Pluto will mark a birthday of sorts on March 23, 2178. No one is likely to be there to celebrate it, of course. Even if humanity is a multi-planet species by then, it would be a decided challenge to visit the tiny, distant world, which measures just 1,477 miles in diameter—or little more than half the coast-to-coast distance of the continental U.S.—lies up to 4.67 billion miles from Earth, and features a surface temperature as low as -400°F. Still that date will be one to circle on cosmic calendars. It takes Pluto slightly over 248 Earth years to orbit the sun, which means that on March 23, 2178, one Plutonian year will have elapsed since the dwarf planet was first spotted, on Feb. 18, 1930.
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“NINTH PLANET DISCOVERED ON EDGE OF SOLAR SYSTEM; FIRST FOUND IN 84 YEARS,” the New York Times announced in a front-page, all-caps headline in its March 14, 1930 edition, the day after the Lowell Observatory in Flagstaff, Ariz., announced its big find. “In the little cluster of orbs which scampers across the sidereal abyss under the name of the solar system,” the Times went on, “there are, be it known, nine, instead of a mere eight worlds.”
This Feb. 18 marks 95 years since the Lowell Observatory hit paydirt, an achievement made not by one of the observatory’s professional astronomers, but by amateur Clyde Tombaugh, who at the time was just 24 years old. Not long before coming to work at the observatory, Tombaugh had built his own telescope with which he had conducted observations of Mars and Jupiter. He made drawings of the two planets—drawings he sent to the Lowell Observatory, hoping the astronomers there would offer comment and critique. Vesto Slipher, the director of the observatory, did Tombaugh one better, offering the eager stargazer a job. His assignment would be equal parts tedious and transformative: scanning hundreds upon hundreds of images of the skies, looking for the elusive world known at that point only as Planet X. Percival Lowell, the astronomer and businessman who built the observatory, had long theorized that a ninth planet existed somewhere out in the cosmic void, reckoning that it accounted for wobbles that astronomers had observed in the orbits of Uranus and Neptune. He searched for Planet X from 1905 until his death in 1916, without luck. It would take another generation before the observatory, aided by the patient Tombaugh, would at last have success.
The 95 years since then have been ones of changing fortune for little Pluto. For decades after the first eight planets were barnstormed, orbited, and landed upon by spacecraft from Earth, Pluto remained the only one of the solar system’s major worlds that never received a visit—a slight that was not rectified until the New Horizons spacecraft flew by it in 2015. In 2006, after New Horizons was launched but before it could complete its nine-plus year journey, Pluto suffered the indignity of being demoted from planet to dwarf planet by the International Astronomical Union (IAU). And it’s as a dwarf planet that Pluto is taught to students today.
But Pluto has in many ways only grown in astronomers’ estimations. It is now known to be part of an entire system of objects, gravitationally anchoring a cluster of five moons; it has a surface marked by mountains and craters and valleys and plains; it is home to abundant quantities of water ice and may even harbor a liquid ocean beneath its surface, making it an improbable—but not impossible—home for extraterrestrial life.
“New Horizons shattered a major paradigm of planetary science,” says Alan Stern, the mission’s principal investigator. “Pluto turns out to have as much complexity as Mars or Earth, so much so that I know planetary scientists who call Pluto ‘the other Red Planet.’”
None of the new discoveries about Pluto would have been made, of course, had the little world not been spotted in the first place—an achievement that took sublime attention to detail. Tombaugh performed his cosmic sleuthing thanks to a telescope with a 13-in. mirror. He used it to gather images of parts of the night sky about as large as a fist held at arm’s length, all in a large area in which the late Lowell had predicted Planet X would be found. Tombaugh captured two images of each spot of sky on photographic plates. The second image of every pair was typically taken several days after the first. Over that relatively short period, background stars would not have moved at all, but a foreground object like a planet would have detectably shifted its position.
During the days, when stargazing was impossible, Tombaugh would analyze the photographic plates with an optical device known as a blink comparator. Beams of light from two microscopes in the instrument would shine through both plates in each pair, and Tombaugh would turn a dial, flipping the focus of the comparator first to one plate and then to the other, looking for a single point from among the spangle of points on each image that had moved. He discovered multiple objects this way—but they were too small and moved too fast to be a planet, and instead had to be asteroids. Finally, on two plates taken on Jan. 23 and Jan. 30, 1930, he found the right-sized point in the right patch of sky moving the right amount for a distant planet. The point shifted its position by just 3 millimeters on the plates, which factored out to a world approximately 43 times farther from the sun than the Earth is. Pluto had been found.
“All observations indicate the object to be the one which Lowell saw mathematically,” said the observatory in a statement.
The new world got its name not long after, thanks to 11-year-old Venetia Burney, of Oxford, England. Over breakfast on the day after the announcement was made, Venetia’s grandfather was reading the account of the new planet aloud from the paper and the young girl straightaway recommended the name Pluto, the ruler of the underworld. Her grandfather mentioned the idea to an astronomer he knew who in turn cabled it to the Lowell Observatory, where it was quickly approved.
“I don’t quite know why I suggested it,” Venetia said in a 2006 interview with NASA. “My grandfather read out at breakfast the great news and said he wondered what it would be called. For some reason, after a short pause, I said, ‘Why not call it Pluto?’ I did know, I was fairly familiar with Greek and Roman legends from various children’s books that I had read, and of course, I did know about the solar system and the names the other planets have. And so I suppose I just thought that this was a name that hadn’t been used. And there it was. The rest was entirely my grandfather’s work.”
Lonely Pluto would eventually turn out to be not so lonely after all. In 1978, astronomers at the U.S. Naval Observatory discovered a bulge in their images of Pluto—one that moved around the planet once every 6.4 days. The 1,477-mile wide world had a 751-mile wide moon—the largest moon relative to the size of its parent body in the solar system. The newly discovered satellite was dubbed Charon, and astronomers would ultimately find that the two bodies were gravitational co-equals, with Charon not orbiting a stationary Pluto, but with both worlds orbiting each other in a loop-de-loop pas de deux. From 2005 to 2012, the Hubble Space Telescope would ultimately discover four more smaller moons—dubbed Nix, Hydra, Kerberos, and Styx.
The complexity of the Plutonian system is reflected in the complexity of Pluto itself. New Horizons discovered that Pluto is home to the largest glacier in the solar system, one measuring more than 386,000 square miles, or larger than Texas and Oklahoma combined. What’s more, the glacier is pristine, with no craters, meaning that it is regularly being resurfaced.
“The glacier was born yesterday, geologically,” says Stern. “We find examples all over the planet of young terrains, middle-aged terrains, and ancient terrains. Pluto has been active for four and a half billion years.”
The secret to all of that activity is Pluto’s probable underground ocean. The water ice on the surface of the world suggests that there should be more water hidden below ground. Over the course of the past 4.5 billion years, that water has been slowly freezing, a process that is likely still underway. That provides the world with energy. “It’s physics 101 that as water freezes it releases latent heat,” says Stern. “That is probably a part of the energy source that’s powering Pluto’s geology. The ocean will continue to freeze for the next one or two billion years, and Pluto will continue to be active.”
The question for planetary scientists and exobiologists is whether Pluto’s ancient ocean may have been able to cook up life. The solar system’s 293 moons include several believed to harbor oceans, including Saturn’s Enceladus, Jupiter’s Europa, and Neptune’s Triton. Enceladus regularly emits frosty water geysers, produced when the gravity of Saturn flexes the much smaller moon. The Cassini spacecraft flew through the plumes in 2015 and detected organic compounds that could be precursors of life.
“I don’t think it’s too much of a stretch to say we might find biology in some of these ocean worlds,” says Stern. “And who knows? Pluto could be one of them.”
For all of this promise, Pluto struggles for respect, with the “dwarf planet” label continuing to rankle Pluto partisans. The IAU defends the definition based on Pluto’s orbit. Unlike other planets that have a more or less circular, equatorial orbit around the sun, Pluto’s is sharply inclined and highly elliptical, with a perihelion, or close approach to the sun, of roughly 2.7 billion miles, and an apehelion, or furthest remove from the sun, of 4.67 billion miles. That, plus Pluto’s small size—smaller than our moon—suggested that the world did not accrete in its current orbit from the primordial gas and dust that gave rise to the sun and the planets, but rather formed farther out, in the Kuiper Belt, a vast band of icy, rocky bodies that surrounds the solar system. From there, it broke free and entered its screwy orbit. Allow little Pluto to keep its planetary status and you would have to confer the same honor on Eris, a Kuiper Belt object of about the same size, as well as any other, similar worlds that might be discovered—raising the prospect of a solar system with an uncounted number of planets.
Stern thinks the dwarf planet distinction is nonsensical—an arbitrary parsing of cosmic definitions. “Small planets are planets too,” he says. “Just because the sun is a small star we don’t call it a dwarf star. We’re not afraid of large numbers of planets; we’re not afraid of schoolchildren having to learn all their names. After all, kids don’t have to memorize every element in the periodic table.”
Further exploration of Pluto is not likely anytime soon. New Horizons has long since soared billions of miles into deep space and no other Plutonian missions are currently planned. Still, the oddball world at the edge of our solar system will continue to intrigue astronomers. “My little saying in public talks is that Pluto defies all of the textbooks,” says Stern. “What that proves is that Pluto doesn’t read the textbooks.”
