Scientific evidence vs. existence of extra-terrestrial life

Hello guys!

I have another question which I’ve been wondering about for a while now: to what extent is scientific evidence necessary to reaffirm our suspicion that extra-terrestrial life may potentially exist? If some arguments could be presented for why it is absolutely necessary, and some others as to why it is not necessary/irrelevant, I would be very happy! Although I’m sure none of you will, it would be great if someone could include some real-life examples they’ve had with (what they would perceive to be) extra-terrestrial life, or if they’ve read something very interesting about it in the news recently.

I know it’s a bit of a mouthful, but any thoughtful comments which critically evaluates the certainty of the claims would be greatly appreciated! :smiley:


You’ve got the Drake Equation, and that’s pretty much it. We don’t know. We don’t know how many “Goldielocks” planets there are, nor how many of them have the right kind of chemistry, nor how many of those eventually do originate life.

We can make estimates, based on assumptions. But our actual collection of data involves one, count 'em one, planet, where life originated a long time ago and has stayed active for billions of years. This suggests that life has a lot of staying power.

As for real-life examples, read a UFO web site, but don’t expect anybody here to take it in the least seriously. We’re kinda evidence-fixated here.

Without evidence, it’s pure speculation. We can argue about what the probability is, and there are plenty of good arguments from many different perspectives, but without solid evidence, we just don’t know.

Note that unsubstantiated reports by unreliable witnesses does not constitute “evidence”.

Is it 100% known that life can’t exist on planets that are much hotter, colder or drier than Earth? Is carbon-based life the only form of life that can exist?

We don’t know those things either.

Damned if we know. Life as we know it is carbon based and requires liquid water. So for the purposes of the Drake equation, we just try to estimate the number of earth-like planets.

We’ve got one data point for carbon-based, water-solvated life. We have exactly zero data points for any other conceivable types of life.

Right now we have a sample size of one, the Earth, so any conclusions we can draw are tentative. Life appeared early in Earth’s history, at least 3.6 billion years ago. This suggests that life is fairly common where the conditions are right (caveat, we don’t know a lot about what constitutes the “right conditions”, and how strict the parameters are). However, it then took 1.6 billion years for multi-cellular life to evolve, another 400 million until the first animals appeared, and another 600 million for intelligent life. This suggests that intelligent life is relatively rare.

We know of extremophile organisms that can survives in very hostile environments. Tardigrades, (water bears) are the poster children for this.

I belive in our galaxy alone, we have hundreds if not thousands of planets that can support life. everyday kepler keeps finding new planets, most of them too close and too far from their star, but a few of them are in the goldilock distance. in order to find out if there is life in them we need more powerfull telescopes than can read carbon, methane and other chemicals that living creatures produce. in conclusion most stars have planets. the chances of finding advance life forms like here on earth are remote but not unlikely.

That couldn’t possibly be proved. It’s just the only example we know of, and we have a hard time positing anything else specific that would work (other than computer-based, but that begs the bootstrap issue).

The vastness of scale in the universe is so huge that it isn’t impossible that there is intelligent life on a scale so small that we couldn’t possibly see it (i.e., subatomic). The limit on small size seems to be the planck length, which is a really tiny fraction of the size of the smallest atomic particles that we know of. Is there a lower limit on the smallest time – planck time? If it’s as relatively small as the planck length, then subatomic civilizations in the untold gazillions could come and go in a second, and we’d never be the wiser. (OK, this is definitely science fiction territory, but my point is simply that it’s very hard to say what doesn’t exist in realms of which we know very little.)

Maybe dark matter & energy could generate intelligence somehow. It doesn’t interact with photons, but that doesn’t mean it can’t create complex structures.

So, we have to stick with the familiar. It might be all we could detect, even if the universe were teeming with civilizations that thrive in places where we just can’t look. Since we can’t look, it’s silly to speculate, other than for a grin or two.

That’s two questions, really. There is no good evidence that anyone has ever had any contact with extraterrestrial life.
But your first question is well stated. “May potentially exist” is just the right phrasing. The Drake equation, as mentioned, has several parameters. We don’t know most of them, but one is the number of planets of any type in the Galaxy, and the results from the planetary search effort is coming up with a number far higher than any I remember seeing back say 20 or 30 years ago. I personally would wager that given our better understanding of DNA and genetics the probability that some sort of life would arise on a Goldilocks planet is also higher than thought. But we’re still in the dark about the number of habitable planets, the probability that intelligent life would arise on one, the probability that this life would create a technological civilization, and the expected life span of that civilization.

But even good numbers on all of these ,does not mean that this likely life is going to be dropping in to see us any time soon if ever.

You’ll also be interested in the Fermi paradox. To put it simply, the galaxy is so vast and old, if the Earth isn’t terribly unusual and if intelligent life isn’t terribly improbable, then we should see evidence of intelligent life from elswhere in the galaxy. But, we don’t.

(The wikipedia article also mentions the universe, but it’s not at all clear that reliable intergalactic travel or communication is feasible, IMHO.)

I believe the distances needed to travel from one planet to another outside of solar systems is too great to ever make the trip. We may never know. No spaceship could ever carry near enough fuel to accelerate it anywhere near the speed of light. So my guess is we are just too far apart to ever really know one way or the other for sure.

We currently have evidence of the existence of a large number of extra-solar planets–so many that we can conclude that planets are pretty common.

It is likely that over the next few years/decades we will find indirect evidence for extra-terrestrial life. We could find incontrovertible evidence of life on Mars. There is currently some evidence suggesting life on Mars, btw, but it is far from strong enough to conclude that there is/was life there; however, it is possible that the Mars Science Laboratory or a future mission will find strong enough evidence.

We could also find extra-solar planets and be able to deduce information about their atmosphere. If we see evidence of, for instance, free oxygen, that would be strong evidence of life–oxygen is so reactive that we would not expect to find much of it in the atmosphere of a planet in the absence of life.

It is possible that we would find scientific evidence of extra-terrestrial visitors to our world–but the likelihood of that is EXTREMELY SMALL.

Actually, as far as evidence for extra-terrestrial life, the closest thing we have to solid evidence is the Alan Hills martian meteorite sample evaluations done by Johnson Space Center. They produced controversial findings of microscopic structures and chemical changes in the meteorite that are proposed to be derived from life. There are people in the JSC lab on both sides of that argument, so it’s far from conclusive.

As far as your question,

It’s the burden of proof and the standard for belief. We have lots of deductions and suppostions drawn from the one known source of life, Earth, and one definition of life we established based upon that one source. Everything else is guesswork.

But the phrasing of your question is vague enough, it sounds like you are asking “how much scientific evidence do we need to think life may exist on other planets besides Earth, anywhere in the universe?” In which case, I would say - zero. We need zero scientific evidence to think life may exist somewhere else. However, if we want to actually confirm that existence, or justify that belief in existence beyond pure whimsy, then we’re going to need something like confirmed habitats similar to Earth, confirmed chemicals in the atmosphere that resemble life, etc.

It’s certainly not 100% known… I have talked to chemists and biologists who think it is highly likely, because carbon is pretty much the only known element that makes such a wide variety of molecular bonds, just as water is the only known solvent that carries so many kinds of molecules in solution.

Isaac Asimov wrote several essays on this and similar subjects. He sought to envision other chemicals than water than might be the basis for seas and oceans. (He coined the term “Thalassogen” for such a chemical – “A former of seas.”) He couldn’t come up with anything that works as well as water.

But never rule out the unexpected! This universe still has lots or surprises waiting for us!

It’s not at all clear that this is so, given that we don’t understand much of anything about the process by which life appeared. It’s again the problem of drawing conclusions from a sample size of one.

Titan has some regions of surface liquid that could be regarded as seas, but the liquid is ethane, not water. Life on Titan would need to be radically different to life on Earth, however.

It could be argued that we’ve also got about a fraction of a percent of a data point for possible electronic life. No evidence for how such life could evolve, other than “first evolve carbon-based life. Then evolve it to the point where it builds robots”, but still, it is, at least, conceivable.

As I said, any conclusions we can draw are tentative. However, in terms of probability, it’s more likely that the Earth is relativity typical rather than an edge case. The Earth is not a single experiment, but many trillions carried out simultaneously under a range of conditions, which is an important distinction.

The anthropic principle torpedoes the above, but doesn’t sink it completely. We can only ask these questions because consciousness and intelligence evolved here. Combining the two suggests the following:

  • Life appeared very early in Earth’s history. This suggests that abiogenesis is a relatively common event when planetary conditions are favourable, unless the evolution of intelligence is such a rare event that only planets where abiogenesis occurs early in their have a realistic chance of developing it.
  • Development of complex life and ultimately intelligence is a low probability event (remember, trillions of concurrent experiments). It’s unlikely the average time it takes intelligence to develop is many times less than it took on Earth. However, due to the anthropic principle, we could be an extreme edge case in terms of evolving it far sooner than the average.

This is slightly more useful than saying “we don’t know”. The evidence (including other factors, such as the prevalence of organic chemisty, existence of extemophile organisms and discovery of life that doesn’t depend on the sun for energy) weakly supports a hypothesis of common life and rare intelligence. It could well be that intelligence is an evolutionary accident. There doesn’t appear to be any particular reason why earlier species such as dinosaurs didn’t evolve it, but our understanding is incomplete.

The exciting thing to me is that there is a chance we could make some significant progress on these questions within my lifetime. While we should never expect to be able to create life in the lab, experiment and supercomputer simulation may be able to give us a better understanding of pre-life chemistry and the probability of abiogenesis occurring. Exploration of Mars and various moons with sub-surface oceans such as Europa and Enceladus might find evidence of life. Detection techniques for extrasolar planets may improve to the point where we can start to categorise planetary systems, and gain some understanding of how typical our own is. This could all help us put a little more meat on the Drake Equation.