Space Saturday XXIII

It has been a while since I have done a Space Saturday post, but I am back. With all the excitement going on in our Solar System, I bring you Space Saturday: the Dwarf Planet Pluto Edition!

Source. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team. Pluto and moons without labels.

There still seems to be a lot of confusion going on surrounding Pluto being reclassified as a “dwarf planet”, so I thought I would try and help clear things up (there will be some repeat of information from previous posts, sorry!). There are often new discoveries in our Solar System and those objects need to be classified somehow. Two objects, Ceres and Eris (formerly nicknamed Xena), didn’t fit with the standard definition of asteroid (or other Solar System object). The IAU (International Astronomical Union) ( “Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation.” - they classify things) met to decided, among other things, how these objects would be defined. During the course of this discussion it became clear that the definition of planet needed to be nailed down. Either Ceres and Eris would be allowed to become planets (along with 50+ more down the road) or they needed to be classified as something else. When Ceres and Eris were fit into the category of “dwarf planet” it seemed that Pluto fit with them and not with the new definition of planet.

This is how the IAU has defined a planet (as described in Resolution 5.1):

5.1) A Planet is a celestial body that
5.1.a) is in orbit around the sun,
5.1.b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
5.1.c) has cleared the neighborhood around its orbit.

Pluto fails criterion 5.1.c in that there are many “plutinos” in its orbit. For a nice visual on this, see this image of Trans-Neptunian Objects. All the red dots are plutinos. For Pluto to be classified as a planet it must get rid of all those objects. Hope this helps!

The Hubble image of Pluto shows its three moons. Charon is the largest (at just over half the size of Pluto [about 1200km compared to Pluto's 2300km diameter]) and is sometimes considered a dual (dwarf) planet because Pluto’s mass is only a factor of 10 larger (the barycenter [center of mass] is actually outside of Pluto). Nix and Hydra are the other two, much smaller (around 45km in diameter), satellites of Pluto which were discovered in June of 2005.

Let me know if you have any questions! See two of my other posts for additional information: Pluto is Not a Planet (a look at the reasons why Pluto is not a planet [and the reasons that have nothing to do with the reclassification]) and 134340 Pluto (a look at the new classifications and names for the dwarf planets and their companions). Also, read NASA’s Chief Historian Steven J. Dick’s article “Pluto, Classification and Exploration” for a nice summary.

Check out my Space Saturday Archive for more astronomy stuff!

Wikipedia - Pluto
Wikipedia - Dwarf Planet
HubbleSite - Pluto’s Two Small Moons Officially Named Nix and Hydra
International Astronomical Union
IAU’s Minor Planet Center
NASA - Pluto: Facts & Figures
The Nine Eight Planets - Pluto
New Horizons - NASA’s Pluto-Kupier Belt mission

Categories: Science
  1. dan
    September 26th, 2006 at 15:36 | #1

    i miss pluto

  2. September 26th, 2006 at 16:06 | #2

    Heh, well fortunately for you, it is still there! :)

  3. Greetje
    October 19th, 2006 at 04:00 | #3

    Thank you. I didn’t really know what was going on with Pluto, but you’ve told me in a very clear way. It’s very comprehendible for a person who is not trained in this field.

  4. John Whelan
    February 24th, 2007 at 02:26 | #4

    Note that it’s not the existence of the Plutinos (and the rest of the KBOs, actually; Ceres is considered to be in the same orbital zone as the rest of the asteroid belt, not just those asteroids with the same period) that’s the problem, it’s that they’re comparable in mass to Pluto itself. That’s why the Kuiper Belt doesn’t disqualify Neptune, and the Trojan asteroids don’t disqualify Jupiter. Actually, Pluto and the rest of the Plutinos are gravitationally dominated by Neptune, which is why there are so many of them in that 3:2 orbital resonance.

  5. February 24th, 2007 at 11:39 | #5

    There are two issues here - why wouldn’t an object be called a planet and why wouldn’t it be called an asteroid… An object wouldn’t be called a planet because it doesn’t have a enough mass to have a large enough gravitational field to “clear its orbit.” And the reason they wouldn’t be called an asteroid is because they do have enough mass to make them round.

    So it IS because of the Plutinos that Pluto isn’t a planet. Pluto isn’t an asteroid because it has enough mass to make it round, but not enough mass to allow it to dominate its orbit. The reason Ceres is a dwarf planet and not just an asteroid is because it is more massive enough to make it round. So for Pluto it is not a planet because it does not clear its orbit (the Plutinos are there) but not an asteroid because it does have enough mass to make it round.

    The Kupier Belt doesn’t disqualify Neptune because Neptune really isn’t in the Kupier Belt. The fact that there is trojan asteroids in Jupiter’s orbit is one of the reasons some people don’t like the definition: it isn’t precise enough (even Earth has objects in its orbit).

    Being in orbital resonance with Neptune doesn’t really have anything to do with the reclassification.

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