Re: Have any of the planets ever gone out of orbit?

Hello there!

As far as we know, all of the planets that we currently see in
the solar system have always been there, in more or less the same
orbits that we see them in today. But we know, from studying 
gravity and the way planets interact in a solar system (we do this
by simulating solar systems on a computer), that it is possible
for planets to be "ejected" from a solar system. They get flung
away from the Sun by the effects of the gravity of the other
planets and never come back. Right now, the predictions say
that the planet Mercury will eventually be ejected, in a billion
years or so.

So it is possible that some planets (probably one or two at most) have
already been ejected from the solar system. There's not really any
way to know, since planets are very hard to see once they're far from the Sun,
and any ejected planets would be far away by now.

Pauline