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Registered: 01-2008
Location: Finland
Posts: 295
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Charon - what the heck is it?


With New Horizons on the way to Pluto and giving us new "best yet" images every other day, I've returned to ponder the issue of Charon.

Back when they created the new definition of Dwarf Planet - demoting Pluto and promoting Ceres and the recently found Eris, Makemake and Haumea - the status of Charon (and therefore Pluto) was left uncertain.

Three options:

1. For now, Charon is merely defined as satellite/moon of dwarf planet Pluto. Generally makes sense, except...

2. Charon doesn't really orbit Pluto. They orbit center of their common mass, which is outside Pluto, different from any other pair of satellite and a planet or dwarf planet. Charon and Pluto can be argued to be a binary/twin planet - a definition that doesn't yet exist.
I'm not quite sure of numbers (or if it's yet known for certain), but I'm under impression that if we count Pluto and Charon as one twin object, it would meet the definition of planet in what Pluto alone doesn't (having "cleared the orbit", meaning having certain percentage of mass of all material on that orbit).

3. Even if we don't want to call them a twin object, Charon itself meets the definition of dwarf planet. Only reasons it's not one are that there's a larger object on the orbit (Pluto) and Charon arguably orbits it (as it's orbit is around Pluto's orbit). Charon is even larger than Ceres, the smallest dwarf planet.

So is Charon a satellite of dwarf planet Pluto (as it's currently officially), are Pluto and Charon a twin planet (I'd say so) or should we just make Charon a dwarf planet?

Options 2. and 3. would have interesting consequence for Earth too. In very very very distant future Moon moves so far that we are in same situation as Pluto and Charon. So if we call them twin planet or two dwarf planets on same orbit (orbiting also common center of mass), it would mean Moon will also eventually become a dwarf planet or Earth-Moon twin planet.

Which makes me think solar system objects need two separate definition systems: one by the size and nature (composition and origin) of the object and second by what it orbits or is in same orbit with.

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2/13/15, 9:38 am Link to this post Email Kaunisto   PM Kaunisto
 


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