If you want to get nit-picky though, we would have known about it all along due to Newton's laws. We account for pretty much everything in the solar system of any size to a pretty exact precision. That's how come we can gravity-assist our probes and stuff and basically hit a hole in one from 500 million miles. This is also how we know the famed Planet-X theories are bunk.
That's why I included the parenthetical statement.