Due to their position in the Gliese system, hypothetical creatures on planet Zarmina might not be alone in their own solar system, the way humans seem to be. And those neighbors could reach Zarmina pretty quickly.
Planet Zarmina, or Gliese 581 g, may be situated happily in the middle of the habitable zone around red dwarf star Gliese 581, but a nearby planet might not have it so good. Gliese 581 d is a big planet, one orbit over from Zarmina. Gliese 581 d was discovered first, so it has an earlier letter, but it is farther from the sun than g is. Gliese 581 d skirts the outer edge of the habitable zone, but its great mass, around six times the mass of earth, may help it have a livable atmosphere.
D and g are separated by less than nine million miles. It's 35 million miles to Mars from earth. In other words, there's a big, cold planet hovering right behind Zarmina, and they might be wanting a nice warm place to stay. Could Zarmina be under attack? Invaded? Or could they, with the more habitable planet with a technological advantage, be the invaders?
Via National Geographic, Wikipedia times two, Universe Today and NASA.