re: How long until we have a self sustaining population on another planet?

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Mars would be easy to populate.

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How would it be self sustaining? It’s incredibly hard just to get a person there, and it’s not just a matter of distance. Once there, Mars offers nothing except the fact it has a solid surface and the temperatures aren’t as crazy as the other rocky planets in the solar system.

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While I agree with the majority of your post, this portion is not necessarily correct.

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Saying why would be great.

If a self-sustaining population of thinking machines counts, it would be much more likely. It sure as hell won't be humans evolved to live here.

I found the article:

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Hollywood movies convey the fun of weightlessness. Interviews with Kelly and Kornienko from the space station hint at the other side. Their faces are puffy, because fluid doesn’t drain out of them. Their arms are folded across their chests, lest they extend straight ahead in the dreaded “zombie pose.” Astronauts can get used to strapping themselves onto a suction toilet and even, Kornienko says, to a whole year of wiping off with a wet washcloth, for lack of a shower. On a much longer, much more hazardous Mars journey, in which Earth is not 250 miles but millions of miles away, with no option to turn back or bail out, what space can do to a human body could be a huge problem. “They’re going to be sick when they get there,” says Jennifer Fogarty, deputy chief scientist for the Human Research Program at NASA’s Johnson Space Center, in Houston.

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Bones waste away in zero gravity: The rule of thumb is you lose one percent of your bone mass per month. Vigorous exercise helps, but the jumbo equipment used on the space station weighs too much for a Mars mission. Some astronauts on the station have also experienced serious vision impairment, apparently because fluid collects in the brain and presses on their eyeballs. The nightmare scenario is that astronauts land on Mars with blurred vision and brittle bones and immediately break a leg. Theoretically the risk could be reduced by spinning the spacecraft rapidly, replacing gravity with centrifugal force. But NASA engineers see that as adding too much complexity to an already challenging mission.

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The long term effects of gravity are a problem. Radiation is another problem.

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Right. I focused only on gravity because it alone is enough to keep us bound to this planet, no pun intended. As the meatbags that evolution has made us, we cannot survive anywhere else.

Well, let me modify that statement a bit. There may be worlds which have gravity approximating ours but we won't be able to get to them. There is no such thing as artificial gravity to hold us over such a long journey.

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You seem to be pretty well versed on this subject, so can you explain why you don’t think the human body would hold up on a planet with less gravity?

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It would depend upon the amount of gravity felt on the exoplanet. The closer it is to earth's gravity, the less the effects on the body. As I mentioned in another post above, a planet with smaller gravity than earth's, such as Mars', would cause a disruption of our bodily functions.

Blood circulation and digestion are a couple of examples of functions that are closely tied to our gravity. There are others, of course. Our hearts are synced with our gravity. They push our blood through our circulatory system with the strength needed to counter earth's gravity. On Mars, our hearts would over-perform and potentially cause bleeding in the lungs, brain and other organs.

Our digestive systems are highly dependent on the assist they get from gravity. The stomach produces a caustic acid to break down the food we eat. If not for our specific gravity, a belch could cause an upwelling of that acid into our throats and mouths. Burping could become dangerous.

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I understand muscle atrophy, but wouldn’t the human body reach an equilibrium such that the body is strong enough for the gravitational forces it’s experiencing?

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Well, still using the Mars example, our bodies are already strong enough for lower gravities. We have to remember, however, that muscles are throughout our bodies, not just our arm, leg and facial muscles.

Our heart is a muscle. So are our intestines and bladder. Even our lungs are powered by muscles that expand and then shrink them like a bellows. Most importantly, our brain's and body's lymphatic system are operated when we use our muscle system. Any atrophying by those muscles will spell great hazards, even death, for us.