FAQ

It takes place in the year 2142. 20 years after Alien, le 8ème passager (1979) and 37 years before Aliens, le retour (1986).

This new weapon seems to be somewhere between the M41A Pulse Rifle and the "Smartgun" both seen in Aliens.

It's possible, due to it being 100 years in the future, that the rounds used in the gun are "expandable" meaning they are smaller projectiles when loaded in the magazine, but once fired. They expand on to a full sized bullet. Thus keeping the weight lower and capacity higher.

It's also possible that it's a newer model rifle than the M41A. As the Romulus station is a secret scientific station. They were privy to more advanced technology. The M41A was the standard issue rifle for the marine core in 2179. But that doesn't mean they weren't already in service by the time of this film. Afterall, the M4 Carbine is the current standard issue rifle for the U.S. military and has been in service for over 30 years. The M4 itself is essentially a shortened and less bulky version of the M16, which was in service since the 1960's.

There are several possible reasons:

1) throughout history, people have disregarded optimal living conditions for the opportunity to make a living. Think of coal Miners in the old west among others. Colonists in the alien universe are sent to distant worlds to make them habitable for humans. Very few of these worlds would have identical conditions to earth.

2) Weyland-Yutani may have understated the conditions of the world to potential workers. By the time they get there, they can't exactly turn around and leave once they see the conditions of the world.

3) There are several places on earth where the sun isn't seen for months at a time and people live there. Some people just aren't concerned with such things.

4) The movie implies that Weyland-Yutani has, in a sense, enslaved the population on the planet. As we see, Rain had completed the required hours to get off the planet, only for the Weyland-Yutani representative to tell her that the company had extended the required amount. So people simply aren't allowed to leave the planet. Rain, Tyler, Bjorn, Kay and Navarro are all the children of colonists. All of their parents died before their required time was fulfilled. The debt transferred to the children. Which is why they are motivated to escape.