planet X

[ plan-it eks ]

noun

, Astronomy.

1. a planet beyond the orbit of Pluto whose existence has been hypothesized but not confirmed.

Word History and Origins

Origin of planet X¹

First recorded in 1880–85

Example Sentences

In 1906, astronomer and businessman Percival Lowell launched a search for "Planet X," a hypothetical giant planet orbiting the sun beyond Neptune.

From Scientific American

Lowell was convinced that Planet X existed based on some supposed irregularities he had observed in the orbits of Neptune and Uranus.

From Scientific American

Today, the Planet X hypothesis is largely considered to be discredited.

From Scientific American

The chances of this are about 7%, and if this is the case, then an object akin to Lowell's long-sought Planet X might be out there after all, though it would still be too far away to influence Neptune's orbit.

From Scientific American

As the communications officer, everything runs through Uhura: She’s the voice of what’s happening elsewhere on the ship, and what’s happening outside the ship, whether announcing the presence of some other spacecraft or relating what’s up with Planet X. Even reciting lines like “I’m receiving Class Two signals from the Romulan vessel” or “Revised estimate on cloud visual contact 3.7 minutes,” she is the picture of the professional.

From Los Angeles Times