Planet Nine
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No, it's not Pluto. Unfortunately for die-hard astronomy fans, Pluto is still languishing in its dwarf planet classification, and now it may become replaced by an even more distant planet, hidden somewhere in the mysterious Kuiper Belt. The supposed planet, creatively nicknamed Planet Nine, has not been proven to exist yet, but astronomers have a wealth of data that points to something about 10 times the size of Earth lurking at the edge of the solar system.

The search for Planet Nine started relatively innocuously with some research in 2014: astronomers Scott Shephard and Chad Trujillo published a paper studying a strange object called Sedna, a 1,000-kilometer-wide trans-Neptunian object (TNO). TNOs are minor planets, asteroids, and other bodies who orbits taken them farther out than Neptune, and include Pluto and 10-30 other objects.

The strange thing about Sedna was that it's incredibly long and eccentric orbit seemed to tie it to an unknown planet somewhere outside the solar system, leading Shephard and Trujillo to hypothesize there may be a ninth planet beyond Pluto.

This led a different pair of astronomers, Mike Brown and Konstantin Batygin, to start investigating other TNOs in hopes of finding a pattern that would show Planet Nine's gravity in action.

In 2016, they announced that they had: several TNOs were shown to have orbits that were perpendicular to the normal orbital plane of the solar system, a phenomenon that can be explained by the existence of super-Earth-sized planet.

Based on the data collected, Brown and Batygin are 99.99% sure that Planet Nine exists.

Unfortunately, spotting Planet Nine has proved harder than they expected-even working with Shephard and Trujillo, Brown and Batygin have to rely primarily on one telescope, the Subaru Telescope in Hawaii.

Inclement weather and bad luck have repeatedly foiled their attempts at observing the planet directly. There's always the chance that Planet Nine is ever farther out than expected, or that it's much smaller than estimated, both of which would make it harder to see.

The other option? Planet Nine might not exist at all.