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Pluto, seen here in a photo taken by the New Horizons flyby of 2015, celebrates 87 years of discovered life on 18 February.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

On the birthday of Pluto’s discovery, Science takes a look back on the dwarf planet’s long, strange history

Happy birthday, Pluto! Since its discovery 87 years ago tomorrow, the dwarf planet has been the subject of near-constant debate, research, and discovery. A look back on Pluto’s life gives us insight not just into how far we’ve come in astronomy, but how far we have yet to go.

The search begins

Percival Lowell founded the observatory that would later discover Pluto, but died before seeing it.

Lowell Observatory/Flickr

It’s 1905 and Uranus is acting funny. Its orbit appears to be disturbed by some gravitational force, and after much calculation, astronomers rule out Neptune as the culprit. Luckily for science, a millionaire Bostonian named Percival Lowell (of Lowell Observatory fame) has another hypothesis, in the form of a far-away, mysterious planet he dubs “Planet X,” which he speculates is the cause of the gravitational tug. Lowell makes it his life’s mission to find the enigmatic solar body but, alas, dies in 1916, leaving the mystery unsolved.

Earth, meet Pluto

These photographic plates, taken 6 days apart, revealed Pluto to Clyde Tombaugh. The arrows point to Pluto’s position on each day.

Lowell Observatory Archives

Thanks to a messy legal battle between Lowell’s widow and the observatory over her husband’s legacy, the search is put on hiatus for a decade. Finally, Clyde Tombaugh, a bright-eyed, 23-year-old researcher, is put on the tedious job, which requires him to take paired images of the night sky days apart and compare them to determine whether any objects have shifted position. On 18 February 1930, after months of the painstaking work, he notices a shift: There, held between his humble mortal hands, is Lowell’s ninth planet.

What’s in a name?

Pluto the planet is rivaled for celebrity with Mickey Mouse’s friend of the same name.


After announcing its existence in March 1930, the Lowell Observatory is flooded with suggestions for names. After picking through hundreds of duds—“Cronos,” “Zyxmal,” and “Minerva,” to name a few—the team decides to go with the suggestion of an 11-year-old British girl. Pluto, the god of the underworld, is a fitting name for a planet so far from the sun that its orbit takes 248 years. Plus, the first two letters–P, L—are the initials of the man who started the search: Percival Lowell. (A few months later another Pluto will be named, this one not a celestial object, but a dog.)

The first of many identity crises

An artist’s impression of a Kuiper belt object.

ASA, ESA, and G. Bacon (STScI)/Wikimedia Commons

Little time is lost between Pluto’s discovery and a flood of scientific questions. Further observations reveal the world is much smaller than predicted, and has a bizarre, oblong orbit. Scientists begin to wonder whether it wouldn’t be better classified “a unique asteroid or an extraordinary comet-like object.” Then, after re-examining Lowell’s original calculations, astronomers realize the wonky orbit of Uranus had no need for a Planet X after all: Pluto’s discovery was a total fluke. Yet the search for Planet X continues to this very day, with astronomers hunting for a suspected gas giant beyond Pluto’s region of small, icy worlds called the Kuiper belt.

Not alone: moons and solar bodies in Pluto’s neighborhood

Charon, the first of Pluto’s moons to be discovered.

NASA/JHUAPL/SwRI/Wikimedia commons

It gets lonely out in the Kuiper belt—but Pluto, at least, has the company of several moons. The first of them to be discovered, Charon in 1978, makes it possible to finally determine Pluto’s mass: At only about 0.2% that of Earth’s, it is impossible for Pluto to be the cause of Uranus’s (later debunked) orbital irregularity. Starting in 1992, new telescope technologies allow astronomers to discover thousands more icy worlds in the region, making it harder and harder for them to defend Pluto’s classification as a planet (though, it is later discovered, Pluto at least remains the largest body in the neighborhood).

RIP planet No. 9

Middle school students in Seattle, Washington, protest Pluto's new classification.

AP Photo/, Joshua Trujillo

The debate heats up on how to classify Pluto, notably in 2001, when New York City’s American Museum of Natural History omits the icy outlier from its solar system exhibit, leading to so much public outcry that celebrity astronomer Neil deGrasse Tyson feels compelled to step in and defend the decision. Finally, in a move that shook the global community and ruined schoolchildren’s mnemonic devices, Pluto the planet is officially killed off on 24 August 2006. Stripped of its former title, it is reclassified as a dwarf planet, of which there are dozens in our solar system. To be a planet, the International Astronomical Union says, it must orbit the sun, be large enough to be rounded by its own gravitational force, and dominate the neighborhood around its orbit. Pluto does the first two, but alas, fails on the third criterion. Consolation prize: Pluto’s demotion leads to the invention of a new term, “plutoed,” which the American Dialect Society votes Word of the Year in 2006.

Ready for a close-up

Mounds of ice rise on Pluto, suggesting an internal heat source. 

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

After a years-long journey across the solar system (including a pit stop at Jupiter) NASA’s New Horizons spacecraft successfully passes Pluto in 2015, revealing views of the dwarf planet in never-before-seen detail. One of the first pictures from the flyby held the record for most “likes” on NASA’s Instagram account (after nearly being lost by the excited scientists handling it). With these new observations, scientists are able to piece together a much clearer picture of the far-flung dwarf planet, picking out ridges and craters in a diverse geography that had once been but a blur. Even more confounding are the mountains of water ice, rising thousands of meters tall at some points; they suggest an internal heat source, and show the dwarf planet is still very much active geologically.

Possibility of life?

Artist's concept of the New Horizons spacecraft encountering Pluto and Charon.

Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Continuing to sift through their New Horizons data, scientists announce in 2016 that the reddish brown cap on Charon’s north pole is composed of tholins—organic molecules which could be the base ingredient for life. This mystery and so many others—how Pluto formed, how its moons formed, and how the many celestial bodies in the Kuiper belt came to be—will remain for the most part unsolved, until science is able to get an even closer examination. New Horizons is still on the move, with a 1.6-billion-kilometer journey from Pluto to another Kuiper belt object underway, hopefully culminating in a flyby New Year’s Day 2019. What will those data reveal about Pluto and the origins of our solar system? We earthlings will just have to wait to find out.