So, How Many Planets are There, Anyway?

I started this entry several months ago, when the latest planet-sized object beyond Pluto's orbit was located. At that time the astronomers were hemming and hawing over whether they were going to call it a planet; but since they called Pluto a planet, they pretty much had to call this discovery a planet as well. I've been waiting for the other shoe to drop. It appears that event has now taken place.

'The other shoe' is the IAU definition of 'planet'. They recently formed a committee to come up with a definition that could be applied to all solar systems, and now they are floating that definition amongst their peers, looking for acceptance.

This new definition would yield 12 planets for this solar system, based on current knowledge. Some astronomers think this number could go as high as 24.

Personally, I think the astronomers are missing a key point in defining what is or isn't a planet. Anyone who looks at the orbit of Pluto (or the Plutons, as the new definition refers to them) and contrasts it with the orbits of the 'classical' 8 inner planets, can probably get the point I'm trying to make here. Pluto is clearly not of the same nature as the rest of the planets. It's orbit describes a body that is more akin to a comet than to a planet. The definition of planet should reflect this.

A planet should first and foremost be formed from the original accretion disk of the star that it orbits, or follow the same orbital pathway that the star's gravity and spin dictates. Anything of planet size that doesn't conform to this plane should be referred to as a 'planetoid'; of planet size, but not truly a planet.

Of course, this kind of level-headed thinking on the subject would yield no new planets for current and future astronomers to hang their names on, so I'm not looking for any of them to notice the argument at all, human nature being what it is.

Call me old fashioned, but I really think these types of defining moments should reflect the need for clarity, not the desire for self-aggrandizement.


  1. Someone asked for a definition that I thought was better. In short, Gravitation *and* conformity to the ecliptic plane pretty much sums it up for me.

    Why ecliptic? Because it's what the original 'planets' had in common. It's something that separates them from the merely round planetoids out in the Kuiper belt.

    As the article I cited notes, the vote is 'yes' or 'no', so there
    isn't any chance to clarify the definition at this point. We're
    probably going to be stuck with 12 to 24 'planets' to memorize. At
    least they can finally work their way through the Greek pantheon, I

    As the photo on this page shows, ecliptic is not limited to Earth/Moon/Sun

    I used the phrase 'Accretion Disk' originally because that is the process by which the planets are formed. 'Plane of the Ecliptic' works as well, though.

  2. And then these objections appear:
    How do you feel about Mercury? Of the ancient planets (to the Greeks, the Moon, Mercury, Venus, Mars, Jupiter, Saturn) it has the highest orbital eccentricity and the greatest inclination to the plane of the ecliptic.

    How do you feel about Uranus? Does it count as "original"?

    How do you feel about its axial tilt? Should it be shunned as a
    non-planet because it is tilted queerly?

    Please feel free to answer.

    Now who's nitpicking? (nitpicking being a charge leveled at "those who don't like the IAU definition")
    Mercury through Neptune roughly occupy the ecliptic plane. They should rightly be considered planets of the sun. Round objects out in the Kuiper belt generally do not orbit on the ecliptic (Pluto as an example) giving them a different origin than the 'classical' planets. I've always thought that Pluto didn't qualify as a planet, because of this fact.

    So, any round planet in the Kuiper Belt or in the Oort clouds that happened to orbit the Sun in the plane of the ecliptic would do?

    I would imagine that the Kuiper Belt itself is a pretty common phenomenon, recurring in other star systems. A different name for round bodies that occur in the belt (Planetoid works for me) makes sense.

    As the article I cited notes, the vote is 'yes' or 'no', so there isn't any chance to clarify the definition at this point.

    Which yielded the objection "voting solves everything, doesn't it".

    I merely observe the rules of the body that has offered the definition for 'planet'(the
    definition that you are defending, BTW. A committee decided on that
    definition. I'm sure they 'voted' on it too) not endorse the concept that majority rule has anything to offer.

    ...To re-iterate, I'm simply of the opinion that important distinctions
    are being glossed over in the proposed definition for 'planet', so that modern astronomers can have their names remembered alongside the great discoverers of history; people who faced persecution and even death in pursuit of real knowledge; and diluting the importance of "the wanderers", which are still visible in the night sky, even for those poor souls confined to large cities.

    But as usual, I appear to be on the loosing side of the argument.

  3. ...and I'm liking the whole idea of a simple "belt object exclusion" myself. Probably have to come up with some definition of 'belt' for that to be useful.


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