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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AFP PHOTO / NASA / ESA

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.

31 comments
Xantrax
Xantrax

Pluto will always be a planet to me, no matter what those egg headed fools at the IAU say.

franklyn.azubuike.174997
franklyn.azubuike.174997

well, cant undastand why pluto is not yet a planet, but the unn website has the answers to the question. please visit the university of nigeria website for more information

Dileep_Sathe
Dileep_Sathe

The recent discovery of the 5th moon of Pluto requires us to reconsider

its expulsion from the solar system in August 2006 by the I.A.U. - on

the basis oif its new definition of a planet. Actually that definition

is having a conceptual problem which can be raised by even an H.S.C.

student - as I have explained in my Letter on p. 19 of http://physics.open.ac.uk/~bwj... Interested readers may feel free to discuss this matter with me using dvsathe[at]gmail.com

dufas_duck
dufas_duck

If planet X were true and it moved into the sun's gravitational field, would it or any other rogue planet that might be caught in the sun's field and begin to orbit same be called a planet by the scientific definition being espoused. If said foreign celestial body was not formed by part of the dust from our sun, the scientists say it's not a planet. It's not a planet because it doesn't follow the excepted orbit that the true 8 planets follow. If it is true true that a body cannot be called a planet less it is borne from it's own star and have a regular orbit, many of the so-called planets with odd orbits discovered around other stars that excite scientists so much are not planets at all.

Purita Fleschhut
Purita Fleschhut

What has been accepted as fact 50, 60 years ago  is no longer true these days. For me 

it's always  planet as we're taught in school and sorry that it took science so long to de-planetized it. This may sound illogical but the fact that Pluto has five moons, no science on earth could contest it. They say it's a "dwarf planet"? In any language, it's still a planet.

ssanam
ssanam

Pluto will continue to be my favorite planet. I am waiting eagerly to see the first closeup photograph returned from the New Horizon.

Smail Buzzby
Smail Buzzby

This is a pretty petty and dumb argument.  If you want to say that anything orbiting a sun is a planet then you just created millions of planets.  In our solar system.

If you want to say that a planet has to have formed around a star then you will probably end up (eventually) excluding something erroneously.  If you say that a planet has to be this close to the star or this close to the orbital plane of any other planets then you will eventually have to make exceptions.

Who cares?  Pluto is a pretty big rock with a few moons.  Maybe it is a rock (or a moon) with moonroids...maybe it is a planet with asteroids.   Let's all chip in and send a rocket filled with PreparationH at Pluto, since there is clearly a bad case of roids happening there.

It hardly matters what we people want to call something, does it?  Does being a planet give Pluto some rights that an asteroid doesn't have?  We treat the Earth like crap, and it is pretty clearly a planet.

Perhaps Pluto is better off just being a dull icy rock with a posse?

PhillyJimi
PhillyJimi

Pluto is classified as a dwarf planet.  Would everyone please get over it.

Ed Bowser
Ed Bowser

A little knowledge of Astronomy, like anything, is dangerous.

As others have pointed out here, Pluto remains a planet, its classification merely further defined as a Dwarf planet. The fact that it is also Kuiper Belt Object is a result of location, not its planetary definition. Earth, in the same orbit, would also be a KBO. So, there was no 'demotion', Pluto, were it aware, would not feel slighted nor insulted.The media is responsible for this overly confused debate, continuing to prod those who object to Pluto's extended classification while ignoring the extended classification of the classical planets; Mercury, Venus, Earth amp; Mars are Rocky planets, Saturn, Jupiter, Uranus and Neptune are Gas Giant / Ice planets and Pluto is one of the Dwarf planets.No confusion, no controversy, so, to the media, no story. And as for the silly notion of New Horizons turning back, its original mission was to fly by and analyze Pluto, then continue to explore other KBOs amp; eventually depart the solar system. That will continue to be its incredibly fascinating mission.No celebrity story here, folks, merely further groundbreaking explorations of our home solar system and the reaches beyond.

I know TIME has been dumping writers like fleas for a while now, but couldn't a science writer who actually knew something about the subject have been retained?

Bobby Rutherford
Bobby Rutherford

I hope the first closeup photograph of Pluto returned from the New Horizon is a giant statue of a 1-raised fingered salute with the words "Screw You Earth and Your Stupid Astronomers Can Kiss My Shiny Cold Frozen Planetary Surface" written under it.

Hollywooddeed
Hollywooddeed

 I'm still upset at the ignominious displanetation of Pluto.  It was my favorite planet.

Blue-eyed Gal
Blue-eyed Gal

Actually, the fact that Pluto is one of MANY "dwarf planets" makes the New Horizons mission more interesting, not less. Instead of just going out to investigate an oddball frozen rock with an eccentric orbit that's not like anything else in the solar system, Pluto turns out to be an excellent representative of a number of special mini-worlds that we know almost nothing about. 

Sedna, Eris, and all the other dwarf planets (some of which have moons ) are just specks of light. Pluto will shed light on a whole class of objects out there, which are too small and too far away to know much about. Pluto is not the largest one of the group, but it's the first one we knew existed. I can't wait for those first photos from New Horizons to arrive.

Pooklont
Pooklont

What Laurel said, adding that if Earth was in Pluto's orbit, it would not qualify for full planethood.  Also, Earth has 19,500 asteroids orbiting it or it and the Sun. So, one can argue it should not be a planet, using the IAU's absurd definition. If you investigate the politics of the 2006 vote in Prague, you will hear from a member who said he was threatened with the destruction of his career if he voted to maintain Pluto's planethood, and, if you watch the video of the sordid session that held the vote, you will see Pluto huggers being cut off in mid-sentence. The debate on planethood needs to be reopened at the next IAU General Assembly this August in Beijing.

Laurel Kornfeld
Laurel Kornfeld

Thank you for the mention of my blog; however, Pluto is not a “planetoid” or “minor planet.” It is a planet. Those terms are synonymous with asteroids and comets, which are tiny objects shaped only by their chemical bonds. In contrast, Pluto is a complex geological world very much like the bigger planets. According to the equally legitimate geophysical planet definition, a planet is any non-self-luminous spheroidal body orbiting a star. Dr. Alan Stern coined the term dwarf planet in 1991 to designate a third class of planets large enough to be squeezed into a round shape by their own gravity but not large enough to gravitationally dominate their orbits. The four percent of the IAU that voted in 2006 distorted his term by saying dwarf planets are not planets at all.

 

Dwarf planets are simply small planets not large enough to gravitationally dominate their orbits. That does NOT make them “non planets.” Furthermore, science does not work through decrees “handed down” from on high. Pluto is as much a true solar system planet as the eight larger ones. It has geology and weather and is geologically differentiated into core, mantle, and crust, just like Earth is. It is estimated at 70 percent rock. Neither does an elliptical orbit disqualify Pluto from planethood. Pluto’s orbit is stable, and furthermore, many giant exoplanets have been discovered with orbits far more elliptical than Pluto’s. Are none of those planets either? The “pitiless” decree was never necessary unless one wants to artificially narrow the term planet. There is no reason objects like Pluto and Eris cannot be dually classed as both Kuiper Belt Objects and as planets. The first tells us where they are; the second tells us what they are. In short, our solar system has three, not two zones of planets—terrestrials, jovians, and dwarf planets. Dwarf planets are indeed part of the “same family.”

Pooklont
Pooklont

 The IAU deplanetized Pluto, Mr. Bowser.  Dwarf planets are not planets in the eyes of the IAU as of 8.24.2006. Just because you say something with relish does not make it true.  In order for the IAU's absurd new definition of a planet to be changed to include dwarf planets, the debate must be reopened and a new vote on the issue must be cast by the membership.

Pooklont
Pooklont

 I prefer the term "deplanetization."  Replanetize Pluto and Ceres!

laurele
laurele

Actually, Pluto IS the largest known object in the Kuiper Belt. Eris was determined to be marginally smaller than Pluto in November 2010 when it (Eris) occulted a star, enabling astronomers to make better measurements of its size.

Pooklont
Pooklont

 What is a dwarf planet to you?  Did you know the man who coined the term feels a dwarf planet is also a planet? 

teviet
teviet

 Actually, the term "planet" has always carried not just an intrinsic physical meaning, but has also included the object's orbital relationship to other bodies.  Not all planetary-mass objects (PMOs) are planets.  The Earth's Moon, Io, Europa, Callisto, Ganymede, Titan, and Triton are generally not considered "planets" despite being PMOs.

Scientists aren't particularly dogmatic about this, but using nonstandard definitions will usually require an extra step of explanation depending on which group of astronomers you are talking to.

Ed Bowser
Ed Bowser

Sorry for you, but you are incorrect. Please feel free to reread the material involved.

SeasiderWR
SeasiderWR

...........and rightly so!  a 'Dwarf' is a small human being.....but a human being nonetheless! :)

LaurelKornfeld
LaurelKornfeld

@teviet Some astronomers have proposed classifying spherical moons as "satellite planets" or "secondary planets." Considering the fact that these bodies have the same complexity as primary planets and could even be locations that host life, this is a good idea.

Pooklont
Pooklont

Actually, they are, Ms. Bartolome.  I visited the museum at the La Brea tar pits in L. A., and there was a skeleton of the missing link between dogs and cats. True story. If you ever go there, you will see it, too. Pluto is a planet.  It is also larger than Eris, we now know per the Paris Observatory and Bruno Sicardy.

Laurel Kornfeld
Laurel Kornfeld

But cats and dogs are both mammals, even if they are different subtypes of mammals. Similarly, dwarf planets, gas giants, and terrestrials are all planets, just of different subtypes. Also, Pluto is both a planet and a Kuiper Belt Object. One does not preclude the other. The first tells us what it is; the second tells us where it is. If we refer to Pluto only as a KBO, we blur an important distinction between the majority of KBOs, which are shapeless, simple rocks, and an object that has geology, weather, and layering into core, mantle, and crust, just like the larger planets do.

LaurelKornfeld
LaurelKornfeld

@Ed Bowser The IAU rejection of 2006 Resolution 5b, which would have established an umbrella category of "planets" that included both "classical planets" and "dwarf planets" was voted down 333-91. As a result, dwarf planets are not considered planets at all by the IAU but an entirely different category. To further muddy the waters, the IAU then gave dwarf planets minor planet numbers, which are reserved for asteroids and comets. Dwarf planets are NOT asteroids or comets, and this only blurred the distinction between two very different types of objects even further.

Elgenio Dervez
Elgenio Dervez

Oh. And apparently neither can Mr. bowser take criticism. That was rude Ed.

Elgenio Dervez
Elgenio Dervez

Mr. Pooklont your replies have gone from relish to salsa. Can't take criticism ?

Since the planets were formed through accretion passing through a stage of being a collection of rocks, why not call Pluto a protoplanet ( not a planet and not a KBO ) especially since it has moons and KBOs don't have moons. This is a question as I don't claim to be an expert.

Pooklont
Pooklont

 That is a cheap punk move to ask me to reread.  If you are saying I am incorrect, then state exactly what facts I have wrong.  For the record, none of my facts are wrong, but I am willing to hear your case.  Go for it, Mr. Bowser, or leave me alone.

Pooklont
Pooklont

 No need to reread as your posts contain no information,  just mere bluster.