Is there an undiscovered planet beyond Neptune?
In plain words
Some very distant icy bodies have orbits that seem lined up in a way that a hidden planet several times Earth's mass could explain. Others argue the alignment is an accident of where telescopes looked.
Precise statement
Determine whether the apsidal and nodal clustering of distant trans-Neptunian objects (semimajor axis above about 250 au, perihelion above about 40 au) is statistically significant after correcting for survey selection, and whether it requires an undetected planet of roughly 5 to 10 Earth masses on an eccentric orbit at a few hundred au. An answer is a detection, or an exclusion of the proposed parameter space by direct search and by unbiased orbital statistics.
What would settle it
Direct detection by deep wide-field surveys, or a large, well-characterized sample of distant objects from the Rubin Observatory survey showing no significant clustering.
Status in the literature
Unverified note
No detection as of 2026; the Rubin Observatory, whose first images were released in 2025, is expected to enlarge the distant-object sample substantially.