The solar system has been stable for some time…but is it true what I read/heard that it is inherently unstable…in that a planet could be flung out of orbit at anytime?
In the short term, the solar system is stable. No planets are going to get flung out in the lifetime of anybody who’s here now, in fact not in the next billion years.
Longer-term stability is a different, and still open, question. When astronomers talk about the debate over the stability of the solar system, this is what they are talking about.
I have heard this and it confuses me.
Say it was a billion years ago and you said this…wouldn’t NOW be the time for a planet to be flung? I mean, it’s not like you can tell instability before it happens, or can you? If you can’t tell, then couldn’t it happen any time?
I’m not exactly an expert (or even very close to it) on this subject, but gravity can’t just “stop”. I’d guess a planet’s orbit can gradually widen and that the further it gets from the Sun the faster its orbit would widen. But it’s not like the Sun is going to suddenly “let go” of a planet and sling it through space.
That’s not the issue.
It’s the (tiny) interactions between the orbiting planets. Given enough time, the gravitational attraction between planets can result in very small perturbations in their orbits. These may lead to larger and larger perturbations, until one planet is, indeed, flung out of the system.
I am not anything close to an astronomer or physicist, but I think the key here is to define “unstable.” To say the solar system is unstable is *not *to say that it’s behavior is utterly unpredictable and a planet could go flying off on a tangent without notice. Each planet has an elliptical orbit but those orbits are not fixed and exactly repeated, revolution after revolution. All the planets (as well as other interplanetary visitors like comets) affect each other’s orbits. These interactions are complex. I do not know how complex a mathematical model would be to predict them but it may be that we can predict ahead by years or centuries or millenia but not by billions of years.
The orbits of the planets can be predicted through numerical simulations, given their positions and velocities now, and their masses. There are limits to the accuracy with which those positions, velocities, and masses are known. When you run the simulation, those position and velocity errors grow a little bit with time. Eventually, those errors become so large that you really can’t say exactly where the planets are, and having a planet flung out of the solar system, or having two planets collide, becomes a possibility.
It wouldn’t be something that sneaks up on you. If it was going to happen that a planet was going to be flung from the solar system in 1.1 billion years, you wouldn’t know now. If that was going to happen in 900 million years, your simulations might give you a 10 percent chance of something happening, but still not a certainty.
If it was going to happen in 500 million years, the chance would be bigger. Possibly what you’d be seeing in that case is two planets getting really close together, and you’d know something would happen, but wouldn’t have enough accuracy to know exactly what. As the time of this event drew closer, your simulations would become more and more accurate.
ETA: Something similar happened on a much shorter time scale with the asteroid Apophis.
Or you’d see one planet’s orbital eccentricity rapidly increasing. In either case, you’d see something that we aren’t seeing now if a planet were likely to get flung out of the solar system or into the Sun any time soon.
We don’t know what you would see if you calculated the long-term behavior of the solar system from 1.1 billion years ago, because we don’t know exactly what positions and velocities the planets had 1.1 billion years ago. Nobody was around to record that data, as far as we know. The best we could do to get that data is run our simulations backwards from today’s data, and we don’t really have a way of knowing if the results of such a calculation are correct. (My guess is that we wouldn’t have much confidence at all that results obtained that way would be correct).
The solar system isn’t so unstable that any planet could suddenly fly out of its orbit at any minute.
Build your own solar system here and see: http://phet.colorado.edu/sims/my-solar-system/my-solar-system_en.html
Great, there goes my afternoon!
Some models of the early history of the solar system suggest that Neptune used to be closer to the Sun than Uranus, and that another giant planet may have been ejected. The data is too sparse to be definitive, though.