5 ms·

This is not terribly surprising because it turns out that nearly every dynamical system with more than three bodies is only marginally stable on a timescale of

This is not terribly surprising because it turns out that nearly every dynamical system with more than three bodies is only marginally stable on a timescale of order its own lifetime. Does this mean that it's highly likely that our solar system had more planets a few billion years ago?

lisper 4y ago
It's 100% certain, modulo your definition of "planet". Our solar system began as a cloud of dust, which over time accreted into larger and larger objects. During that accretion process, many objects large enough to be called planets accreted by colliding with each other. That's how earth's moon was formed, and it's very likely the cause of Uranus's extreme axial tilt and Venus's retrograde rotation.
- cperciva 4y ago
  Ok, I'm thinking of the "cleared its orbital neighborhood" definition. So not the initial accretion process but after it reached a more modern-looking system of planets separated by mostly empty space.
  - lisper 4y ago
    Well, like I said, it all turns on how you define "planet". Comet Shoemaker-Levy impacted Jupiter in 1994, so before that there was one more body orbiting the sun than after. But I'm guessing that doesn't count because it was a "comet" and not a "planet". Meteors hit the earth and moon constantly, but again, not "planets". Orbiting bodies are eliminated regularly, either by impacts or by being ejected, the former happening much more often than the latter because the ejection window is smaller than the impact window. And the bigger the body, the less likely it is to happen because to be eliminated it has to hit or do a flyby of something bigger, and the bigger you get the fewer things there are that will do the job. Jupiter, for example, ain't goin' nowhere. At some point in the past we had a lot more Mercury-to-Mars-sized things floating around than we do now. How long ago the last one got eliminated is anyone's guess, but it was almost certainly a few billion years.
    - AnimalMuppet 4y ago
      > At some point in the past we had a lot more Mercury-to-Mars-sized things floating around than we do now. How long ago the last one got eliminated is anyone's guess, but it was almost certainly a few billion years. The last one hasn't been eliminated - we still have Mercury and Mars. You probably don't think of it that way because we think of the existing planets as constants, not variables.
      - lisper 4y ago
        I meant: no one knows how long the last one that was eliminated was eliminated. But that sounded awkward (and I thought it would be obvious).
AprilArcus 4y ago
You might be interested in the "Five Planet Nice Model": https://en.wikipedia.org/wiki/Five-planet_Nice_model https://en.wikipedia.org/wiki/Five-planet_Nice_model
FreeFull 4y ago
The most popular theory about the Moon's formation is that Earth collided with a Mars-sized planet, so if that's true there definitely used to be at least one more planet than what we have now.

lisper 4y ago

It's 100% certain, modulo your definition of "planet". Our solar system began as a cloud of dust, which over time accreted into larger and larger objects. During that accretion process, many objects large enough to be called planets accreted by colliding with each other. That's how earth's moon was formed, and it's very likely the cause of Uranus's extreme axial tilt and Venus's retrograde rotation.

cperciva 4y ago
Ok, I'm thinking of the "cleared its orbital neighborhood" definition. So not the initial accretion process but after it reached a more modern-looking system of planets separated by mostly empty space.
- lisper 4y ago
  Well, like I said, it all turns on how you define "planet". Comet Shoemaker-Levy impacted Jupiter in 1994, so before that there was one more body orbiting the sun than after. But I'm guessing that doesn't count because it was a "comet" and not a "planet". Meteors hit the earth and moon constantly, but again, not "planets". Orbiting bodies are eliminated regularly, either by impacts or by being ejected, the former happening much more often than the latter because the ejection window is smaller than the impact window. And the bigger the body, the less likely it is to happen because to be eliminated it has to hit or do a flyby of something bigger, and the bigger you get the fewer things there are that will do the job. Jupiter, for example, ain't goin' nowhere. At some point in the past we had a lot more Mercury-to-Mars-sized things floating around than we do now. How long ago the last one got eliminated is anyone's guess, but it was almost certainly a few billion years.
  - AnimalMuppet 4y ago
    > At some point in the past we had a lot more Mercury-to-Mars-sized things floating around than we do now. How long ago the last one got eliminated is anyone's guess, but it was almost certainly a few billion years. The last one hasn't been eliminated - we still have Mercury and Mars. You probably don't think of it that way because we think of the existing planets as constants, not variables.
    - lisper 4y ago
      I meant: no one knows how long the last one that was eliminated was eliminated. But that sounded awkward (and I thought it would be obvious).

AprilArcus 4y ago

You might be interested in the "Five Planet Nice Model": https://en.wikipedia.org/wiki/Five-planet_Nice_model https://en.wikipedia.org/wiki/Five-planet_Nice_model

FreeFull 4y ago

The most popular theory about the Moon's formation is that Earth collided with a Mars-sized planet, so if that's true there definitely used to be at least one more planet than what we have now.