Because to go through plane like that they need to match our solar system speed relative to galaxy.

Universe is big and full of random small rocks floating around everywhere.

Why should I believe some object was _intentionally_ thrown here? Maybe it is just one of those random rocks.

> Floating around everywhere.

Sorry to be pedantic. But space is really, really, really... empty. That's why the best name for it is, space.

	▲	alganet 2 days ago \| parent [-]
		You can go outside at night and see big rocks floating. If space is so empty, how is it possible that you can see them with your own eyes? We live in a patch of space that's not that empty. Maybe that interstellar rock floated from other patch of space that's not that empty all the way over here, all on its own. Most rocks we see in our patch of space, as far as we can possibly know, were not intentionally launched.