[ExI] new ninth?

Dan TheBookMan danust2012 at gmail.com
Mon Jan 25 16:47:51 UTC 2016

On Jan 25, 2016, at 8:13 AM, John Clark <johnkclark at gmail.com> wrote:
>> On Sun, Jan 24, 2016 at 7:41 AM, Anders Sandberg <anders at aleph.se> wrote:
>>> http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system
>> ​> ​Would it actually count as a planet? I seem to recall that the Pluto-killing definition includes "has cleared its orbit of planetesimals".
> ​I've never quite understood ​that definition. Neptune hasn't cleared its orbit of Pluto so why is Neptune a planet? 

This one I agree with you on. And let's say by some change another currently accepted planet's orbit became uncleared -- say the asteroid belt shifted into Mars orbit -- would it be scratched from the list of planets?

And how clear is clear? There are Mars-crossing asteroids now, to stick with my example.

I don't see why the decision couldn't have been along the lines of:

There are three types of planets we now know of:
1. Rocky ones like Earth and Mars,
2. Gassy ones like Jupiter and Neptun, and
3. Icy ones like Pluto.

I don't see why 1 plus 2 alone must be planets, must be where Nature made the joints, while 3 is not.


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