Photosynthesis, not respiration (that uses up oxygen), but yes, any significant amount of elemental oxygen in an atmosphere would be pretty much ironclad evidence of life on that planet. As for “defended”, though, it could mean anything from blue-green algae to supreme alien overlords, and for most of our planet’s life-bearing history, bacteria were the pinnacle of evolutionary complexity, so most life-bearing worlds probably wouldn’t be capable of defending themselves.
Well, 50/50 on top of that that it’s defended well enough if we have the technology to get there 
I had a few more thoughts on physiological problems with higher gravity. First, the heart will have to work a lot harder to pump blood to the head. That may be ok for an explorer with a very strong heart, but it may lead to chronic high blood pressure and all of the problems that leads to. Plus, you know how you get dizzy after laying down for a long time and standing up suddenly? Magnify that effect in increased gravity. And it’s not something that a stronger heart will help with; it’s more an effect of the elasticity of blood vessels. In really high gravity, it could be so much of a problem that even the most fit humans will have to move around like fragile old people.
I remember long ago reading this one book on astronomy where the last couple of pages described a hypothetical gas giant where blimp-shaped and spear shaped lifeforms flourished (the latter type being predatory). They would be extremely unlikely to develop advanced technology, but I wonder if it’s still plausible with current knowledge that such life could exist.
As for temperature, if we allow that water-based life is the only possible kind (and maybe it is), then any temperature at which water is liquid should seem to be capable of having something evolve in it. DNA unravels at a certain temperature but perhaps another self-replicating molecule could emerge with a higher temperature tolerance.
Even today, there are extremophiles that live in the boiling water of hotsprings, in places like Yellowstone (that’s what gives the hotsprings all their pretty colors). And in fact, as I understand it the current consensus is that the first life on Earth was probably a lot more like those extremophiles, thriving at extremely high temperatures, than like the lower-temperature critters.
What requirements a colonist might have aren’t the same as what their grand children or great grand children would require. People would adapt quickly to their new environments. Only the healthiest people would survive long enough to reproduce. Several generation later humans would have difficulty coming back to Earth if the planet was different sufficiently. Low gravity effects on the heart and bones might not be a problem if you constantly live in a low gravity field. Hearts would grow stronger in high gravity conditions. While .9-1.2g might be tolerable for colonists, their offspring might acclimate to .5-2g without too many generations. If a trip to a new colony takes 100s of years, you could slowly change the conditions in the ship from earth like to more mirror the conditions on arrival.
The complex molecules possible with carbon at the liquid-water temperature range are what allow life to happen. Despite Star Trek and others, silicon for example does not give itself so readily to a similar set of processes and arrangements.
It’s possible that there may be analoguous chemical systems and processes in the extreme low temperature, hydrogen - helium - litium - boron world, but I have never heard of any serious scientific literature about it.
There are two kinds of adaptation that would occur, individual adaptations and evolutionary ones. For the individual adaptations, the colonists themselves would see at least some, and their children born on the new planet would be as adapted as anyone’s going to get. But it looks like you’re talking about evolutionary adaptations, where those who are inherently better suited to the planet will have more success in contributing to the next generation. That’ll happen, too, but it’ll take far longer than grandchildren for it to have any noticeable effect.
The indians in the extreme highlands of the Andes are specifically adapte for their condition, according to a few articles I’ve read. Given that they have been there about 14,000 years or less, there’s another clue as to evolution and adaption. The less extreme adaption of Samoans (they have a culture revolving around eating and big) is a result of about what, 1500 years or less of evolution?
Greetings! I am new here.
Given that, I feel I am able to add a bit to this discussion.
Given our current abilities as a culture, it is not difficult to project that within 50 years or so, we will be able to adjust the capabilities of our progeny to adapt to any number of extreme environments. Whether this will be an adaptation that requires them to live in a nonterrestrial environment is one of those kind of scenarios that good SF is built around.
If we do find a habitable planet, given our current and projectable level of tech we will be required to use a generation ship kind of transport to get some of us (or more properly our progeny) to this habitable planet. If we posit a fusion or matter/antimatter drive to get the passengers there at a 1 G acceleration, time dilation effects would begin to become a factor, but even then those passengers would be subjected to pressures beyond those experienced as yet by our species.
This is not to say that I don’t think we need to be off-planet to increase our viability as a species and a global culture named ‘Humanity’, but our explorers may diverge from those who stay at home more than at any other time in our known history.
Vernon Vinge and Gene Wolfe have both dealt with these themes and have some interesting concepts about this that may of some interest to the other worthies here.
Again, I greet you all and hope I can find a place here among you.
Here on Earth? :eek:
Oh, you meant here on the Dope. Welcome Gagundathar.
Maybe so - but I’d suggest that this may be more complicated problem than just working out. You’ve got a lot of guts and stuff that are held in place by structures that evolved to carry a certain load. They won’t necessarily get stronger over time.
Science Fiction author Larry Niven speculated about that. His Known Space stories feature a colony world names Jinx, a very massive rocky moon of a gas giant in the solar system of Sirius “A”. Quoting from his short story “The Ethics of Madness”: