Can planets form from asteroids?

04 October 2011

Question

I've got a space question for you – so as far as I know, our solar system has got a pretty big asteroid belt. Is it possible to bring some of those asteroids close together enough that their combined gravitational pull would start gathering stellar asteroids and eventually form a planet in the place of the asteroid belt?

Answer

Dominic - This is one of those really good questions where I have to say we don't completely understand the answer. The way that we think planets formed in our solar system and elsewhere is by solid particles of grit-like material which came together and formed into small asteroids, and then those asteroids gravitationally attract one another and come together to form large masses that turned into planets. Now for some reason, we have this gap in the middle of our solar system between Mars and Jupiter, and it's a big gap in which there are no planets, but there are lots of these small asteroids. So, given that no planet has formed in the last 5 billion years since the solar system formed, we can probably say there is some good reason why a planet has not formed there, and probably the reason is that those asteroids are too close to Jupiter, and Jupiter is stirring up the orbit of those asteroids and stopping them from gravitationally attracting to one another and coming close enough together to form together into a planet in that gap between Mars and Jupiter.Chris - There was a terrific paper in Science about two years ago by David Minton who was I think in Arizona and he did a mathematical model, another computer model of the Kirkwood Gaps in the asteroid belt. There are all these holes in the asteroid belt which can and can't be accounted for and they worked out where Jupiter had come from and where it had gone to, because it didn't always end up in the same position it's in now. It's migrated a bit during its evolution of the solar system. And this has created, when they put it into their model, what they dubbed 'gravitational resonances'. So it lined up with all the other planets in the right sort of place, it shook the asteroid belt a bit and dislodged some of these other bodies from the asteroid belt, kicking them onto earthbound orbits actually and probably accounted for the late heavy bombardment that rained in on the planet 3.9 billion years ago. But their point was that the gravitational tug there is so unpredictable and so intense that it actually stopped all these things accumulating, and accreting into a planet, or even if they did at one time, they fell apart again.Dominic - Yes, that's absolutely right. The Kirkwood Gaps are really the forensic evidence that Jupiter is the problem why no planet has formed there, because the Kirkwood gaps are lined up with places in the asteroid belt where you would be orbiting twice as fast as Jupiter or perhaps 1.5 times as fast as Jupiter. And for some reason, there were no asteroids there and that suggests that there is some reason why you can't be in orbit at that point.

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