Twel… no, it’s eight.

Personally, I gotta say hats off to the IAU for taking the hard decision and evicting Pluto from the list of planets. I really didn’t think they’d do it.

That doesn’t mean that Pluto becomes uninteresting… it just means that, like Ceres, we now know that it’s an unusually large member of a large belt of debris–which is its own kind of interesting. I’ll still be interested to see what the New Horizons probe has to see.

Pluto and Ceres–and 2003 UB313–now become members of a new class: “dwarf planets”.

The rules are now: “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a … nearly round shape, and has cleared the neighborhood around its orbit.” Pluto loses because of the eccentricity of its orbit, which brings it sometimes inside that of Neptune (as it was until only a few years ago)–it hasn’t cleared its own orbit. However, I suspect that the eccentric exoplanets–many of which are gas giants–have cleared their orbital space because of their size, and will not need a new definition.

I approve. It’s a simple rule, easily and fairly applied. Kudos to the astronomical community for taking the unpopular–but correct–stand.

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4 comments so far

  1. jamess_fox on

    Personally, I think the defintion sucks. The whole ‘cleared the neighborhood thing’ is incredibly confusing vague, as others have pointed out, and neglects such object as trojan bodies, possible large objects is resonant crossing orbits (in other solar systems), and other such minutae.

    I think the Original Counter Proposal makes much more sense. It would have given essentially the same result for the solar system, but using terminology that is less confusing (though just as vague) , and more in line with the most persuasive argument against Pluto: that it is just a large Kuiper belt object.

    This new defintion also leaves out such things as binary pairs, the maximum mass of a planet, and so on. It is a mess.

    • The Rev Dr Sherwood Forrester on

      Hadn’t seen the counterproposal before, but yes, that does make more sense to me. What we got, I suspect, was what was most palatable to the greatest majority at the conference. In comparison to that counterproposal (and I like the no fusion criterion), what was passed isn’t perfect, but it is better than the status quo. I suspect future exoplanet discoveries will force a clarification in the next ten years or so that will bring the adopted definition closer to the counterproposal. Good point about Trojans, too.

      And we don’t have to look past our solar system for a resonant crossing orbit (if that means what I think it does): Neptune and Pluto are in a 3:2 resonance, so that even when Pluto crosses the boundary of Neptune’s orbit, Neptune is always way off somewhere else in its orbit. They never get anywhere near each other–certainly never close enough for major gravitational perturbations.

    • thattallguy201 on

      And you left out the aspect of this defintion that bugs me the most: it will be decades, if ever, before we can identify any extra-solar planets. Why? Well, how can we see if there’s debris in the same orbit as a suspected planet around stars so far away we can barely see the planets??

      • The Rev Dr Sherwood Forrester on

        Well, that kinda raises the question of whether it should be considered a protoplanet until the orbit is essentially cleared… I’m just glad they made the tough call, even if the definition needs work.


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