Position of the asteroid belt

  • #1
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Main Question or Discussion Point

The asteroid belt is present between mars and Jupiter but why is it spaced out like that? Why not between some other planets?
 

Answers and Replies

  • #2
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Because Jupiter and the sun are the two largest objects in the solar system. In these kinds of orbital dynamics, there are only so many gravitationally stable locations, called Lagrangian points. You can't simply float around the sun when you have a big bully like Jupiter pushing you around, you have to stay in a sweet spot.

https://www.exploremars.org/trojan-asteroids-around-jupiter-explained

There were asteroids in other locations, but over time the big planets tossed them away through gravity.
 
  • #3
Bandersnatch
Science Advisor
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@newjerseyrunner the question asks specifically about the asteroid belt, i.e. the largest concentration of asteroids this side of the Kuiper belt, not about Jovian trojans.

The asteroid belt is present between mars and Jupiter but why is it spaced out like that? Why not between some other planets?
From what I gathered (I'm not following this area closely), the best, if still hypothetical, current understanding of the formation of the solar system has a period of planetary migration, where the giants migrate both outwards (Saturn, Uranus, and Neptune), and inwards (Jupiter). As those large masses slowly change their orbits, they act like gravitational sweepers, clearing their neighbourhood of the remains from planetary formation.
The migration itself is driven by planetesimals initially present outside the orbit of giants being passed from giant to giant, inwards, until finally being ejected from the solar system by the last and largest mass (Jupiter). This means that the migration stops when the supply of planetesimals gets depleted - in other words, once most of the debris from beyond the orbits of giants is cleared out. What remains of those planetesimals is the Kuiper belt.
So any debris below the final orbit of Jupiter, and sufficiently above inner planets, would have been saved by the sweeper finally stopping. It's the last large safe spot for asteroids inside the orbits of planets.

With inner planets, I think they're already too close to each other to allow for a gravitationally-stable belt between them.
But as @newjerseyrunner 's link shows, there are some other, gravitationally favourable places where smaller amounts of the ancient debris material have survived.
 
  • #4
russ_watters
Mentor
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Perhaps the opposite question: why isn't the asteroid belt a planet? Because it's too close to Jupiter to collect into a planet. Jupiter basically stirs it up.
 
  • #5
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Perhaps the opposite question: why isn't the asteroid belt a planet? Because it's too close to Jupiter to collect into a planet. Jupiter basically stirs it up.
Both are very good questions.

It has been suggested that Jupiter's migrated inward from where it was formed (3+ AU) to approximately 1.5 AU, and then back out again to its current position (5.2 AU) during the early formation of our solar system. This would not only have kept the asteroids in the asteroid belt from forming into a planet, it also kept Mars from becoming bigger. Jupiter would have passed through the asteroid belt twice. Jupiter's migration not only effected the inner solar system. Both Neptune and Uranus were pushed further out, and it is also suggested that this migration had an effect on the distribution of the Kuiper Belt.

Sources:
Jupiter’s Decisive Role in the Inner Solar System’s Early Evolution - Proceedings of the National Academy of Sciences, Volume 112, Number 14, February 2015 (free preprint)
Outward Migration of Jupiter & Saturn in 3:2 or 2:1 Resonance in Radiative Disks: Implications for the Grand Tack & NICE Models - The Astrophysical Journal Letters, Volume 795, Number 1, October 2014
Stochastic Effects in the Planet Migration and Orbital Distribution of the Kuiper Belt - Monthly Notices of the Royal Astronomical Society, Volume 336, Issue 2, October 2002
On the Migration of a System of Protoplanets - Monthly Notices of the Royal Astronomical Society, Volume 313, Issue 4, April 2000
Jupiter's 'Smashing' Migration May Explain Our Oddball Solar System - Space.com, March 2015
 
  • #6
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But why did it migrate inwards?
 
  • #7
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But why did it migrate inwards?
It has been suggested that in the very early formation of the solar system planetesimals exchanged angular momentum with the surrounding protoplanetary disk and their orbits began to be reduced. Such a theory would also explain the "hot" Jupiter exoplanets we have been finding. They apparently lacked an adjacent Saturn to pull them back out again.
 
  • #8
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It has been suggested that in the very early formation of the solar system planetesimals exchanged angular momentum with the surrounding protoplanetary disk and their orbits began to be reduced. Such a theory would also explain the "hot" Jupiter exoplanets we have been finding. They apparently lacked an adjacent Saturn to pull them back out again.
To conserve angular momentum?
 

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