I’m watching a tape of the Nova series Origins, and they repeatedly refer to the Asteroid Belt as basically leftovers from the time when planets were coalescing.
Now, granted, my knowledge of science is largely limited to that I’ve acquired via Sci-Fi. But maybe from reading too much Heinlein and Hogan, I had always been under the impression that there was a very high liklihood that the belt was the remnant of a broken up planet rather than one that never formed in the first place.
So what’s the straight dope?
While we’re on the topic of origins, can anyone give me an intelligent but geared-for-an-utter-layman explanation of how we get from amino acids to life (or at least the best theory extant)? I mean, amino acids, while necessary (at least for life as we know it here on earth), are not sufficient, right? Life is self-perpetuating; it needs energy to sustain itself, and its mission in sustaining itself is to reproduce itself. It seems strange that things would go from an equilibrium situation (i.e. no energy needed) to one that needs energy input to self-perpetuate.
You would need an astrophysicist specializing in solar system gravitational dynamics to explain this – fortunately, we have a couple of people who come close to qualifying on that ground, and I hope one of them will expand on what I say. However, I’m not one of them.
But as a layman interested in the subject, I had been given to understand that Heinlein was writing fictionalized versions of the “straight dope” on the origin of the Asteroid Belt as it was understood in the 1950s – that a planet had existed between Mars and Jupiter and had broken up, the asteroid belt being a part of the remnants. (Why, is of course a different question.)
That theory was replaced by the following because there was no mechanism (short of a planet-busting explosion by an intelligent race, an assumption that gave Ockham severe razor burns) that would explain the breakup, while simple gravitational dynamics would lead to the failure-to-coalesce hypothesis.
As I understand it, Jupiter was a very early and giant eddy in the protosolar-system accretion disk, which continued to sweep out the area within a reasonable distance of its orbit as it continued to form and grow. Hence it became the #2 influence in the Solar System gravitationally.
Because it formed early and was so large and hence exerted such a strong gravitational pull, the material within a certain distance inwards from it was not able to accrete into a protoplanet, but rather was spread, scattered, and forced into harmonic orbits by the influence of Jupiter’s gravity sweeping past them once every 12 years. Hence the coalescence was limited to stuff in the immediate vicinity of the handful of small eddies that did exist, and none of them grew to any appreciable size – Ceres being of course the largest, and that small by comparison with any planet and many moons.
The total mass contained in the asteroid belt is pretty insignificant compared to the mass of any of the major planets–probably on the order of 1/1000th the mass of the Earth.
The Kuiper Belt and Oort Cloud each likely have many times the mass of the asteroid belt. In fact, Sedna, 2004DW, and Quaoar (the three largest Kuiper Belt Objects found thus far) are likely by themselves more massive than the asteroid belt.
There’s also some evidence that there might be a large dark body out in the Oort Cloud with a mass about three times that of Jupiter, though it’s debatable. It’s sometimes referred to as “Sol b” even though it’s not massive enough to really qualify as a companion star or brown dwarf.
However, it’s also been estimated that the asteroid belt may have once contained two to ten times the mass of the Earth, the gravitational influence of Jupiter and the solar wind having flung or blown much of the mass out into interstellar space (or into the Kuiper Belt or Oort Cloud, if you think about it).
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You would need an astrophysicist specializing in solar system gravitational dynamics to explain this–
At your service. The dynamical evidence shows that Jupiter stirred up the asteroid belt. The velocities were high enough that when two asteroids collided, they had a larger chance of shattering each other than sticking together. This prevented the growth of a larger planet. This, in addition to the effects that Poly mentioned, kept the planetesimals of the asteroid belt from forming a planet.
There is also some compositional evidence to bring to bear. The majority of asteroids are C-Type asteroids. The meteorites that come from these asteorids, carbonanceous chondrites, are clearly a mishmash of rock particles and tiny flecks of metal. These look very different from any rock you would come from a planet, because planets are big enough to melt, and once you melt the rock, the metal sinks to the center of the planet and rock floats to the top. This process is called differentiation. When you break up a larger, differentiated body, you get rocky S-Type asteroids and metallic M-Type asteroids are in the minority.
Where did the asteroids come from? Well, let’s just say you do not mess around with the Martian Old Ones, even if you do got a two piece custom-made pool cue.
How would a planet ‘break apart’? Some external force would have to act on it. A huge collision, for example. But such a collision would send the fragments into elliptical orbits. The material in the asteroid belt is fairly uniformly dispersed in a relatively circular orbit.
The actual answer is no one knows for certain. The first step was probably some kind of self-replicating polymer, possibly an RNA or peptide. This page from the talk.origins archive has more on the topic.
The dynamical evidence shows that Jupiter stirred up the asteroid belt. The velocities were high enough that when two asteroids collided, they had a larger chance of shattering each other than sticking together. This prevented the growth of a larger planet. This, in addition to the effects that Poly mentioned, kept the planetesimals of the asteroid belt from forming a planet.