The Search for Planet Nine

Written by Hayley Bricker, Science Libraries Research Assistant

Astronomers haven’t yet discovered Planet Nine, but they did discover asteroid 2015 BP519, also known as Caju-- an enigmatic Kuiper Belt Object (KBO) orbiting the Sun in an incredibly puzzling orbit. Planet Nine may explain why.

Depiction of solar system with Planet 9

Solar System with Planet 9: By Tomruen - Own work, CC BY-SA 4.0,


Caju is approximately 55 times the distance between the Earth & the Sun (also known as 1 astronomical unit, or “AU”) away. This may seem too far out to be influenced by the Sun’s gravity, but Trans-Neptunian objects, bodies found past Neptune’s orbit, have been known to orbit up to 200,000 AU. Another piece of the puzzlie the orbit of Caju. The asteroid’s orbit is inclined at a 54 degree angle from the rest of the Solar System, and has an orbital eccentricity of 0.92, with 1 indicating it will only pass through rather than orbit. The rest of the Solar System generally orbits in one plane, called the ecliptic. However, the Sun has been known to capture interstellar objects, such as 2017’s Oumuamua, which tend to have highly irregular orbits compared to the rest of the System.

Artist conception of Planet 9 against space background

Artist’s conception of Planet 9: By nagualdesign; Tom Ruen, background taken from File:ESO - Milky Way.jpg - Own work, CC BY-SA 4.0,


So is it possible that Caju is a captured interstellar body? Not according to the astronomers studying Caju. In their study, available on, they look at the long term stability and likely evolution of Caju’s orbit, which lends credence to the idea that it has lived within the confines of our Sun’s gravitational pull for the majority of its life. Either it was captured very early on, or it was formed in the Solar System.

Artist conception of solar system with Planet 9

Artist’s conception of Solar System including Planet 9


Astronomers operate under the assumption that Caju was formed in the plane of the ecliptic, and was greatly perturbed by an object of great mass and gravitational influence. The hypothetical Planet 9 fits this bill of circumstantial evidence. Astronomers are persuaded by the fact that the hypothetical Planet Nine most easily explains the orbits of anomalous asteroids and comets in the Solar System, particularly Caju’s.


While there is mounting evidence for the existence of Planet Nine, it won’t be easy to observationally track down. It could be that Planet Nine is very far away-- thousands of AU away, which may mean it would be very faint or barely detectable. More objects like Caju in highly eccentric orbits can guide astronomers to select regions of the sky to point their telescopes. In the meantime, the search for Planet Nine remains a waiting game.


Related resources in the UCLA Library:

The Planets: Different Worlds:

Eccentric Planets around Evolved Stars:

Dawn of Small Worlds:


Resources Used

Planet Nine from outer space: is there another world beyond Neptune?

A Sedna-like body with a perihelion of 80 astronomical units by Chadwick A. Trujillo & Scott S. Sheppard