|Pluto and it's moon, Charon|
So, let me offer a definition of a planet from the viewpoint of a consumer of astronomy. To be a useful definition, let's make it more like the way the APA describes mental illnesses. It gives some options and the requirement is that the disorder (or planet in this case) meet 3 of 4 or 4 of 7 of the criteria to be that mental disorder or astronomical object. This leaves room for variation within a type. So here's my offering for a definition of a planet.
1. Is a celestial body moving in an elliptical orbit around a star.
2. Is spherical in shape and has a shape that is stable under its own gravity.
3. Is large enough to dominate its orbit having cleared neighboring space of debris.
4. Has at least a diameter of 2,000 km.
5. Has a satellite that orbits around it.
6. Has an atmosphere.
To be a planet, it must meet four out of six of the above criteria.
Pluto would be a planet (1,2,4,5,6)
Eris might be a planet (1,2,4?,5,6?)
Makemake would be a dwarf planet (1,2,)
Quaoar would be a dwarf planet (1,2,5)
Orcus would be a dwarf planet (1,2)
Sedna would be a dwarf planet (1,2,)
Ceres would be a planetoid or asteroid (1)
I might even make a further distinction.
1 criteria = asteroid (mostly because it's not spherical)
2 criteria = planetoid
3 criteria = dwarf planet
In which case Orcus, Makemake and Sedna would be only planetoids.
Just my own humble opinion. It just seems that the requirement that a planet meet every criteria in a list is too rigid. It doesn't allow room for something odd to be a planet even though everybody thinks it ought to be one because it looks like one. That's how things get named out in the real world.
Scientists should remember that before they go around willy-nilly making a kid's model solar system obsolete.
Yeah, it makes me mad. I'm getting too old to relearn the solar system.
© 2014 by Tom King