50 Fun Facts About Pluto

Who discovered Pluto in 1930?

Clyde Tombaugh was a Kansas farm boy with no degree who won his observatory job by sending in sketches of the planets made with a telescope he had built himself, and he was 24 when he found Pluto.

What device did Pluto's discoverer use to spot it shifting between two photographic plates?

A blink comparator flips rapidly between two photographs of the same patch of sky, so the stars stay put while anything that has moved appears to jump back and forth.

Pluto was discovered in 1930 at an observatory in which US state?

Arizona: Percival Lowell had built his observatory at Flagstaff in 1894 to study Mars, and its staff were still hunting his predicted 'Planet X' when Pluto turned up on their plates.

Who first suggested the name Pluto?

An 11-year-old English schoolgirl, Venetia Burney of Oxford, offered the name over breakfast; her grandfather passed it to an astronomer friend, who cabled it to the discoverers in America.

Pluto's traditional planetary symbol is a monogram of which two letters?

P and L: they open the name Pluto and are also the initials of Percival Lowell, who began the hunt for a ninth planet, a coincidence that helped the name to win the observatory's vote.

Pluto is named after the Roman god of what?

The underworld: Pluto was the Romans' name for the Greek Hades, and the dwarf planet's moons have since been named to match, from the ferryman of the dead to the three-headed guard dog.

Pluto's largest moon was found in 1978 when an astronomer noticed what on photographs of Pluto?

A bulge on one side: James Christy saw Pluto's image stretch and shrink in a regular rhythm, then found the same bump on plates taken years earlier, back to 1965.

Plutonium was named after Pluto in the 1940s; which chemical symbol did Glenn Seaborg give it as a joke?

Pu: Seaborg passed over the obvious 'Pl' for letters that sound like someone holding their nose at a stink, expecting to be overruled, but nobody got the joke and it went into the periodic table.

How is Pluto thought to have gained its family of moons?

A collision with another world is the favoured answer: the moons' neat, circular orbits suggest they grew from the wreckage instead of being caught later, much as our own Moon is thought to have formed.

Which body voted in 2006 to strip Pluto of its status as the ninth planet?

International Astronomical Union: the vote came at its meeting in Prague, and some planetary scientists, the Pluto mission's chief Alan Stern among them, rejected the result.

Which of the three official planet criteria does Pluto fail?

Clearing the neighbourhood around its orbit is the test Pluto fails: it is round and it circles the Sun, but it shares its zone with a swarm of icy bodies that it is far too small to sweep up.

The discovery of which more massive object in 2005 forced the debate over Pluto's status?

Eris, hailed in the press as a tenth planet, was later named after the Greek goddess of discord, a fitting choice for the find that ended with Pluto and itself both classed as dwarf planets.

The American Dialect Society's word of the year for 2006 was 'plutoed'; what does it mean to be plutoed?

To be demoted or devalued, as the ninth planet had been that August: the new verb beat 'climate canary' in the society's run-off vote.

Pluto is a member of which region of the Solar System?

The Kuiper belt: Pluto was its first known member, and once a second turned up in 1992, followed by hundreds more, astronomers began to ask whether Pluto was a planet or simply one of a crowd.

How long does Pluto take to orbit the Sun?

About 248 years: Pluto has not yet finished a single lap since it was found in 1930 and will not complete its first until 2178.

For twenty years from 1979, Pluto's lopsided orbit put it where?

Closer to the Sun than Neptune: Pluto's stretched orbit dips inside Neptune's for about twenty years of each circuit, so through the 1980s and 1990s Neptune, not Pluto, was the outermost planet.

What keeps Pluto and Neptune from ever colliding, even over millions of years?

A 3:2 orbital resonance: Neptune circles the Sun three times for every two circuits by Pluto, a rhythm that always keeps the pair billions of kilometres apart.

After its 2015 flyby, how long did the Pluto probe take to radio all of its stored data back home?

About 15 months: the probe recorded more than 50 gigabits during the flyby and then sent them home across more than five billion kilometres, the last arriving in October 2016.

How long does sunlight take to reach Pluto at its average distance?

About 5.5 hours, because Pluto lies some 39 times farther from the Sun than we do; the same light reaches our own planet in roughly eight minutes.

How heavy is Pluto compared with our Moon?

About one-sixth: the world once counted as the ninth planet is outweighed by seven moons of the Solar System, our own among them.

Which mountaineer has an ice mountain range on Pluto named after him?

Edmund Hillary and Tenzing Norgay, the first to reach Everest's summit and return safely, each have a range, Hillary Montes and Tenzing Montes, because Pluto's official names honour pioneers of exploration.

Pluto's surface area is roughly the size of which country?

Russia is the nearest match: a globe 2,370 km across has about 17.7 million square kilometres of surface, yet the flyby found that small world crowded with mountains, glaciers and frozen plains.

Pluto's four small moons are Nix, Hydra, Kerberos and which other, named after a river of Greek myth?

Styx, named for the river the dead had to cross, was the last of the four to be found, spotted in 2012 while astronomers checked the approaching probe's route for hazards.

Charon is so large beside Pluto that the two are often described as what?

A double planet: Charon is half as wide as Pluto, so the pair's balance point lies in open space between them rather than inside Pluto itself.

Which telescope discovered all four of Pluto's small moons between 2005 and 2012?

Hubble Space Telescope: it picked out Nix and Hydra in 2005 and two fainter moons in 2011 and 2012, during searches made to scout the way ahead for the approaching Pluto probe.

Charon's discoverer chose the name partly as a tribute to which member of his family?

His wife: Charlene Christy was 'Char' at home, so her husband proposed 'Charon' for the moon, and the mythical ferryman who shares the name made it fit Pluto's dark theme.

What did the discovery of the largest moon finally allow astronomers to measure?

Pluto's mass came out at about 0.2% of our own planet's, far too slight to have tugged the outer giants off course as the hunters of Planet X had supposed.

Which spacecraft flew past Pluto on 14 July 2015?

New Horizons was launched towards what was still the ninth planet; seven months into the flight astronomers reclassified its target as a dwarf planet.

What unusual cargo, honouring the 1930 discovery, rides aboard the Pluto flyby spacecraft?

Some of the discoverer's ashes, about an ounce, ride in a small canister on the probe, and his 93-year-old widow, Patricia, was at the launch site to watch it leave.

Which planet gave the Pluto probe a gravity assist in 2007?

Jupiter's gravity flung the probe onward about 14,000 km/h faster, which cut three years off its journey to Pluto.

The bright region the 2015 flyby revealed on Pluto is famous for being shaped like what?

A heart: the region is named for Pluto's discoverer, and it stands out so pale because its western half is a thousand-kilometre plain of bright, fresh ice.

What did scientists find no trace of on Pluto's vast ice plain, a sign that its surface is young?

Impact craters: not one shows on a plain a thousand kilometres wide, because its ice slowly churns and wipes away every scar, leaving a surface less than ten million years old.

Pluto's bright plains are made mostly of what?

Nitrogen ice is soft enough to flow like a glacier, whereas the mountains beside the plains are blocks of water ice, frozen as hard as rock in Pluto's cold.

Looking back at Pluto after the flyby, the probe saw the haze of its thin atmosphere glowing which colour?

Blue: sunlight breaks up the gases of Pluto's thin air into soot-like particles, and that haze shows blue in pictures made to match what a human eye would see.

How long is a day on Pluto?

About 6.4 Earth days, exactly the time its big moon takes to circle it, so the moon hangs over one spot of Pluto and never rises or sets.

How does Pluto spin?

Tipped on its side, like Uranus, which gives Pluto extreme seasons: each pole in turn spends years facing the distant Sun.

After Pluto, the same probe flew on to which snowman-shaped body, reaching it on New Year's Day 2019?

Arrokoth, nicknamed Ultima Thule at the time, turned out to be two lobes that had gently merged into a flattened snowman, a shape that was the biggest surprise of the flyby.

Which moon, slightly larger than Pluto and much like it, is thought to have been captured from Pluto's home region?

Neptune's Triton circles its planet backwards, the mark of a captured body, and an early theory had the story the other way round, with Pluto an escaped moon of Neptune.

Voyager 1 could have gone on to Pluto, but in 1980 NASA sent it past which moon instead?

Titan: exploring it was a primary goal of the mission, and the close pass flung Voyager 1 up out of the plane of the planets, which ruled out any later visit to Pluto.

Bodies that share Pluto's orbital rhythm with Neptune are known as what?

Plutinos are thought to have been herded into step with Neptune as the giant planet drifted outward long ago, and Pluto is simply the largest of the flock.

Roughly how long did the flyby probe take to reach Pluto?

About 9.5 years, even though no spacecraft had ever been launched faster: it crossed the Moon's orbit within nine hours of liftoff.

In 1931, going by its supposed tug on the outer planets, Pluto was reckoned to be as massive as what?

Earth was the 1931 answer; each new measurement then shrank the figure, to a tenth of that within twenty years and to less than a hundredth by 1976.

Scientists suspect Pluto may hide what beneath its icy crust?

A subsurface ocean of liquid water may survive because heat from decaying radioactive elements in the rocky core could keep a deep layer from freezing, far below a surface at about minus 230 degrees Celsius.

A 1991 US postage stamp that rides aboard the Pluto probe carries which caption about Pluto?

Not Yet Explored: the stamp came from a set that paired every planet with a probe that had visited it, and Pluto alone had none, so one was fixed to the spacecraft that would put it out of date.

Pluto's largest ice plain is named after which pioneering spacecraft?

Sputnik 1, the first artificial satellite: the mission team borrowed the names of pioneering space missions for Pluto's lands, and astronomers made this one official in 2017.

Seen from the surface of Pluto, what would the Sun look like?

A dazzlingly bright star: at 39 times our distance the Sun shrinks to a point, yet it still lights Pluto's noon hundreds of times more brightly than a full Moon lights our nights.

In 2007 which US state, the longtime home of Pluto's discoverer, declared that Pluto is still a planet in its skies?

New Mexico: its legislature resolved that Pluto 'be declared a planet' whenever it passes through the state's night skies, and named 13 March 2007 Pluto Planet Day.

Why did the search for 'Planet X' stall for ten years after Percival Lowell died?

A lawsuit by his widow: Constance Lowell contested the will that left most of his fortune to the observatory, and the fight tied up its work until 1929, when the hunt for a ninth planet resumed.

When Pluto's two smallest moons were named in 2013, which name topped the public vote but was turned down?

Vulcan won the poll, but astronomers ruled it out because the name had long been attached to an imagined planet inside Mercury's orbit, so names from lower down the vote were used instead.

Pluto's mountains Wright Mons and Piccard Mons are thought to be what kind of feature?

Ice volcanoes: a 2022 study concluded the region was built by many eruptions of icy material rather than molten rock, which implies Pluto's interior held more heat than anyone expected.