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The search for Planet Nine

Artist's impression of Planet NineArtist's impression of Planet NineCaju, an asteroid discovered in the outer Solar System in 2015, 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)


Asteroid 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 planetNeptune. Since its discovery in 2015 it has been carefully 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—as well as 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 NineOrbits of planets, TNOs and Planet Nine

A super-Earth?

Some astronomers think that Planet Nine, if it exists, could be the core of a giant planet that was ejected from its original orbit by Jupiter during the early years of the Solar System. Others propose that the planet may have captured from another solar system.

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