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Is there a ninth major planet lurking in the outer reaches of our Solar System?

Artist's impression of Planet NineArtist's impression of Planet NineAn asteroid recently discovered in the outer Solar System is proving of great interest to astronomers. They believe that its strange orbital path around the Sun can only be explained if a giant planet—so far undiscovered—is lurking in the distant reaches of the Solar System, far beyond Neptune. The astronomers think that the planet, known as Planet Nine, if it exists, would be about four times the size of Earth and 10 times its mass. It lies so far away it has so far remained out of sight. If Planet Nine is ever found, it would be the first discovery of a new “true” planet in our Solar System since Neptune in 1846.

Orbits of Caju, TNOs and Planet Nine (lime green)Orbits of Caju, TNOs and Planet Nine (lime green)


The new asteroid, nicknamed Caju, is officially named 2015 BP519. It is described as a “trans-Neptunian object” or TNO, because it lies beyond the orbit of the outermost planet, Neptune. Discovered in 2015, Caju has been tracked by a group of astronomers led by Juliette Becker at the University of Michigan.

The asteroid’s orbit is tilted by a huge angle (54°) to that of the planets—which all lie on roughly the same plane as they circle the Sun—and most other asteroids. The Michigan astronomers have found that Caju’s weird orbit could be explained if the gravitational pull of a large unseen planet has pulled it into that position.

Orbits of planets, TNOs and Planet Nine Orbits of planets, TNOs and Planet Nine

A super-Earth?

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