It wasn't the planet. It also wasn't the moon that was explored in the first Alien. That moon was LV-426 in Alien, while in Prometheus it was LV-223, however in the scene where they show the moon LV-223 on the hologram, LV-426 can also be seen. So by that logic, its safe to assume that where Prometheus took place was on a moon or some rock caught in the planet's gravitational pull.