Is there a hidden Planet Nine lurking in the outer solar system?

The answer

The Planet Nine hypothesis, proposed by Caltech astronomers Konstantin Batygin and Mike Brown in 2016, suggests an undiscovered planet of 5-10 Earth masses orbits the Sun at 400-800 AU with a period of 10,000-20,000 years. The evidence comes from the peculiar orbital clustering of extreme trans-Neptunian objects like Sedna, whose elliptical orbits align in ways that have only a 0.007% probability of occurring by chance without an unseen gravitational influence. Skeptics argue the clustering may result from observational bias, as surveys preferentially detect objects in certain sky regions. In 2024, the LSST camera at the Vera C. Rubin Observatory began operations, expected to discover thousands of new distant solar system objects that could confirm or refute the hypothesis. If Planet Nine exists, it may have been captured from another star or ejected from the inner solar system during early planetary migration.