There might be an undiscovered ninth planet in the solar system. Though it has yet to be observed and remains only a theoretical possibility, the case for its existence is strong.

Caltech astronomer Michael Brown—the scientist behind Pluto’s demotion to dwarf planet status—and his team argued in a new paper that the strange orbits of certain objects in the Kuiper Belt could be explained by the presence of a large planet. How large? Calculations put it at ten times the size of Earth. And the scientists have made the task of identifying the planet much easier: they’ve already mapped its possible orbital path. Whether the planet is finally observed by Michael Brown or some amateur astronomer is of little consequence. Its discovery will join our long history of planetary sleuthing.

In 1846, two astronomers independently calculated that Uranus’ orbit was being disturbed by an unknown object, and predicted where it might be found. Subsequent observations led to the discovery of Neptune. Fifty years later, Dr. Percival Lowell used similar observations on Neptune’s orbit to predict yet another planet, which he dubbed Planet X. In 1930, years after Lowell’s death, the supposed Planet X—Pluto—was observed for the first time.

To be fair, these successes were countered by some spectacular failures. Urbain Leverrier, one of Neptune’s co-discoverers, famously and erroneously believed in Vulcan, a nonexistent planet somewhere nestled between Mercury and the Sun. Not to be outdone, 20th century astronomer William Pickering predicted no fewer than seven Trans-Neptunian planets, which he dubbed O,P,Q,R,S,T, and U (a Trans-Neptunian object refers to any minor planet orbiting the Sun at a distance greater than Neptune). But he was hardly alone. Many astronomers of his day falsely predicted the existence of one or more additional planets.

In 1972, Joseph Brady proposed that Halley’s comet was being disturbed at its perihelion, the point of the comet’s orbit nearest the sun. Brady believed that a large Trans-Neptunian object was responsible. Brady’s work has since received considerable criticism, but it is worth mentioning that the unusual orbits of the Kuiper Belt objects that helped theorize the existence of Planet Nine are also the strangest at the perihelion.

Michael Brown and his team are reassured by the mass of astronomers and amateur planetary sleuths who have turned their telescopes to the band of sky where the alleged new planet may be in orbit. In a few years time, if all goes well, we may be rewriting the textbooks yet again.



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The Scientific Monthly, Vol. 34, No. 1 (Jan., 1932), pp. 5-21
American Association for the Advancement of Science
Publications of the Astronomical Society of the Pacific, Vol. 84, No. 498 (April 1972), pp. 314-322
Astronomical Society of the Pacific
Isis, Vol. 67, No. 4 (Dec., 1976), pp. 551-564
University of Chicago Press on behalf of History of Science Society