Was it a planet that got broken up?
Why is it there?
Probably not.
The most widely accepted theory is that Jupiter’s gravity disrupted any potential planets in that region of the Solar System, so that instead of a planet you have assorted left-over debris.
A) Probably not. The entire mass of all the asteroids in the belt is much less than our own moon, IIRC.
B) It’s probably rocky debris left over from the formation of the Solar System, that never coalesced into a planetoid. Why? Someone else can take over here, but possibly because it was caught in a tug of war between Jupiter and the Sun.
Interesting thought: All the major bodies in the Solar System have rings: Saturn, Jupiter, Uranus, Neptune–and the Sun! If you consider the Asteroid Belt a “ring.”
According to the theory subscribed to by the BBC series The Planets, the early inner solar system (the part occupied by the terrestial rocky worlds) is thought to have consisted of many large bodies, possibly as many as one hundred. It is believed that these “planetoids” collided with one another for a considerable period of time, with recurrent episodes of fragmentation and accretion. While much of the total matter has coalesced into the four inner planets that we know today, a considerable quantity is thought to contibuted to the asteroid belt. Of course, some of the debris would have been sucked in by the Sun’s gravity, while the remainder would have dissipated in a variety of ways: sucked into one of the gas giants, accumulation into comets, nebulous gas and debris etc.
However, like all cosmological theories, this scenario is the subject of much conjecture and disputation.
While we’re on the subject, what is the formal definition of a planet, anyway? Why is Pluto a planet while Ceres isn’t? Size is the obvious answer, but what’s the cutoff, and where do we get off having an artbitrary size requirement? What empirical, objective qualities does Pluto (or any other planet) have that Ceres does not?
-Ed
You know that doughnut-shaped thing you have to sit on when you have asteroids? That’s an asteroid belt.
Really.
No, really.
Not exactly on the OP, but following the thread…
This site discusses recent efforts by (some) astronomers to “demote” Pluto from its status as a planet. Seems it should be an asteroid or comet or something. Objective data suggest the demotion, subjective data (is that an oxymoron?) - like years of learning The Sun Has Nine Planets - work against popular acceptance.
I don’t believe there is one. In fact, there is a growing tendency not to regard Pluto as a true planet, as more about its characteristics become known, but instead just the largest of many “Kuiper objects” - or maybe it’s an escaped moon of Neptune.
One definition I have seen is that a planet:
- Orbits the Sun directly, not some other object
- Has a gravitational field strong enough to form it into a spherical shape
These folks define “planet” as such:
‘A “planet” is an object that has a mass between that of Pluto and the Deuterium-burning threshold* [*about 12 x mass of Jupiter–SK] and that forms in orbit around an object that can generate energy by nuclear reactions.’
The Pluto lower limit seems rather arbitrary to me, and it does not account for large bodies which form in orbit around a sun and are later trapped in the gravity well of an object other than the central gravitational point–the “capture” theory.
My own definition of a planet might be an object large enough for its own gravity to make it roughly spherical in shape, and which traces an independent orbit around a sun, but which is not massive enough to generate its own fusion heat. If two such bodies are in a double-planet configuration, as Pluto/Charon and Earth/Moon* are, the larger of the two gets the nod as the “planet.”
The definition allows for both Pluto and Mercury, which really aren’t much larger than the Moon, while excluding large sperical bodies which clearly are subservient in their motion to a larger body, like the Moon, Ganymede, and Triton, whose origins are still (weakly) in dispute.
Ceres, an asteroid orbiting in the Belt, may also meet my own definition. I can’t remember if it is sperical or not. If it is, I will lobby tirelessly to allow tiny Ceres into the club, just as I now tirelessly lobby to have the tomato promoted back to a fruit. Either that, or I’ll drink some beer and watch some television.
*Yes, the Moon is said to orbit the Earth, but in fact they share a common center of gravity measurably distant from the center of the Earth, but still inside the planet.
See? If Colibri quoted it from another source, my idea must be good. On to the tomatoes!
I agree. The upper threshold seems obvious enough, but the Pluto lower extreme bugs me. I am aware of efforts to revoke Pluto’s planetary status for a variety of reasons, but that still doesn’t leave us with a satisfactory lower size cutoff.
Again, I agree. Ceres is indeed a sphere. It strikes me that an objective lower boundary for planethood would be, as you say, sufficient mass to collapse the object into a sphere. That being said, is the “real” definition of planethood the one you note at the top (haven’t checked out the site yet, it’s my next stop)? It seems like we should be able to drum up at least much support for overturning it as those anti-Pluto weenies have.
-Ed
A body doesn’t have to be very large for gravity to force it into a roughly spherical shape, perhaps a few hundred kilometers in diameter. Ceres is spherical, but there are several spherical asteroids smaller than Ceres.
The definition I propose is that a planet has to be larger than the largest moon in the solar system. This would disqualify both Mercury and Pluto. There are three moons bigger than Mercury (Ganymede, Titan, and Callisto) and there are seven moons bigger than Pluto (those three plus Io, our own Moon, Europa, and Triton). In comparison, Ceres is only about the 26th biggest body in the solar system.
Well, heck, I can’t even spell “spherical” right, so don’t take my word for it.
I chose spherical as a definition on the assumption that such a body would aglomerate in the early phases of solar development, while other bodies might later come together after having cooled and crystalized in their individual subsections, then “bumped” and coalesced into a more traditional asteroidial body.
Frankly, those “hybrids” are the ones in which I’m most interested (as a guy who does Amerincan Indian Affairs for a living). I think they could give some valuable insight into the early orbital mechanics of the Solar System by comparing their component parts against each other.
Yeah, but it’s only the tenth largest object that directly orbits the sun (twelfth if you want to be fussy and include the Moon and Charon as double planets – am I missing any comets?). The definition you propose is fair and objectively measurable, but would necessarily vary from star to star. What about a system that has no moons at all? Would every object in it be a planet? Also, “bigger than Ganymede” is only marginally less arbitrary than “at least as big as Pluto.” If Ganymede, Titan and Callisto suddenly dropped out of their orbits and were destroyed, Pluto would suddenly be a planet, without any of its own properties changing (would that make Pluto a Vice Planet?).
Of course, now that I’ve slept on it, the sphere definition troubles me too. Vesta is a sphere, and is way smaller than Ceres. Plenty of pebbles are observably spherical, pretty much by coincidence. It may be that there is no empirical quality that can firmly set a minimum size for planetosity. Perhaps the definition is or should be, “I can’t explain it, but I know a planet when I see one.”
Is there some official group of astronomers who get to decide stuff like this?
-Ed
Yes, the International Astronomers Union.
And just to throw in some other orbiting bodies, don’t forget the Centaurs.
Yeah, there were a couple of sites about Pluto I looked at that used that definition. However, I didn’t check to see if the largest asteroids were spherical or not. If they are, that pretty much cans that definition (unless you want to promote them. And then, how “spherical” does an object have to be?)
Basically it all boils down to a continent vs. island type of distinction. Is Australia the smallest continent or the largest island? We fit our definition to include objects that have “traditionally” be classified that way, and exclude those that have not.
According to Ptolomian astronomy, of course, the Earth wasn’t a “planet” either. It’s just that over time the distinction of planet has been extended to other objects pervieved as being similar to the original five: Mercury, Venus, Mars, Jupiter, and Saturn.
Pluto is definitely a Kuiper belt object, but there’s nothing that says it can’t also be a planet: The two are not necessarily mutually exclusive. Depending on your definition of “planet”, I’ve seen figues quoted for anywhere between seven (demoting Pluto and promoting Jupiter to a brown dwarf) and fourteen (promoting the four biggest asteroids and Luna) planets in our System. Any definition is of necessity a bit arbitrary, and the most common one is “one of those nine things that we’re calling planets”. Obviously, this presents problems with the discovery of extrasolar planets (or are they extrasolar binary brown dwarfs? What about when we do eventually find another Earthlike body?).
By the way, Jupiter is far enough from spherical (due to its rotation) that the flattening can be seen by eye, through a half-decent telescope.
Where have you seen these figures? Are there books or websites that talk about this stuff? Personally, in case I haven’t made it evident, I’m inclined more towards inclusion than exclusion (hey, the more the merrier), and I’d love to see someone who agrees with me.
-Ed