It was the sort of thing that, like the extraterrestrials, just has to be out there:
The presence of Planet Nine was proposed in 2016, when astronomers Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena observed that the orbits of six trans-Neptunian objects (TNOs) — part of the Kuiper belt, a collection of small bodies orbiting the Sun beyond Neptune — seemed to be clustered together.
This clustering, they said, had to be due to the gravitational influence of a huge planet hiding somewhere in the outer Solar System, at least 400 times as far from the Sun as Earth, or around 10 times as far as the most famous TNO, the dwarf planet Pluto. If proved to exist, the distant world would be a major discovery — a giant beyond Neptune that would unquestionably be classed as a planet…
A team led by Kevin Napier, a physicist at the University of Michigan in Ann Arbor, has now taken this analysis further. Combining 3 surveys to examine the orbits of 14 ‘extreme’ TNOs (ETNOs) — those orbiting far beyond Neptune — the researchers found that the objects’ orbits could be explained without the presence of a nearby planet.
Jonathan O’Callaghan, “No sign of Planet Nine? Trail runs cold for hypothetical world” at Nature
Darn. But there’s always extraterrestrial light sail Oumuamua to liven things up.
Trans-Neptunian objects: