[Sirius Lee]

The population of humans left behind will become extinct, sooner or later, leaving the emigrants as the survivor population/s.

But perhaps it's much more difficult to adapt to living on even the most Earthlike words we find - so much so that on each new planet we have to take some of the native lifeforms and start incorporating their genetics into the humans who will become the first generations that can survive on the surface. The rest of us have to terraform underground bases that maintain air that is more Earthlike, grow Earth foods, and shield ourselves from the constant alien microbial onslaught, all the while perfecting and adapting our species to life on that particular planet. If there's a moon, perhaps set up underground shop there, first.

Because of the constant threat of meteor strikes, gamma ray bursts, or just plain nasty animals with big shreddy teeth, advanced civilizations may prefer building underground and terraforming "up".

[WhiskeyX]

You’re talking about exoplanets located in other star systems. We’re concentrating at the moment on the nearer future in our own Solar System. Mars is an extremely difficult environment to master, and its utility maybe more trouble than it is worth in comparison to the asteroids. Mars’ gravity and magnetic field are too small to maintain a terraformed atmosphere of sufficient pressure. Aerospace vehicles entering the atmosphere have trouble decelerating, yet the weak gravity is sufficient to make lifting of cargoes prohibitively expensive as it is from the Earth. The atmosphere is also not very effective at shielding against meteorite and asteroidal strikes, high energy particle radiation, or high energy electromagnetic radiation. Basically, a society needs exceptional reasons to accept the problems of establishing a community on mars and maintain it there until it suffers the same kinds of fates awaiting the Earth and its inhabitants.

[GeronL]

To terraform Mars (by the way, that would involve making most of the atmosphere methane to replace all that CO2 since I don’t know how to get Nitrogen there) would involve crashing ammonia asteroids into the atmosphere, inflatable solar mirrors, spreading dark dirt over the polar caps to trap heat etc etc... hopefully this would warm it up a bit (its as cold as Antarctica - in its summer) and thicken the atmosphere (although how do we keep that from escaping??

eventually humans might be able to wander around with just a light oxygen mask because the air pressure would still be too thin

Sure if we warmed it up and thickened the atmospheric pressure and replace most of the CO2 with oxygen and methane (think of that smell) we might eventually live on Mars

BUT... we would also need to put a bunch of satellites into orbit that do nothing but emit an electromagnetic field, to help guard against solar radiation and stuff.

Terra-forming is HARD!!

[chimera]

Good point about the microbial problem. NASA developed an elaborate protocol to deal with just that when the first lunar explorers returned (isolation garments, the LRL at JSC, etc.). The lunar environment turned out to be a self-sterilizer, but any kind of life-supporting world we discover will have its own alien (to us, when in fact we'll be the aliens) bacteria, viruses, and microbes that our systems have never had to defend against. No doubt they will, to some extent, but will it be enough? Remember what happened to the Martians in War of the Worlds. It could very well happen to us if we go somewhere that has germs and viruses we can't cope with.