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Ever since its discovery in 1930, Pluto has been mysterious—and divisive.

Ever since its discovery in 1930, Pluto has been mysterious—and divisive.


'A New Planet Beyond Neptune': The year we discovered Pluto

The news headline in the 21 March 1930 issue of Science was simple and muscular: “A New Planet Beyond Neptune.” And so was the three-page story beneath it. Astronomers at the Lowell Observatory in Flagstaff, Arizona, had spotted an unknown planet close to where the observatory’s founder, funder, and eponym, Percival Lowell, had predicted before his death 14 years earlier. Lowell had based his forecast on Herculean calculations involving glitches in the orbit of the planet Uranus. Earlier researchers had also “discovered” far-out planets by crunching numbers, but their work had fallen by the wayside. Lowell’s had stood the test of time—and his own successors had borne it out.

Then the “yes, but”s began. By mid-April, “Planet X” was looking disturbingly unplanetlike. Observations showed that it was a lot smaller than Lowell had predicted, and its orbit was strangely oblong. “[I]t is now thought that it may be proved to be a unique asteroid or an extraordinary comet-like object,” Science reported. No worries, the director of the Harvard College Observatory rejoined: Such a weird, far-flung object could be “perhaps of greater importance in cosmogony” than another run-of-the-mill planet would.

Things got even murkier in May after an astronomer analyzed Lowell’s original calculations and concluded that they would have given the same result even if “Planet X” didn’t exist. In short, the discovery had been an accident. Still, a reviewer pointed out, the accident was still a discovery, too: “The value of a scientific hypothesis is not to be judged by its truth but by the impulse which it gives to the search for truth.” (He also speculated that other small trans-Neptunian bodies might turn up—as indeed they did, more than 60 years later.) The upshot: partial credit for Lowell.

The pro-Lowellites rallied. By 13 June, “Planet X” had a name, chosen in a worldwide contest. The winner, submitted by the astronomically well-connected 11-year-old daughter of a University of Oxford professor: Pluto, the Roman god of the underworld—like the outer solar system, a place of Stygian gloom. (A popular runner-up in the contest, Minerva, the goddess of wisdom, was already the name of an asteroid.) Pluto, the Lowell Observatory astronomers proclaimed, was definitely a planet and definitely Percival Lowell’s—or close enough on both counts. “This remarkable trans-Neptunian planetary body,” one wrote, “has been found as a direct result of Lowell’s work, planning and convictions and there appears present justification for referring to it as his Planet X.” That “all’s well that ends well” consensus held sway for the rest of the century.

Now to see what’s really there …

*See Science’full coverage of Pluto, including regular updates on the New Horizons flyby.