I think at that point it becomes an issue of short or long term satisfactions. Whether we use that planet for its resources and kill the sentient life, or allow that life to prosper and grow (just like a previous alien race could have done to us). I think we are going to have to make some tough choices (I mean, we need habitable planets to colonize). There should be a system of classifying a planet as likely for sentient life to exist, and if not (but is still habitable) then that planet can be used for colonization.
I agree with that. If we have choice we should never destroy extraterrestrial life to make room for our own. We most likely will be faced with those decisions at some point in our future, though. Many things could happen to threaten the Earth, but the ultimate and unavoidable one will be when our sun dies. When that happens the Earth will be reduced to a bit of ember circling inside of it. If we find only one habitable planet in that time and it happens to be inhabited by other sentient life confrontation would be hard to avoid.
I do not remember completely, but I remember hearing in some science documentary that Mars was actually at one point a miniature earth, but the magnetic field was significantly smaller due to the smaller size, and at some point it collapsed and the sun's radiation wiped the planet of life
I believe before we venture on to settle on other planets we should work on a more peaceful world here... I know it's near impossible, but separating the human race like that wouldn't be good w/o world peace... I for one believe there is bacterial life on mars, I've always feared one of those rovers would come back with a sample and it'd be some disease that wiped out everyone(maybe what happened to mars, hmm?). I think the best thing to do is explore the rest of earth(oceans, etc) first, get world peace(or closest possible thing), then the technology...
What we can do, is try to understand that sentience is all a matter of critical mass. Every other animal has neurons and brains, but only we have a connection to a soul. If there are other species that have a similar conection to the soul, EG not space squirels, we should treat them as equals. And chances are, they won't be carbon based.
As far as I'm concerned, all organic molecules are carbon based. So right now, in our definition of ALIVE requires carbon molecules. That thought is sci-fi. Look at how we evolved. We became the way we are for a reason. We evolved because our bodily form is the best way for a species to live because we are the DOMINANT species. If we find another Earthlike planet, chances are, animals on it we be similar to animals here, but will just look foreign.
I agree that the biology will probably be the same. But what if the available molecules were very different than the molecules found in our solar system or even galaxy? Then other life forms could be very different. The conditons would also have a big impact on evolution. What if a distant planet had two stars? That would be really interesting. Back on topic, I do believe that life must have a arose at leat at some point on Mars. If it did billions of years ago, then the evidence could be burried far below the visible surface. I think we will have more info and answers to this once the next probe lands to start digging deeper. I also heard that the moon Europa could have life under the ocean. That would be amazing. We prob won't be able to send a ship to land there for at lest another ten years though ; (
The chemical chance of the most common molecules being drastically different is rare when it boils down to fundamentals such as water, simple hydrocarbons, etc... but what could theoretically be different is the types of proteins and amino acids that exist. The most probable differences could exist on earth already, such as those few species that use an alternative to hemoglobin that contains copper in the place of iron, making oxidized blood blue (blue crabs etc...) Oh and about the distant planet that might have two stars, the way a solar system works, all orbits are ellipses and not perfect circles. This means that there are two foci to evey galaxy. A star is commonly located at each foci, and our solar system is actually a very rare occurence having only one star (note: if you haven't learned the sun is a star, you don't really belong in this thread) Back to mars. There, at some point in distant history, could easily have been life. The probability is huge. As I said earlier, it was a mini earth whose magnetic field collapsed.