AFAIK there is word of god that the planet in Prometheus and the one in Alien are not the same (different Numbers, Atmosphere, etc.). I'm not as sure about this but I think I read somewhere that there is also word of god that they are both in the same solar system or even they are both moons of the same planet.
Indeed, Ridley Scott has implied (if we interpret his words a little) that LV-223 (the moon visited in Prometheus) and LV-426/Acheron (the moon visited in Alien and Aliens) orbit the same planet, Calpamos, in the Zeta 2 Reticuli system. He actually says they both orbit the planet Zeta 2 Reticuli, which doesn't make sense, but let's assume he misspoke. At the very least, they're in the same star system.
However, there are some discrepanies too: The color of the planets the moons orbit are different in the films (perhaps different planets in the same system), and there are some issues with their distances from Earth. Here's a good write-up on what fits and what doesn't. But let's assume Ridley either meant what he said, or that – again – they're in the same star system but not orbiting the same planet.
And why is neither in Alien I nor in Alien II mentioned that there is an earth like planet with breathable atmosphere next by?
Well, the obvious question is: Because Prometheus hadn't been made yet. If we try to find an in-universe reason, however, perhaps Weyland-Yutani kept that information from the colonizers? The following will be conjecture, to try to fit the retconned existence and position of LV-223 into the canon.
If this is the case, why do the humans in Alien II try to terraform and colonize the Alien planet if the nearby Prometheus planet has a breathable atmosphere available?
Presumably Weyland-Yutani knew about the Engineer outpost on LV-223 – Peter Weyland went there, after all. Maybe they set up Hadley's Hope (the colony) on LV-426 to have an outpost close to LV-223, but didn't want to set it up on LV-223 itself because they were apprehensive of what was there?
Weyland-Yutani were not aware of the derelict ship on LV-426 until they were told of its existence in Aliens (this can at least be inferred by the fact that they immediately erected a base around the ship in Aliens: Colonial Marines when they learned of it after the fall of Hadley's Hope), so settling on a nearby moon would probably suffice for a while. So presumably the derelict was not the reason for the colonization.
On the other hand, extras on the Prometheus Blu-Ray reveal that Peter Weyland and David did know about the derelict's distress signal, but chose to ignore it. (I can't confirm that information as I don't own the disc, but I found several other references to it.) If so, he probably didn't share it with anyone but David, and the future Weyland-Yutani company didn't learn of this information.
