Why Pluto Now Has Five Moons, but It’s Still Not a Planet

It's rapidly becoming the little planetoid that could, with more satellites than the inner four planets put together. But that doesn't mean Pluto's getting any more love.

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This image, taken by the NASA/ESA Hubble Space Telescope, shows five moons orbiting the distant, icy dwarf planet Pluto. The green circle marks the newly discovered moon, designated S/2012 (134340) 1, or P5.

If ever a spacecraft had the right to turn around and come home, it’s NASA‘s little New Horizons ship. Launched on Jan. 19, 2006, it was dispatched on a mission no other ship had ever dared attempt — fly out and reconnoiter Pluto, the most distant and mysterious planet in the solar system. Little more than seven months later, however — on Aug. 14, 2006 — word came down from the International Astronomical Union that, oops!, Pluto isn’t a planet after all. It’s a dwarf planet or a minor planet or a planetoid, or, most insultingly, a plutoid. But whatever you want to call it, it had been summarily busted down from one of the sun’s little princelings to a mere pretender — nothing more than a refugee from the Kuiper belt, the band of comets and other rocky, icy debris that circles the solar system. And for this New Horizons was supposed to travel 4.5 billion miles? Talk about bait and switch.

But Pluto’s been getting a lot of love — and a lot of redemption — lately, thanks to observations by the Hubble Space Telescope, which just discovered yet another moon circling the lonely little world. That brings Pluto’s total to five, which — if you’re counting — is four more than Earth has and five more than moonless Mercury and Venus.

The fact that Pluto had any moons at all was something of a surprise. Just 1,412 mi. (2274 km) in diameter, it’s less than half the size even of tiny Mercury. Pluto was discovered in 1930 and for nearly 50 years, there was no evidence at all of anything orbiting it. That changed in 1978, when astronomers at the U.S. Naval Observatory in Flagstaff, Ariz. discovered the moon Charon, which, at 1,206 mi. (1,940 km) was practically a Plutonian co-equal. In 2005, the Hubble Space Telescope turned up two more moons: tiny Nix, no more than 42 mi. (68 km) across; and her big sister Hydra—52 mi. (84 km) in diameter. Just last year, Hubble found yet another member of Pluto’s litter, a flyspeck of a satellite that measures just 21 mi. (34 km). The little bit of rubble doesn’t even have a formal name yet and is now known unlyrically simply as P4.

And this week, again courtesy of Hubble, comes P5, which is the smallest of all, perhaps 15 mi. (24 km) wide. The Plutophiles love it. “Are there still any doubts that the IAU vote in 2006 was premature?” asks Laurel’s Pluto Blog, hosted by a writer who proudly ID’s herself as Plutogirl. Adds @NickAstronomer, via Twitter: “when is a planet not a planet – oh yeah …when it has 5 moons,” And, Tweets the venerable @Newscentist: “Important enough to have 5 satellites? You are a planet!”

Nice sentiments, but nuh-uh, as the equally venerable TIME Magazine concluded when the debate was still raging back in 2006. As we (alright, I) put it then:

…let’s be clear: Pluto has to go. Clean out your locker, turn in your playbook and go see the coach. Oh, and on your way out, tell the other walk-ons and wannabes that the roster is frozen. We’re sticking with the original eight.

There are sound astronomical reasons for such a pitiless verdict. Pluto’s elliptical, sharply inclined orbit — not to mention its icy, rocky anatomy — clearly mark it as a rogue object, gravitationally lassoed from the Kuiper Belt. This is an entirely different origin from the eight true planets, which swirled into life from leftover dust as the sun itself was forming, and for that reason orbit neatly in the solar plane. Planets and Kuiper Belt objects can certainly play nice, but so can some cats and dogs. That doesn’t make them part of the same family.

But all that is academic  — especially for New Horizons and its mission planners. The spacecraft is intended to barnstorm Pluto at a distance of just 7,000 mi. (11,000 km), and doing that while dodging a swarm of moons can be tricky. The rendezvous is not set to occur until July 2015, which gives navigators plenty of time to refine their course and take corrective maneuvers in advance – not to mention look for still more moons that could complicate things even more. The spacecraft’s flyby will be fleeting, since it will be moving at 30,000 mph (48,000 k/h). But the data it collects will be exponentially more than we’ve ever had before. And the pictures it beams home will be a lot lovelier too — images not of a lonely nonplanet, but a parent world and a brood of pups, whatever their exact species may be.