Thirteen planets and counting | Hacker News Reader

Thirteen planets and counting

blogs.egu.eu

25 pointsokket8 comments

I like the term "world" to denote a roughly spherical body with evidence of interesting dynamics happening, or at least having happened. It's poetic and evocative—but not excessively so. Emily Lakdawalla of Planetary Society fame uses it in that sense.

Although this is explicitly at odds with the author's intentions, I don't think there should be an overlap of any kind between "moon" and "planet." So I'd modify his definition of "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" to add: "excluding any object that orbits a point located within a planet." Thus our Moon would remain a moon but Pluto-Charon could be a double planet because the point they revolve around is not within either body.

It's a weird issue because really it's all vaguely arbitrary distinctions.

There's nothing really different between the various celestial bodies and at the same time they're all so vastly different from each other. You have planetary satellites larger than some planets. A couple of planets that are pretty much failed stars.

Even things like comets and asteroids, they're really just defined by behavior. I mean the definition of asteroid is basically "not a planet or comet".

It's basically all "stuff in space".

> arbitrary distinctions.

Exactly, the current definition of planet is not that useful. If a moon happens to exist in the goldilocks zone it could easily support life even if it's sitting next to a Jupiter sized planet that does not.

Jupiter and the Sun orbit a point located outside of the Sun. Using your definition would this make Jupiter a double planet with the Sun? A double star, and thus not a planet?

Since the Sun isn't a planet, it shouldn't change anything for those.

For a more apt example: In a billion years when the moon's orbit has expanded an additional ~118,000 km it would suddenly stop being a moon?

The problem with barycenter is it is equally changed by distance as it is mass ratio.

I don't disagree with your point but it's an issue that plagues definitions in general. If something is "near" and slowly backs away, at some point it becomes "far," but there's no clear dividing point. On the other hand, according to law and custom, someone does suddenly stop being a minor when they become a legal adult. In that case we choose a clear arbitrary cutoff for simplicity's sake. A third type of definitional change is one where there is a clear non-arbitrary cutoff, e.g. a star turning into a black hole, or an irregularly shaped asteroid accreting enough matter through collisions to turn into a dwarf planet. Arguably, the barycenter moving from within one planet to outside of it would constitute this third type of change. So I think if two former planet/moon objects evolve to orbiting around an external barycenter, it's not arbitrary to state that the less massive no longer a moon of the other, even if, at some point in the past it was, and in the case of our Moon, will likely be again.

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