Re: Panspermia, anyone?
High. Heavier elements are more massive. If pre-solar nebulae form in the first place (and we can actually see them forming in lots of places, notably the Pleides), then heavier elements are less likely to escape the gravitational pull.
Um...so?
Leading us to the conclusion that space has lots of rocks in it. As it happens we've looked at space, and it turns out that space has lots of rocks in it.
Ah but that's not the issue. The issue is whether an even more improbable event is preferable as an explanation to one which merely seems improbable. Panspermia can't be ruled out at this time but there's no real support for it either. We haven't found earthlike-life in space, and there are reason to think that space would be very inhospitable to it so until we do, there's no particular reason to think that Earth-like life didn't originate on Earth.