Some waterworlds might be more than 50% water, and should have detectably lower densities than the Earth. But there are probably many more where the water layer is less deep.
There is probably a whole planetary zoo up there, of many different types of world that we have never seen, or even imagined, yet. I suspect, although I can’t be sure, that if we expect to find many Earth-clone worlds in the stellar neighbourhood, we will be disappointed - but if we are looking for planetary diversity our search will be rewarded.
This may follow for many people, but doesn’t apply to those of us who simply argue that you can’t assign a probability, or even make a reasonable ballpark estimate of the order of magnitude of the probability.
I bet most of us, if we had to go on our hunches, would agree with you and guess that life probably exists elsewhere. That’s a far cry from stating that it’s definitely likely that there is life elsewhere. I think this is the only distinction we’re arguing here (amongst other interesting sidebars).
This is very straightforward. Take the most powerful laser we have, with a pinpoint aperture. Aim it carefully at the nearest candidate system, Alpha Centauri A, a yellow dwarf similar to the one nearby and give it a blast. By the time this very coherent beam of light reaches that system, it will be well over a billion miles across, just due to beam spread. And that is not accounting for 4+ lightyears of fluff between here and there, which contribute to beam scatter. Not much to detect, even if you are looking really, really hard.
And that is just a nearby star. Run a program called “Celestia”, line it up on the sun and move away until the sun is no longer visible. I lost sight of it after just 58ly, much closer than I would ever realistically expect to find another inhabited system. That is the most powerful beacon in our system, and it becomes background noise in less than 20pc.
As sophisticated as our search hardware is, the distances over which it is trying to scan are just beyond comprehension. We will simply not find EM evidence of distant civilizations because there is no way such weak signals would carry far enough. There is just too much background noise.
It seems rather difficult to come up with a specific definition of “life” that is sufficiently broad to cover all the bases yet narrow enough to be useful. I would suggest the following constraints:
[ul]
[li]it should be a natural process (but, then, what is not a natural process?)[/li][li]it should have a distinct boundary with respect to its environment[/li][li]it should consume available natural resources, affect their composition, and expel waste (metabolism)[/li][li]it should have a replication mechanism[/li][li]it should have the potential for mortality[/li][/ul]
By this measure, the Red Spot does not constitute life because it lacks a distinct boundary from its environment (it is just a reconfiguration of the stuff around it) and it has no metabolism (anything that enters it also later leaves essentially unaltered).
Yet, this definition, being satisfactory to describe an oak, is not sufficient to exclude stars. They consume available resources (the nebulous media from which they form), they metabolize those resources into something else, they are distinct from their environment, they eventually die, and some of them beget new stars (which is true of life as we know it: not all living thing do, or even can, reproduce).
If we mandate some sort of aggregate structure (like cells), stars appear to be defenestrated, but OTOH, we are not entirely certain what all goes on inside them.
Galaxies are generally thought to have a “goldilocks” region, simplistically portrayed as an annulus that excludes the outer reaches (inappropriate balance of elements) and the bulge (too much radiation and violence). But that relies on life being in a form we can [del]eat[/del] recognize. Perhaps life could involve a process that thrives on or even requires large amounts of ionizing radiation. It would obviously be pretty hard to study or interact with, but those beings would find our form of life similarly difficult to imagine or study.
If life is a wide enough category as to possibly be beyond what we can definitively discover, much less study or have encounters with, does the whole thing become a matter of, “hmm, that’s interesting, let’s look for something more familiar”?
The definition of life has been discussed before. Schrödinger’s What is Life? (32 pages) is worth reading; it makes several important points. I hope someone else will post a good synopsis; I’ll just note here that he concludes that redundant ensembles of complex molecules are the right scale.
IMO, a #1 requirement for “life” is that it create order, i.e. manufacture negative entropy. The 2nd Law of Thermodynamics then implies that waste entropy will be created; that must be carried away by some “conveyer belt” apparatus. (The lowest-level “conveyer” is water in the well known implementation of life). Creating much order, requires consuming much energy. Need for persistence may dictate some form of replication, perhaps supplemented by repair, etc. This leaves five key requirements:
[ul]
[li] Consume energy[/li][li] Create order[/li][li] Dispose of waste[/li][li] Replicate[/li][/ul]
The five most common non-inert elements in the Universe are Hydrogen, Oxygen, Nitrogen, Carbon, Silicon. The top two here form water, a rather magical solvent. The top four are the main constituents in all organic compounds. I’ll ask experts whether the chemical richness of fifth-place Silicon can compete with Carbon’s.
One way to come up with a “living” creature not based on molecules is via (magnetic?) crystals, perhaps obtaining the high energy needed to support high-tempo life via incoming lightning bolts. An even less likely proposal is replicating storm systems, e.g. in ocean or atmosphere. Perhaps a powerful and stable community can be designed, but how would it have evolved?
(Does anyone have a cite for the plausibility of lifeless waterworlds with atmospheric oxygen? Oxygen does react with hydrogen, ammonia and many other molecules.)
Hydrogen will combine readily with O2, so on worlds where gravity is high and Jeans escape is difficult then abiotic O2 will be insignificant. Where the escape velocity is low enough, H2 will escape before the O2 can recombine.
Ammonia is a reasonable option, but doesn’t occur frequently in the inner system. It is very rare compared to water in warmer regions, and on a waterworld will probably occur in solution if at all.
And yet, if it sent us a message saying “Hi, how’s the weather?” would it not be life? (Good post overall, btw .. thanks.)
My definition for life is simpler than a list of characteristics, though I’ll admit it has its weaknesses:
Life is that which evolves itself
Most of the entries in lists of what life requires are probably valid, though I don’t know whether I agree with the boundary part. (I think of a colony of bees as the organism, more than the individual bee. I was delighted to read Goedel Escher & Bach to find that Hofstadter agreed with me, to some extent.) Beside the point, though. In any case there’s definitely a difference between life and intelligence, but I would call any naturally occurring intelligence “life” and expect it would have to have arisen from “that which evolves itself”.
Yeah, I can’t imagine life that doesn’t do those things. Nor can I imagine how ‘storm life’ could have arisen, evolved, or even persist. We’re having a hard enough time coming up with ideas how the life we know so much about arose!
All this aside, I’m still curious about the requirements life imposes on its substrate. What kinds of parts and principles are necessary to create life? Or even an automaton, for that matter. Schrodinger’s list is definitely a subset. I look forward to reading the paper; thanks for posting!
[li]it should be a natural process (but, then, what is not a natural process?)[/li][/QUOTE]
I think that caveat is just to exclude stuff invented by humans, which might meet the criteria, but which we would not call life. It raises the question of at what point could stuff we create take on a “life” of its own, but for current practical purposes I’m OK with it.
I agree with this, but think it’s a little too optimistic. One factor that many don’t give enough weight to is TIME. Human civilization has only been around a very brief time on a cosmic scale. So, there may have been other life/intelligent life, but it may be dust by now; or we may be dust before intelligent life develops elsewhere. Space is big, but so is Time, and it’s totally unclear how long we’re going to be around.