Does NASA, or JPL, etc., have some definition a heavenly body in our solar system (for starters) must meet to be considered a planet and not an asteriod? For example, I have read about some asteroids found beyond Pluto’s orbit. And, there a better-known asteroid between Saturn and Uranus’ orbits.
Yet, Pluto got a lucky break to be classified as a planet - long before its moon was discovered. Although, some still debate if it should be called a planet or not.
Anyone have any knowledge about this, or an educated WAG?
There was an article in Smithsonian a few months ago about Pluto that had a good discussion of this matter. If I remember right, it said that since what we call planets are such a heterogeneous bunch of objects, there wasn’t really a definition of “planet” that would include Pluto without also including some of the bigger “asteroids,” and probably some Kuiper Belt objects no one’s seen yet.
I just found the magazine in question and it said the International Astronomical Union’s definition of a planet is that it has to orbit the Sun and be big enough for its gravity to pull it into a sphere. By that definition, both Pluto and the asteroid Ceres are planets. So it’s easy to see why there’s controversy.
(One criterion they mentioned that some people want to use is that a planet has to have a strong enough gravitational pull to clear all of the other miscellaneous debris out of its orbit, which apparently isn’t true of Pluto or the asteroids.)
I would suggest that Pluto’s a planet and not an asteroid for the same reason that Australia’s a continent, and not merely an island. We choose to say it is. Some categorizations which have only a small number of objects in them don’t have to be well defined, because you aren’t often going to run into new candidates. Attempting to come up with an objective measure ex post facto that validates the existing notions may be difficult.
The reason Aussie is called a continent is that Greenland the next sized “island” is so much smaller. Though a lot smaller New Guinea still is close enuff in size to Greenland to be comparable.
Therefore it makes sense to call Greenland the largest Island.
Except that we’re up to what, sixty-something extrasolar planets, now? None of them are remotely close to being “too small” to be a planet (although most of them are pushing it on “too big”), but it’ll happen eventually.
There’s potential to be other Kuiper Belt objects as big as Pluto that haven’t been discovered, isn’t there? And thus the question of Pluto’s status.
Jinx, right now nobody has a good definition. This was solidified in the previous debates by the International Astronomical Union on Pluto’s status a couple years ago. Amateur astronomers have fun drawing up their own criteria and debating the points, but nothing official has been declared.
Couple to the discussion on the small end what happens on the large end. What is the division from Jupiter type planet to brown dwarf star? What about free roaming brown dwarfs? Objects orbiting brown dwarfs?
I just read an old article in The Planetary Report (The Planetary Society’s magazine) about comet-like asteroids and asteroid-like comets, so even the little guys are muddying the waters.
Basically all the conventional terminology is obsolete.
The answer to the OP is “no”. There is no definition for planet, or asteroid. There is likely to be none, though some astronomers have argued for one, and even printed papers about it. I wrote an article about this here. Look for the link titled “Is Pluto a Planet?”
The consensus on that one seems to be settling on deuterium-burning. An object about 17 times the mass of Jupiter can’t fuse ordinary hydrogen, but it can fuse deuterium. In addition, I think that it’s generally agreed that to be a true planet, an object must be orbiting a true “star” (i.e., not just a brown dwarf). Anything else would probably just be lumped into the MACHO category (but what if it isn’t a halo object?).
