Planet Nine

Planet Nine is a hypothetical ninth planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs)—bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth, over 250 astronomical units (AU). These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Nonetheless, some astronomers question this conclusion and instead assert that the clustering of the ETNOs' orbits is due to observational biases stemming from the difficulty of discovering and tracking these objects during much of the year.

Based on earlier considerations, this hypothetical super-Earth—mini-Neptune sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400–800 AU. The orbit estimation was refined in 2021, resulting in a somewhat smaller semimajor axis of 380 −80 AU. This was shortly thereafter updated to 460 −100 AU, and to 290±30 AU in 2025. Astronomers Konstantin Batygin and Michael Brown have suggested that Planet Nine may be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others suggest that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.