I was thinking the deep freeze might be useful in preserving some potential food sources, assuming it happens quickly enough. But I agree sterilizing the planet provides some advantage as well.

Aliens probably would not be able to eat Earth life.

You are correct, but it's capitalism and the adrenaline kick. We are culturally conditioned folk, and all scripts about alien invasions that end up in movies follow a completely implausible scenario. Most don't even bother any longer to give a reason for invasion. I would blame the writers of those scripts, but movies without violence don't give the adrenaline kick and don't sell well.

Here is an idea for any writers reading this who want to break the violence cycle: whenever you feel you need to have a character kill another in a gratuitously violent way, twist the plot so that they instead end up having steamy sex. The readers will get an intense hormone rush that way too.

American viewers love gory violence, but get offended at steamy sex scenes. That makes no sense to me.

Ahh yes famously American media like Squid Games and Alice in Borderland are so filled with gory violence I can barely handle it. And American movies are famous for having so few steamy sex scenes in them that people are not constantly rolling their eyes at the “sex sells” mantra to explain their frequency.

> a really big rock ... nicely sterlizes the planet

Have you seen Earth? We've been hit with a big rock, we've turned out atmosphere into poison, and had several other mass extinctions, none of which sterilized the Earth.

Unless you mean a Theia-level rock to turn the entire surface into lava, but you risk changing the mass, composition, and satellite system of the planet. At that point it's a different planet entirely, so I don't know why you would expend all that energy. Just find another planet to live on.

It came pretty close, remember life slowly bounced back afterward. Something a little larger would almost do the trick. You just clean up what survives.

No I don't think it did. I'm not sure you're understanding the scale of things here. I don't believe it came close at all, if "close" means that a ground team could have sterilized the planet afterward. Even if 99% of life were destroyed, that doesn't mean there's one little tide pool with some barnacles surviving, that means there's extremely hardy single-celled organisms living in the deepest nooks and crannies all over the planet.

We can't even "just clean up what survives" in the cleanest rooms on Earth, using techniques specifically intended to do that. Life is incredibly tenacious, I really don't think you're going to wipe it off a planet's surface without destroying the surface (and atmosphere, probably) of the planet in the process. And if that's acceptable - again, why wouldn't you just pick a barren planet.

> Honestly it's all kind of silly because distances are so vast. It makes little sense to travel to a distant star - much better to build space habitats in your own star system.

Maybe their own star system is not safe for them. Say a political or religious minority that the majority would like to exterminate.

> And even if you were going to do that for some crazy reason, there's no shortage of uninhabited planets out there. There's no need to commit planetary genocide of an alien race.

If they've got cheap and easy FTL, sure. But suppose they don't have FTL. They are going the slow way, using a generation ship. At launch they may only know that their destination has a habitable planet but not know if it has intelligent life already.

Their generation ship environment might not be reliable enough for them to want to risk trying to move on to another star if it turns out the first habitable plant already has intelligent life.