Idk if to put biology or planetary science so ye.
In: Biology
It doesn’t need to.
However, conditions like earth is the only example we have that we are 100% certain can support life.
Until we find another non earth like environment that supports life, they are considered less likely to harbour any life, as far as we know.
When you lose your car keys, do you travel to the Antarctic to look in the Antarctic permafrost and scour every inch of the planet to find them? Or do you look in the places you were most likely to lose your keys?
Searching for life is like that. We are aware that life could’ve come about in many different ways. But our only example is Earth life. Space is really big, so we are prioritising looking for hallmarks of Earth, because that’s a scenario we’re 100% sure could possibly make life. We know that because we are that life. We could try scouring every inch of space, but that would take so long that we could miss Earth-like carbon based life in the time we were searching for silicon-based life under the 248 millionth rock on the moon.
Additionally, we don’t really know what non Earth-like carbon based life might look like. When you’re looking for your car keys, you have an idea of what that looks like. Roughly car key shaped. Knowing what a thing looks like and where it likely could be makes finding it far easier.
In other news, I’ve lost my keys.
We think of space like “anything can exist out there”, but chemistry is the same everywhere, and the elements available to life on Earth are pretty normal for what’s available all over the place. Living things have to do a series of chemical reactions to get energy out of their “food”, and we can understand the rules that govern these to have a good idea of what’s possible in different conditions. The Earth already has a huge variety of single-celled things that use every possible chemical pathway to extract energy and live.
Water is an example of this. Stuff dissolves really easily in it, it makes tons of chemical reactions happen much more easily, and it’s plentiful and liquid at a temperature that’s reasonable for other molecules to be stable. All known life needs water for these basic reasons, and something else would have to also do what it does.
When we do find other liquids on other planets/moons, we can work out how on paper how life that uses that instead of water might work, and what the evidence for that we might be able to see. There’s plenty of hypothetical, exotic life chemistries that people have thought up, but it’s most likely that if life is out there, it’s doing something that something in Earth is doing.
Right so, mix of things, and there’s enough to write quite a lt more to answer these questions, but in the interest of ELI5 I’m going to summarize very hard.
* We don’t know exact requirements for alien life, and while yes we only have ourselves as examples, we have good reason to think some aspects we have on earth are a hard requirement. Like how there are many ways to build a box but usually having edges and walls aren’t optional no matter the shape it takes.
* Water is a common and abundant solvent. See, one of those core things potentially 100% necessary to life are chemical reactions (a lot of them), which usually take place best when molecules can freely diffuse around each other in close proximity–such as when they’re dissolved or in solution. There’s other solvents out there that are theoretically possible for some more unusual forms of life, such as liquid ammonia and the like, but water is our safest bet, so usually that’s what people get excited about.
* Reactions also depend a good bit on temperature. When temperature is low, things tend to turn solid, AND reactions tend to not have the energy to make transitions between states. On the other side, if temperatures are too high, these states may not be stable since there’s so much available energy that crossing energetic boundaries is pretty easy. Complex molecules like those that form the building blocks of life here on earth tend to do best somewhere in the middle. So yeah: if said planet is so far from the sun that’s it’s frozen solid and gets very little energy input, you’re unlikely to find anything.
They don’t have to. But doing things in real life isn’t a computer game. You don’t have a button that says “tell me everything about this”. Every question asked has to be specific “is there oxygen?”, “is there CO2”, “is there carbon?” etc all come with an associated cost and time. Each detector might use a different telescope. Each telescope and the associated equipment costs millions of dollars. Every question asked needs someone, ultimately, to analyze the data collected.
Given finite resources, finite time and finite knowledge, and not living in a computer game, real life research requires a narrowing of search criteria and to focus on “most likely candidates”. People can have differences in what they consider “most likely” but they too need to get approval to use the resource needed to search using their own ideas.
If someone said “life is most likely pink unicorns made out of cake and pooping out diamonds”, they’d probably not get anyone to agree to use valuable search resources based on this criteria.
Life is really difficult. We have reason to believe that
a) most examples of life in the universe will have achieved it in something close to the easiest way possible, though there may be some rare examples, 1 in a billion say, who did it a harder way, e.g. methane not water, silicon not carbon.
b) life on Earth is a typical example, that took the shortest path to life.
There are many shapes that complex life can take, aliens will likely not appear “humanoid”, but basic chemistry does not have a lot of easy alternatives to what we see.
It’s the last bit.
We currently only know of one planet that hosts life. Ours.
Space is REALLY REALLY big, things are really really far away. So it’s difficult to investigate any particular thing in great detail.
So if we want to look for life, the first place we look is where we’re pretty sure we have a good shot find some, since we already know it’s possible.
Can life exist in different conditions? Who knows. Not us, that’s who. If we ever find out, brilliant.
It can, life could theoretically be based on other substances than “earth” life. But as far as we currently know, the only place life has worked is earth. So we sort of assume other life will be similar in composition, that might be wrong, but with lack of any other examples we have to go with what we know.
If you can imagine a chemistry that works with physics, AND has the materials needed available, then propose it. The fact is, carbon is one of the more widely available compounds with high valence so is an obvious framework.
Think of it like this: space is extremely hostile to biological lifeforms that have DNA:
– cosmic rays that pierce through DNA and cause mutations and decay (necrosis)
– solar radiation that could literally cook any DNA that’s exposed to sufficient quantities, causing cancer and other maladies
– exposure to vacuum prevents any respiratory functions
– nucleonic radiation from, say, a nearby gas giant or super nova, again causing necrosis
– plasma winds from nearby super nova that could strip the atmosphere and the top layer of soil across the entire planet
– extreme cold or heat that prevents any metabolic functions
The list goes on and on. Basically, the only reason we have a biosphere is because the Earth protects us from all those effects:
– our magnetosphere deflects cosmic rays
– our atmosphere diffuses the most harmful blue and UV radiation from our sun
– we have a breathable oxygenated atmosphere, mostly on account of our magnetosphere protecting it from being stripped by the sun
– our magnetosphere also protects us from nucleonic radiation
– there are no nearby super nova
– we are the right distance from the sun to not receive too much radiation, but enough to sustain photosynthesis
There’s a reason why every other planet in our solar system is sterile: they don’t meet any of those criteria. A bit of observation bias due to detection techniques, but nearly all the exoplanets we’ve detected are also likely sterile because either they’re orbiting a red dwarf closer than our moon orbits the Earth, or because they orbit some blue giant that would cook them at any distance.
So what we’re left with is a very narrow range of unlikely conditions:
– a strong magnetosphere
– high water content, but not enough to ensconce every landmass
– high metalicity, rich in things like calcium, phosphorous, and iron
– Goldilocks zone from a stable main sequence star
Find one of those, and you just might find aliens.
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