There are probably a lot of planets that would be more-or-less habitable by humans, but I think it would be rare to find among those planets a planet
that is "very" Earth-like (i.e., one which we can easily inhabit without the aid of technology to allow us to survive).
The term "Earth-like planet", as it is used toady, does not mean a planet that is exactly like the Earth. It simply means a rocky planet that may
have a temperature for water to exist as a liquid. There are many, many other factors that would makes so-called "Earth-like planet" one that most of
us would call "just like Earth"
I think finding a planet in which:
-be able to move, unaided, in the potentially different gravity
-the air pressure is similar to the earth's
-we can breathe the atmosphere unaided
-not be affected over time by potential radiation
-not be affected by the difference in day-night cycles
-not be affected by the native pathogens
-we are able to grow food in soil that may contain compounds harmful to Earth life.
I'm not saying every habitable planet is going to have a combination of these potential problems, But I think it would be quite rare to find one that
we can just inhabit without any of these stumbling blocks.
I think gravity will be one of the biggest problems. A planet that is even a modest 150% of Earth's gravity will make it very difficult for us to
move around unaided and do even the simplest of tasks. The same goes for a planet with , say, 75% of Earth's gravity, but for different reasons.
Sure -- with 75% gravity, it would be easier to move around, but the human body will begin to atrophy after a while. The low gravity would be harmful
to the unaided human body. Something would need to be done to offset these problem. The higher-gravity planet would require the use of a
gravity-assist device, and the lower-gravity planet would require some sort of muscle/bone loss mitigation.
Even something as seeming simple as a difference in the 24-hour day could potentially be a lot for a human to overcome. Some studies suggest that it
is difficult for humans to adjust to a different circadian rhythm -- although those studies are not conclusive. Oddly enough, one study suggested
that without any external stimuli at all, the human body wants top function on a 25-hour rhythm. I'm not saying it would be impossible for humans to
adjust to a different day/night cycle, but, say for example, if we tried to inhabit a planet that had 7 house of daylight and 7 hours of night (14
hour days), it may take a full generation or two to finally become "in-sync" with the planet's day/night cycle.
Even air pressure, which would most likely be different, will affect us greatly.
In the vastness of the universe, I'm sure another "One-G", "one-atmosphere", 24-hour day, oxygen-nitrogen atmosphere planet with a magnetic-field
radiation shield exists somewhere
, but its much, much, more likely we'll find not-so-perfect but still "habitable" planets out there first.
edit on 4/25/2011 by Soylent Green Is People because: (no reason given)