You're sayihg that not only do aliens exist, but that the second law of thermodynamics is wrong? Or the universe is non local, and these aliens have developed ftl teleportation?

In order to get here, aliens have to have traveled here, and while traveling, and alive, they'll give off heat -- glowing brightly if they have to turn or accelerate to avoid crashing really hard into the earth.

An alien arriving before they traveled breaks a lot of things, and makes them much closer to magic than some single cell equivalents on a distant moon