Reply to post: Re: Large Satellites?

Amid new push to make Pluto a planet again... Get over it, ice-world's assassin tells El Reg

VinceH Silver badge

Re: Large Satellites?

With a little thought, the definition could be updated in such a way that allows Pluto to be defined as a planet, but excludes the Moon (and those of other planets).

If we take the proposed definition quoted in the article as a starting point:

"A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

The article notes two requirements from the 2006 definition which are the cause of all Pluto's problems:

* Clearance of all other objects in the same orbit

* Orbits the Sun

The first of these is clearly imperfect because small objects can still exist on the same orbit as larger objects having not been cleared - even larger ones if they're sufficiently distant in that orbit, as this highly scientific documentary makes clear.

Orbiting the Sun is also a problem because it doesn't allow for rogue planets, as noted in the article - and although we think of it as the moon, that time it went rogue (as detailed in this documentary) it should arguably have been considered a planet.

So the definition could perhaps scope out whether or not it is orbiting a larger body, and if so the nature of that body. If it is orbiting a larger body that has never undergone nuclear fusion, it is a moon - otherwise it is a planet.

That makes Pluto the planet, and Charon a moon - and because it doesn't specify that it must be orbiting a larger body, rogue planets can be planets too. If you want to take the barycentre into account, then it is the larger body of those that share a barycentre.

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