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rlagmagm in astronomy

About Pluto...question.

I hate to sound rather dumb-but anyhow. Is Pluto still considered a proto-planet or is it now considered a dwarf planet? I may sound a bit behind the times but last I heard was that Pluto is a proto-planet.


Dwarf planet. It probably formed around the same time as the other large trans Neptunian objects, but none of them ever got past the planetesimal stage of planet formation - the timescales and gas densities at those orbital radii are just too unfavourable. It may be a little bit *like* what we'd expect of a proto planet at that distance, but it's certainly not going to grow any more.
I consider Pluto a planet! Mystery solved!

The entire dwarf planet thing is a little arbitrary, because one of the criteria for a dwarf planet being a dwarf planet and not a full planet is that it has not "cleared its orbit", that it doesn't have enough gravity to have absorbed debris in its orbital path.

The problem with that is that what does that mean? Since every planet has some debris still entering, what is the difference between say, Earth and Pluto? For that matter, since Pluto intersects with the orbit of Neptune...is Neptune not a full planet because it hasn't absorbed Pluto yet?
I think the unstated clause in "hasn't cleared its orbit" is "of objects of close to its own size or larger, excluding moons." Hence Ceres is only a dwarf planet because it shares the asteroid belt with other decent-sized small rocky bodies such as Vespa, which, while it may not be big enough to be round, is not that much smaller than Ceres. Pluto crosses much-larger Neptune's orbit and shares the Kuiper Belt region with slightly-larger Eris and other decent-sized bodies. Earth, on the other hand, is a full-fledged planet because the near-Earth asteroids are more than an order of magnitude smaller than Earth.

Many astronomers still think the distinction is stupid, though...it shouldn't matter so much whether the object shares an orbit with larger or only slightly smaller objects or not. I would have preferred the 12-planets-and-counting solution myself. But the 8-planet solution at least prevents us from having to revise the textbooks every year with a new dwarf planet discovery in the Kuiper Belt, so maybe that's why they went with creating the intermediate category for these large-ish, round belt denizens.
Thirteen. We have five dwarf planets: Pluto, Ceres, Eris, Haumea and Makemake. Plus a number of Trans-Neptunian Objects that are almost certainly dwarfs.
Well, it was "12 and counting" at the time of the debate and the definition of dwarf planets. I noticed about Haumea later, but it was depicted as a prolate spheroid, so I'm not sure how well it fits the roundness criterion. (Then again, the giant planets tend to be a little oblate.)
Roundness does have a definition: basically the surface is shaped by gravity, rotation and tides. (Hydrostatic equilibrium) So it allows for things like Haumea's shape from its really fast spin as still being round.

Vesta is a nice example: it's very close to hydrostatic equilibrium, but if it was slightly bigger, the craters on its south pole would have rebounded, restoring its shape, and it would be a dwarf planet. So, it's not a dwarf planet, though it's got some features of them (like a mantle and core).
Ok, that definition of roundness makes sense. And Wikipedia confirms my suspicion that "hasn't cleared its orbit" means that "of other things close to its size." Which means if an extrasolar world the mass of the Earth were part of a belt with the next-biggest object being Mars-sized, both would be considered dwarf planets in that solar system.

August 2015

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