There is a mystery in our solar system surrounding the orbits of Kuiper belt objects. More than one trillion icy objects smaller than our moon orbit the sun in a donut-shaped ring beyond Neptune. Oddly, a cluster of outer Kuiper belt objects all ellipse in a similar way, as though being gravitationally pulled in one direction. The leading hypothesis is that an unseen object, five to 10 times the mass of Earth, is causing the pulling effect. The mystery object has been dubbed Planet 9.
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Source: Phys.org