planets, suggesting a strong source of gravity. Batygin and Brown’s observations confirmed the predictions made by their computer models; that a super-Earth with a highly elliptical orbit best explains the confluence of these anomalies.
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