It's possible to capture a rogue planet even with quite a high speed, it simply requires a closer near-miss with a gas giant already in our system. So less likely, even if still possible.BDanielMayfield wrote:I was looking at this as not just an orbital mechanics problem (which could only be possible if the speed of the rogue was low enough), but also from the planet's composition issue. Such a planet would most likely be quite unusual. Also, if it came from the core of our galaxy it wouldn't be orbiting in our ecliptic plain.Chris Peterson wrote:Well, I think we'd have to accept that it's logically (and even physically) possible. But given how easy it is to demonstrate the statistical rarity of such a capture, it most certainly isn't reasonable.
However, I don't know what to think about composition. If one of our gas giants came from elsewhere, I doubt we could figure that out from composition, unless we could access a direct sample. Terrestrial planets might be easier to distinguish, although they may all be similar enough that it would be hard to tell based on superficial composition. It might take something like looking at isotope ratios in zircons, or some similar method allowing us to identify material older than the Solar System. And so far, we've only had good access to the Earth, and a very limited sampling from the Moon (and Mars if we count meteorites). So realistically, I don't think anything we know about the other planets excludes their having been captured from other systems (with the possible exception of Mars, where we have some reasonable dating that's right in line with when we would have expected it to have been created).