For this ninth planet, astronomers would most likely assume that: it originated in another star system and was gravitationally captured.
A solar system is a collection of both outer and inner planetary (astronomical) bodies alongside the Moon that usually orbit around the Sun, in slightly elliptical orbits.
Based on astronomical science, there are nine (9) planets in the solar system and these are;
Astronomers would most likely assume that a planet that orbited the Sun in an opposite direction in contrast to the other planets, originated in another star system and was gravitationally captured.
Read more on the solar system here: brainly.com/question/1286910