How big is Pluto? (and is it a planet?)

How big is Pluto? And is it really a planet? if not, what is it?
Question posed by William

Pluto is the smallest planet in our solar system. It's a bit more than 2300 kilometres in diameter*, which is about one-sixth the diameter of the Earth. To put that in perspective, if you could shrink the Earth down so that it was about the size of a football, Pluto would be about the size of a table-tennis ball. Pluto is even smaller than our Moon.

I went into a bit more detail on why Pluto is no longer a planet in an older blog post, but I'll go over the basics here as well: Pluto used to be called a planet, but that was when nobody had really decided what a planet actually was. Back in 2006 the International Astronomical Union (IAU) decided on a definition which meant that Pluto was no longer included. The IAU also decided on what Pluto (and other objects like it) should be called, and because of that, Pluto is now known as a 'dwarf planet'. There are currently five known dwarf planets in our solar system, including Pluto.

This image (nabbed from another website- click to visit) shows the size of Pluto in relation to the Earth and its Moon, as well as a couple of other solar system bodies (both Sedna and Quaoar are candidates for the title of 'dwarf planet', but we need to collect more information about them before we can be sure).

* The diameter of a planet is the distance from one side to the other if you went straight through the middle of the planet.


  1. You are doing readers a disservice by presenting only one side of an ongoing debate as "the truth." To many scientists, Pluto IS still a planet. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. One reason the IAU definition makes no sense is it says dwarf planets are not planets at all! That is like saying a grizzly bear is not a bear, and it is inconsistent with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. Also, the IAU definition classifies objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is essentially useless. Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity--a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned.

  2. Hi Laurele, and thanks for commenting.

    Your issue is, of course, a semantic one rather than a scientific one. Yes, there are many people who take great issue with the 'degradation' of Pluto from a planet to a dwarf planet. Currently, though, the accepted definition of the word 'planet' does not include Pluto and similar astronomical bodies. This, with debate, may be changed (again) in the future (and I would have no problem whatsoever with the idea), but as it stands, the accepted definitions put Pluto in the category of dwarf planet rather than planet.

    If I may play devil's advocate, part of the reason for developing a definition that would classify Pluto as a planet involved much deliberation around the fact that if Pluto was to be included as a 'planet', then there would be potentially hundreds (maybe thousands, eventually?) of planets in our solar system. The term 'planet' is felt by many to be something which should apply to a small number of objects in order to keep its 'special' status. After all, if there are hundreds of 'planets' floating around, what is there to distinguish the eight considerably larger and more dominant bodies in our solar system from the rest? Indeed, if your definition by location argument were to be employed, many of the moons in our solar system (including Earth's own Moon) would have to be upgraded from 'moon' to 'planet'. But again, this is an issue of semantics.

    I would have no problem, personally, with reclassifying the Earth-Moon system as a binary planet rather than the current planet-with-moon, as would be necessary to fall in line with your proposals, but one issue that I can see would be an unnecessary amount of upheaval in terms of reclassifying astronomical bodies. As it stands, only Pluto has had to suffer reclassification under the current accepted definition.

    Personally, it matters not to me whether Pluto is referred to as a planet, a dwarf planet or a banana. Whatever we choose to call it, it is an astronomical body that plays its part in the clockwork of the universe; it is beautiful in its way, and no change of definition can affect that: /that/ is what matters to me.

    I must, however, take issue with your accusation that I am doing my readers a disservice, and your insinuation that I am peddling untruths: It is perfectly true that in 2006 the IAU released a definition for the word 'planet' that no longer included Pluto. Whether or not you personally agree with that decision has no bearing at all on whether or not it actually happened: It did, and that's The Truth. Whether it was a good idea or not is not something that I have not commented upon here or elsewhere.

    Finally, I take issue with your assertion that the Earth would not be considered a planet if it were in Pluto's orbit. This does not, to me, appear to be consistent with the IAU's 2006 definition of what a planet actually is.


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