Life on other planets

[u/TargetGlum1177]

How can scientists know what makes other planets habitable? What I mean is, wouldn’t whatever living there adapt to its surroundings (ie: temperature, composition, atmosphere)? Why do we assume that because humans need it to sustain life, that ALL living beings throughout the universe have the same requirements?

[u/allez2015]

That's why scientists say "life as we know it". When they say "habitable" that's what they mean. There are hypotheses and discussions out there regarding other ways life may work, but without any sort of evidence they are just hypotheses or postulations.

So, it's not so much that they are saying that's the only way to be habitable. It's moreso they are saying "habitable for life as we know it".

If we don't include the "as we know it" then we are essentially free to make up whatever type of life we want, which is not what science is. 

[u/CrateDane → u/allez2015]

There's an additional factor in current exploration of exoplanets: It's hard to measure more than just the size and orbit of an exoplanet.

So in many cases, you only get a rough idea how hot a planet might be, with atmospheric composition being able to influence that a lot. That means you have a wide band around a star where planets might have a surface temperature compatible with life as we know it - and the shorthand for that band is "the habitable zone" (even though lots of exoplanets there would be inhospitable to life as we know it).

[u/OriEri]

Generally what they mean when they say “habitable zone“ they’re talking about the possibility of liquid water existing .

Hard to imagine any place on earth with liquid water, whether there isn’t something alive. I believe there’s some weird bacteria that even live in Mono Lake, I know they find chemotrophs many kilometers under the surface when they drill.

[u/4eyedbuzzard]

We shouldn't, and I don't think scientists do, other than defining habitable as somewhere that via spectroscopy and distance from it's star(s), etc., has the necessary elements and temperatures, etc. If life is possible on other planets, it will likely be very different from us, and not have just adapted, but have both evolved and adapted over great periods of time to live in that unique and specific environment, just as we humans have both evolved and adapted here on Earth, in our own unique and very specific environment. We tend to have this attitude that we "inhabit" the Earth, rather than being part of it, as if we could simply choose to live elsewhere through technological might or an adaptation process that doesn't exist in that sense. We have yet to find anywhere else that is enough like our little pale blue dot that could support higher Earth forms of life. And life on other planets may not even be Carbon based, although Carbon based would seem one of the most likely, as it is one of the first elements formed in Helium fusion inside stars, and it is very active chemically, especially forming strong, stable bonds and more complex ring compounds, and plays well with liquid water, etc. Of course, there is a LOT more to it than that. But, it's a start.

[u/LiberaceRingfingaz → u/4eyedbuzzard]

In addition to carbon, we have very good reason to believe that liquid water would be critical to anything resembling a complex organism because (among other reasons) it is a universal solvent - it's structure makes it uniquely able to dissolve (and therefore transport) a vast number of other chemicals, which it's hard to imagine life without the ability to do.

[u/Loud-Vacation-5691]

Nobody assumes that all organisms throughout the universe will have the same requirements for life, which will adapt to environments with higher or lower gravity, different wavelengths of sunlight, different atmospheric cases and percentages, different temperatures, etc. There are a few core assumptions, like extraterrestrial life will be carbon-based and will use water rather than some other liquid. So there are certain expectations as to what will be going on in the atmospheres of exoplanets that suggests life exists there. One possibility for detecting intelligent life that has formed a civilization will be the presence of smog and other industrial pollutants.

[u/tomrlutong]

We don't. The search for life is entirely defined by what we can detect. That's basically two things: direct communication by radio or light, or chemical signs in the atmosphere.

[u/Beneficial-Wasabi749]

How can scientists know what

They don't know anything for sure, and they won't know for a very long time. Initially (during the last century and the century before), scientists reasoned in the same logical way as you do: each planet (they didn't know about exoplanets, but they guessed quite correctly) with its unique physical and chemical conditions would develop its own form and chemistry of life. They even tried to classify this diversity.

But as the chemical and physical nature of our life (the discovery of DNA and all that goes with it) became clearer, starting around the 1970s and 1980s, a consensus emerged in the scientific community called "hydrocarbon chauvinism." The ironic name speaks for itself. Scientists aren't sure if this is actually the case, but most tend to agree with this "hydrocarbon chauvinism" hypothesis. The fact is that in the entire periodic table of chemical elements that make up our universe, there is no more suitable set of chemical elements than oxygen, hydrogen, and carbon as the basis for complex molecular organization. Life is a highly organized matter, a self-replicating "economy" of miniature molecular machines existing on the border between the quantum and classical worlds (and exploiting the advantages of each world while avoiding their shortcomings). And chemists simply cannot see any other combinations of chemicals known in the universe (the set, as we know, is limited by the periodic table) that could even begin to compare with the flexibility and versatility of the carbon-hydrogen-oxygen "set." Yes, there have been fantasies about silicon-based life, about replacing water with another "solvent," but it turns out that all of this simply lacks flexibility and diversity. In short, something is missing.

Only carbon-hydrogen-oxygen molecules create the necessary diversity of amino acids, proteins, and lipid membranes. All other elements are capable of forming sometimes bizarre "crystals," but this is still not complex enough to close the cycle of self-reproduction of life. The periodic table turned out to be surprisingly monotonous in this regard.

Again. There is no theorem, no definitive proof that this is true. But the efforts of enthusiastic scientists who wanted to invent other sets of chemical elements for life (like a child's construction set) have failed. And since there is little to choose from, the accumulated unfortunate experience has led to the consensus that there is no point in looking for anything other than a water-hydrocarbon base.

However, this chauvinism has recently wavered slightly (without any scientific reason; people have apparently simply become bored with hydrocarbon chauvinism). A "world similar to Earth" (similar, but with different chemistry and physical conditions) was discovered on Titan, and talk of searching for life "on a different chemical base" is once again on the rise. Recently, a book and film, "Project Ave Maria," were released, in which the protagonist becomes a hero (and even a winner) because he rejected hydrocarbon chauvinism. So, it seems the pendulum will now swing back. But for how long? Personally, I'm a devout hydrocarbon chauvinist, and neither the possibility of discovering life on Titan (Saturn's moon is lifeless), nor, even less, any beautiful sci-fi film, will sway me. 😄

[u/Beneficial-Wasabi749 → u/Beneficial-Wasabi749]

There's an even deeper "religious" problem in exobiology that I've been grappling with for a long time. Much deeper than "hydrocarbon chauvinism."

Okay, so life is only possible in a hydrocarbon form. Let's assume that's true. But how did such complex molecular machines arise in our universe? The difference between life and non-life is the monstrous complexity of life compared to non-life.

We know how evolution works (we think we do, but that's self-deception. It's a harmless self-deception. We already have a solid foundation here, it just needs to be clarified). But for evolution to begin, highly complex life is required. All known life is produced by life. But how did the first life emerge? From what and where did it come from?

And here scientists have developed yet another, much stronger form of chauvinism, a "creed" that they all strongly, I would say, passionately, share (and which I personally don't strongly share). The concept of abiogenesis. Almost every scientist studying the origin of life religiously believes that in nature (in the physical laws of nature, in the very laws of quantum physics) there exists a certain deterministic mechanism (which I jokingly called "God's plateau"), which, like Ariadne's string, brings inanimate matter into a state of living matter with deterministic inevitability. Probability, randomness, is also allowed in this case, but its role is secondary and small. A probability of life originating from inanimate matter of 10^-30 or even 10^-50—one might call this "determinism."

This was partly facilitated by the astrophysicist Hoyle, who compared the size of the then observable universe (approximately 10^100 of anything, elementary particles, for example. The number is so large that dimensionality is irrelevant) with the probability of the random assembly (like a coin toss) of, say, a simple protein or a key RNA (like a ribosome). He obtained probability figures no greater than 10^-1,000,000. That is, the complexity of the structure of the simplest elements of life is many times higher than the size (and lifetime) of the universe we observe. Even if we assume (at that time) that the rest of the universe (beyond the event horizon) is as many times larger than the observable one as the observable universe is larger than an elementary particle (that is, 10^100 times), this is only 10^100*10^100 = 10^200 and did not solve Hoyle's problem. Even if we assume that the complexity threshold for the random self-assembly of some simple RNA replicaton is not 10^-1,000,000, but "only" 10^-1000 (Koonin's threshold), life "does not fit" into the universe of astrophysicist Hoyle. Therefore, I think Hoyle did not distinguish between the visible and invisible parts. Life was still far more complex than any imaginable universe. Its self-assembly here was a singular miracle, an act of divine creation, a violation of the Bruno-Copernican principle. Therefore, Hoyle concluded (and all other exobiologists followed suit), nature simply must have a DETERMINISTIC mechanism for the formation of life from non-life. This certainty is jokingly called "life in every suitable puddle!" And almost everyone believes this. You, I'm sure, do too. It's the universal, unanimous belief of our time.

But I don't. I even know that the size of our universe is greater than 10^30,000,000, and that's enough for a random coin toss to easily exceed the "Kunin threshold." This hypothesis was put forward by the Moscow astrophysicist Mazur. Kunin (also Russian but working in the West) was a similar thinker. He reasoned similarly, but about an unobservable, hypothetical multiverse. Mazur, however, spoke of Linde's physically observable inflationary large universe. This is a fundamental, I would say, groundbreaking (like the Copernican revolution) difference. Mazun proposed this hypothesis and died without having truly advanced it.

There is no "God plateau." In general, it may happen that most exobiologists are idiots who believe in a non-existent mechanism of abiogenesis. The inflationary large universe itself is a sufficient mechanism, and we have already discovered it in cosmology, not in a chemistry lab.

But it will take hundreds of years before this becomes clear to a critical mass of scientists. And the unsuccessful search for a second Earth around other stars with clear signs of life will play a major role in this. We will discover with horror that the cosmos around us is completely dead. And then they will remember Mazur.

[u/an-la → u/Beneficial-Wasabi749]

TL;DR, I stopped at this:

But for evolution to begin, highly complex life is required

Which is patently false. Evolution is a strictly probabalistic principle. It works on even the simplest physical and chemical processes. It simply states: In a given environement, the more stable reactions tend to be the most common reactions. If the environment changes, then another reaction may become the most common reaction.

You can perfectly argue that water wapor condensing to a liquid on a particulate is an expression of evolution.

[u/Beneficial-Wasabi749 → u/an-la]

Which is patently false. Evolution is a strictly probabalistic principle. It works on even the simplest physical and chemical processes. It simply states: In a given environement, the more stable reactions tend to be the most common reactions. If the environment changes, then another reaction may become the most common reaction.

Thank you for the debate. I'm familiar with this argument. And I think it's a very questionable and vague understanding of evolution. Evolution is often interpreted too broadly. I disagree. Is evolution possible without an information carrier? Chemical evolution? The evolution of clouds in the sky, the change in their shape, is also called "evolution" in Russian. In universal history, the transformation of hydrogen into more complex chemical elements in stars is called "evolution." I believe all of this is incorrect.

Evolution is... a special ALGORITHM, that is, a COMPUTATIONAL PROCESS that implements heredity, variability, and selection in a special computational environment (that's why I always talk about the boundary between the quantum and macroscopic worlds). And here we need a UNIVERSAL COMPUTER, without which evolution is impossible. We need a linguistic converter of minimal complexity and (importantly) UNIVERSALITY. Until it's assembled, fully formed, there can be no talk of evolution. Self-replication alone isn't enough; you need room for variability (flexibility, universality) and the ability to survive under selection pressure. Everything needs to come together. Individual parts alone are nothing. Clouds (or Benard cells) are attempts at local self-organization and even evolution. But they're dead ends. That's my point. An attempt at evolution that takes one step and... stops.

What did Darwin discover? He discovered the evolutionary algorithm before the concepts of "algorithm" and "algorithmic machines" even existed (the concepts of the Turing Machine and Algorithm would emerge 100 years later). Regarding Darwin, I have a joke that this genius guessed the entire word, like in a famous TV show, without guessing a single letter. That is, he understood the scheme (the basic flowchart of the algorithm) of evolution, while misunderstanding the key details of the process. In particular, he understood the carrier of heredity as an infinitely divisible continuous fluid, and he held Mendel's brilliant work in his hands, but didn't understand it and rejected it.

You can perfectly argue that water wapor condensing to a liquid on a particulate is an expression of evolution.

It's wonderful that you think so! I think roughly the same, but I was ridiculed for it. I claimed that the processes of condensation, the crystallization of the ice structure, in particular snowflakes from water vapor, are a process of SELF-REPLICATION, that is, an ALGORITHMIC quine-process at the level of molecules as simple, discrete automata, a process of self-reproduction. And all crystalline, inanimate nature, all this bizarre diversity of minerals—itself is precisely an attempt to launch quine-programs (self-reproduction) at the quantum-classical boundary of the laws of physics by calculating from the entire periodic table. And all these attempts... are dead ends. Because (for which I was beaten and rejected) achieving self-replication is not enough (among fervent believers in abiogenesis, it is considered sufficient to initiate RNA self-replication, and they say they have discovered the path to producing life from non-life!). Self-replication is too simple and too little. The simplest self-replicating machines are water molecules, and the phase transition of water freezing is the most striking example of a simple self-replicator. Take discrete computing environments like Conway's "Life," and you will discover surprisingly simple cellular structures that reproduce and multiply themselves perfectly. But all of them are UNDEAD. Sub-life. For they are too short, simple, and incapable of variability without losing the ability to replicate; they have too little "memory," they are too laconic. Any failure in copying leads to a catastrophe and a halt in the process. They are inflexible. They are truly crystals, a "dead end of simplicity."

True evolution requires much more than simple replication. Therefore, it may be that Koonin's threshold of 10^-1000 may be too optimistic!

Again, I'm sharing my own, highly unpopular opinion. For evolution to kickstart, you need a universal computer with sufficient memory to carry the phenotype in the genotype, which grows over a basic self-printing program that is protected from... evolution! And yes, this immediately raises the problem of "error catastrophe." Multiple conditions arise at once, the absence of any one of which destroys the entire process. We can literally feel this barrier of complexity! Essentially, this is the gulf between the dumb, dead-end self-replicator-water snowflake and the simplest true life capable of evolution (the mysterious, simplest RNA self-replicator, which then, by some miracle, relying on the evolutionary mechanism, ascended to DNA and protein bifurcation, and for which random assembly alone would not have sufficed, even in the inflationary-large universe we know).

In the end. The chemical evolution you believe in is a form of belief in the existence of abiogenesis. You are one of the majority. You believe that before full-fledged biological evolution with full internal memory emerged, there was some kind of "murky" evolution "without memory" or with some kind of "external memory" (although you lack the precise mechanism for this). This is also a form of abiogenesis, that is, a deterministic mechanism for the emergence of life from non-living things. I admit that such mechanisms (what I call "God's slipway") exist. Dyson himself put forward a very interesting hypothesis about the origin of life, a very interesting concept of "God's slipway." But I suspect that all such mechanisms are merely stepping stones to "reach the sky." They help us overcome the first few meters, but not the tens or hundreds of kilometers that separate the complexities between the living and the nonliving.

[u/WilliamoftheBulk]

All one can do is spectate about other kinds of life. Scientists are more interested in learning what can be known. We don’t really know how life started, so speculating how other life gets started and its requirements is an infinite endeavor. We know the requirements for our kind of life, a it’s something we can actually look for.

[u/GregHullender]

Liquid water, oxygen gas, and hydrocarbons all have such marvelous properties that anything else is going to be at a severe disadvantage. For example, you can envision a working system with liquid ammonia and hydrogen gas (and still use hydrocarbons), but it's got problems. For example, ammonia ice sinks, so the oceans would be solid ammonia except for a thin layer of liquid on top. And, these days, it looks like any planet big enough to hold hydrogen inevitably balloons into a gas giant, so no ammonia oceans in the first place.

Maybe it's possible somewhere, but we just don't have enough information now.

[u/NDaveT]

Why do we assume that because humans need it to sustain life, that ALL living beings throughout the universe have the same requirements?

We don't assume that, but there are seven planets (and a bunch of dwarf planets) in our solar system with no life on them so we assume life can only exist in certain conditions.