There may be a ninth planet lurking at the edge of the solar system

Since the 2016 paper by Konstantin Batygin and Mike Brown (the astronomer who demoted Pluto to dwarf planet), evidence has mounted for a ninth planet roughly 10 times Earth's mass orbiting the Sun at 400–800 AU — far beyond Neptune. Its proposed existence is inferred from the anomalous clustering of orbits of six distant trans-Neptunian objects, which statistically would be expected to be randomly distributed. The planet, if it exists, would take 10,000–20,000 years to complete one orbit. No direct detection has been made yet; surveys with the Vera Rubin Observatory, coming online in 2024, may either find it or rule it out definitively.

Source: Batygin & Brown, The Astronomical Journal (2016)