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Why Pluto is No Longer a Planet

This article was originally written in 2008, but we created a cool video to go along with it yesterday

Let’s find out why Pluto is no longer considered a planet.

Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next.

After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).

The Solar System now had 9 planets.

Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.

Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun).

Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.

Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that same classification.

Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt.

And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto.

With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic.

Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto?

In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf planet”.

Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:

  • It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
  • It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
  • It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.

It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could qualify for planethood status. Then our Solar System would have 9 planets again.

Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf) planet’s surface.

Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able to appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.

If you’d like more information about Pluto, we did two podcasts on this topic at Astronomy Cast. The first discusses the IAU’s decision, and the second is about Pluto and the Icy Outer Solar System. Check them out.

Here is much more info about Pluto, including pictures of Pluto.

NASA Solar System Exploration Guide


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

  • gewr March 23, 2009, 9:05 PM

    pluto is still a planet, i dont care wat u say!

  • shondie March 24, 2009, 7:46 AM


  • Katie March 24, 2009, 9:12 PM

    the comments that have been left for you are very retarded and immature. wow. who knew that someone actually cares about a ball of ice that hardly anything is known about. get a grip people.

    this has been a very interesting debate. but Pluto has a moon, right? then again, Mercury has no moon, and its still a planet,so who knows. i’m going with the “dwarf planet” theory.

  • Lauren March 27, 2009, 8:41 AM

    pluto was my favorite planet though DD:

    i still love it <3

  • marie March 28, 2009, 6:16 PM

    Last time in school was the teacher telling a planet storey about Pluto. Many years Pluto was very small then she told me it was destroyed a long time ago I wanted more information so I looked online. One of the coments I saw said like pluto is a planet I wonder if it is true. Wait! I forgot pluto is a planet but not any more my teacher said that pluto was been destroyed It is no longer a planet. I saw in a map in school wait or was it online oh ya it was online I saw a video all about pluto.

  • Jackiie March 31, 2009, 5:56 PM

    i liked Pluto too because we have so much in common like we all both small lol … but yeah i am a little mad that it is not a planet =\

    lol but youu people are funny withh the comments =D haha

    because it is nnot even on the subject or you people are just sayin ohh tht suck that it is not a planet.. or talk bout ut sexually life whichh is really weird but whateverr lol

    POOR OLD PLUTO =[ ii lovee youu pluto lol =p

  • Season April 1, 2009, 1:08 PM

    My boyfriend and I are having an argument. He says that Pluto was once a planet but now it is no longer a planet.

    I say Pluto never was a planet in the first place, only that it was once incorrectly considered to be a planet. (for example, the Earth was once considered flat and then discovered to be round; does that mean that the Earth was once flat but now is round?)

    The title of this article is Why Pluto is No Longer a Planet. Does that mean Why Pluto is No Longer Considered a Planet, hence validating my argument? Or is it literally Why Pluto Used To Be A Planet but Changed Into Something Else, validating his argument?

    Please help.

  • Season April 3, 2009, 9:18 AM

    We have been spending most of our “lifes” learning how to spell “babies” correctly. We apologize for not spending our “lifes” trolling Internet comments.

  • Kakashi Hatake April 6, 2009, 5:37 AM

    I think that pluto is a planet, because well, when i was in school, they tought us that it was a planet, and everyone says you should turst school with their knowledge, so I think that Pluto is a planet! (^\)

  • danj4chilis April 6, 2009, 1:24 PM

    i think this is a very good article.
    I can’t believe that something this trivial has put so many people out.i mean..so what if it’s not a planet.it doesn’t change the fact that it’s in the sun’s orbit,it doesn’t change the fact that its billions of miles away from us and will neve bother us.
    As for the “we grew up learning this therefore it will forever remain true” argument…an atom was once thought to be the smallest particle in existance.indivisible.then we discovered there are 57 different types of subatomic particles.people took this on as a fact and there was no more debate about it.this is exactly the same.Pluto is not a planet and that’s a fact,no matter what you grew up thinking it was..in a few years everyone will just have to accept this fact…
    i know it’s unfortunate that we have to take time from our daily lives to learn a new rhyme so that we can remember all the planets.i feel that pain too.
    i guess i’ll leave it at that.
    and pluto…don’t be sad…i’m not a planet either.

  • Kakashi Hatake April 7, 2009, 5:18 AM

    Now that you put it that way danj4chilis, i now see what people are getting at, thank you for opening my eyes and showing me that we do need to move on to something better like, are ghosts real, stuff like that, but we will have to learn a new saying to figure out the planets Mercury, Venus, Earth, Mars, Jupiter, Saturun, Uranus, Neptune.

    hmmm…does that work???

  • Chris April 8, 2009, 12:13 PM

    I get all the arguments about why Pluto doesn’t meet the definition for a planet. What I don’t understand is why they didn’t just grandfather it? That way they still get their new, more sensible definition, and the public stays happy.

  • Dave R April 11, 2009, 1:26 PM

    Its interesting that some guy feels its his mission in life to decommission a planet and that’s probably because he wasn’t the guy who discovered it. Just my opinion! But why stop there! Why should we call Phobos and Deimos moons? They are just two little rocks that orbit Mars. Aren’t they too small to be considered as moons?

  • Cole Wentworth April 14, 2009, 8:00 AM

    :) i >:( that pluto is not a planet!!!! i also :'( that pluto is not a planet
    from Cole Wentworth

  • george matthew nebres April 14, 2009, 11:21 PM

    I was editing a video about solar system, and included pluto as one of the planets. I felt very sad when someone told me that pluto is not a planet anymore. Maybe someday i will not be that surprised when they’l demote Earth as well.

  • leslie April 16, 2009, 11:55 AM

    i just want to say thanks, i learned a lot, rather than reading those other unexplained sites. this is very nice & understandable. and very cool. :)

  • BRANDON April 18, 2009, 1:26 PM


  • Jack April 19, 2009, 8:36 AM

    I personally do not agree with the whole “clearing of the neighbourhood” thing, it doesn’t really make enough sense. There could be a system out there (as space is rather large) that has, say, two planets in the same orbit around a star. Lets say both are roughly the same size as Earth. If they’re both in the same orbit, that means they haven’t “cleared their neighbourhood”, so are now defined as Dwarf planets? Even if they are slightly bigger/smaller than EARTH? Because I really don’t think that we live on a Dwarf planet.
    Also, I agree with Laurel Kornfeld in saying that the definition of a planet should be broad, even if it does mean that there could be over 100 in our solar system, does that matter? Not really.
    Ok then, Pluto can be a Dwarf PLANET, along with many others, and there can still be 4 terrestrial planets and so on. I reason that if a object achieves hydrostatic equilibrium and it orbits a star it should be shoved under the umbrella that is “Planets”, and sub categorized further. (Exo-planets included)

  • ruvim udodik April 21, 2009, 10:18 AM

    i think that pluto is a planet becuz it is round like a planet so i think there is 9 planets becuz pluto is round

  • ruvim udodik April 21, 2009, 10:21 AM

    i think also that pluto can not be a planet becuz it small like my my penis