Why hasn't life evolved on other planets?

Who says it hasn't. Life first has to be recognized as life--we as moderately intelligent monkeys--are only recently starting to recognize the extremeophile life on this planet, much less the extreme potential conditions on other small rocks in tis solar system.

BTW there is much interests in the water Oceans under the ice crust of Io.

Rap

It's a big mystery.
So far,from observation, evidence suggests that life and water are inseparable. This mainly has to do with the most fundamental molecule that we currently consider to be a requirement for something be "alive," and that is DNA. DNA structure can not exist without the influence of water (H-bonds is very important in Earth that allows for life to exist). Furthermore, all observed life forms are so far found to be carbon bases life forms that is dependent on water, as it as also a reaction solvent to such life forms, besides shaping DNA. In this universe, it could be possible that a life form may exist (likely non-carbon based) that uses a different chemical than water as a reaction solvent, but so far we have yet to find such a life form.

What raprap said. Who says life hasn't evolved on other planets? How would they know?

You should look into Circumstellar Habitable Zone or CHZ that exist not just our solar system. It's an interesting hypothesis. This suggest a good potential for life forms to exist out there in the universe besides earth.

@ Enzo: Remember the NASA fluke of non-carbon based life form, specifically arsenic based life form. It would've been very interesting if there really existed such life forms. I wonder if other scientists are testing these microbes in the lab for themselves. This happened in 2010, and I've yet to hear any new leads on this subject.

Such earthly bacteria that incorporate arsenic into their routine, familiar chemistry were found in a geologically young lake so the bacterium wasn't even primeval, and so it doesn't even offer us any insight into cell evolution. This only gives an understanding of something along the lines that the internal works of cells are tolerant to the differences between phosphate and arsenate that it can keep on working to some degree no matter which one is present. An example of an interesting adaptation mechanism, nothing more. The story was quite blown up by the media, as I remember.

Before a biosystem can begin to evolve it needs to start. And while the startup process is probably related to evolution it is not exactly the same thing. Chemical precursors may undergo some form of replication and selection, but it probably begins as more of a chemical process involving some type of naturally occurring bridging mechanism which exists in the environment (clay being one possible example).

And there are other considerations to your question. Life as we know it requires water and a very specific collection of compounds, but we don't yet know the full range of what is possible for "life". SciFi writers have explored just about every extravagant form of life imaginable, but while almost anything is possible, we have no other examples to go by other than our own form of biology.

In general though, I would speculate that the highest probability for life to form would be in "medium" environments which are not too simple and stable and also not too complex and dynamic.

Our moon for example is a very simple and stable environment so there isn't much to work with. While other environments may be too harsh and change too rapidly for any chemical process to take hold.

Planets and some moons (larger than ours so that they can retain an atmosphere and are almost like planets) however have a nice balance of complex chemicals available for accidental associations, and they are dynamic enough to allow for variation and selection to have an effect.

I suspect that there is no life on our moon. But I would put the odds of life on Mars or Europa or Titan or Enceladus at 50% (just my humble opinion/guess). I also wouldn't expect to find anything fancy on any of those places, probably something more like bacteria. But if something does exist it would be incredibly important to learn what it was made of (DNA or something else).

I hope I live long enough to get some answers.

This ought to be a question of why hasn't life evolved on other planets in our solar system. There are billions of satellites of stars in our galaxy alone, and the possibility of life on those planets is undeniable. The probability is another matter. One of the limiting factors of which we know is cosmic radiation. Mars has no radiation belt to protect its surface from cosmic radiation, which may account for why we have found no life there.

There also needs to be liquid water to support anything like the life that evolved on Earth. This rules out most of the planets in our solar system.

You're skipping over an important point here: a change of what in response to the environment? At the very least, evolution needs a replicator to act on. But some planets do not have the geo-chemistry in place that would allow for the emergence of self-replicating molecules.

There are extremophils which inhabit the antarctic wastes without access to liquid water. Whether or not they could evolved without liquid water being available to an ancestor, i couldn't say.

HOW do u know that it has NOT ??





David

By the way, both carbon and silicon have four valence electrons. Some scientists have speculated that it is possible that there may be silicon-based lifeforms reminiscent of carbon-based lifeforms. Would we recognize them?

Whether or not we would recognize lifeforms which are, from our experience, exoctic is an important question. I've read a novel and a novella (long short story) which were dissimilar except that they both posited stars as being sentient life forms.

Is boredom a threat to sentient stars ?

We'll definitely recognize it or see the potential for life forms to exist if we stumbled across it. All it has to do is replicate, evolve and adapt, has an input of some chemical ("food") and output energy, and have traits that are passed along to the progeny through a molecule, analogous to chromosomes in carbon-based life form.
But there will be disagreements in the scientific community whether the presence of a DNA like structured molecule is what differentiates between living and non-living outside our solar system, at least until proper evidence is accrued.

In lab, an interesting thing in a test tube was made called The Immortal Molecule that can and does replicate all by itself under a carefully controlled environment. This RNA molecule can copy itself over, and over, and evolve over time through mutation, and it can pass information from parent to progeny.
If we atleast observe such a similar phenomenon in another planet besides earth, we can at least interpret this prebiotic evolution to mean a very large potential for life to exist.

I consider this an unwarranted statement, for which i do not know you have any authority.

Well, if scientists can't even agree to that, then it becomes a philosophical enigma as to what are the properties that differentiates the living from the non-living.

Marvelous . . . you missed the point altogether. Your statement is based on some vauge premise that anyone leaving the planet will be possessed of an omnibus perceptive ability which will enable them to look around in an alien, exotic enviroment and immediately see "life processes" taking place. You make us sound like supermen.

Well I had hoped my words "stumble across" sufficed to address that context.
But you're right, our observable skills are limited, and it is certainly a large possibility that we may altogether miss to see some living organism while exploring some "alien, exotic environment."

If intelligent life does evolve elsewhere this would not necessarily mean that they would try to contact us or even have any motivation for interplanatary travel. We tend to project our cultural traits onto other science fiction creatures. They are usually assumed to be technologically motivated as we are.