[yitchelle]

Is a plant being round a subjective thing? Just trying to understand how roundness can be measured.

[ProblemFactory]

In more detail it's not about the shape, but mass. Planets are massive enough for gravity to overcome the internal bonds in rocks and metals, making even these materials flow into a round shape.

An asteroid could be perfectly round, but its shape is based on material strength and not just gravity.

[simonh]

Yes, but if we want to distinguish between say asteroids and planets, or orbital pebbles and planets, we need criteria with which to do so and those are going to be at least somewhat arbitrary.

The 'cleared it's orbit' thing is also arbitrary. Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit? It's mainly to exclude Ceres, which is pretty close to spherical.

The problem is any rules that exclude Ceres and also exclude the other trans-Plutonian planetoids also end up excluding Pluto. If Pluto is a planet and this new thing, why not Eris, Haumea, Sedna, orcus, etc, etc, etc.There are many dozens of them.

We do have applicable terms - planetoid, dwarf planet, so why not use them?

[KnightOfWords]

> The 'cleared it's orbit' thing is also arbitrary.

Not really, the degree to which a body has cleared its orbit can be calculated: https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

> Pluto is sometimes closer to the Sun than Neptune for example but does that mean they share an orbit?

Pluto is locked in a resonant orbit with Neptune, a position it reached due to the influence of Neptune's gravity:

https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptun...

[lmm]

Why does one want to exclude Ceres?

What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself. If a planet is ejected from its orbit, it becomes a ... well, we don't even have a name for what it would be. (This problem exists for "moon" to a certain extent, but a moon that's thrown out of planetary orbit would become a planet if it was big enough and an asteroid if it wasn't). It's also hard to apply: a lot of extrasolar planets we're discovering will have to be reclassified once we get better at finding smaller bodies, just like in this solar system.

[KnightOfWords]

> What I don't like about the "cleared its orbit" criterion is that it's something you can't tell from looking at the object itself.

Dwarf planet is a helpful term in some contexts but not others. The solar system is a dynamic system, when talking about the whole thing or its evolution it's useful to distinguish between planets and dwarfs.