I think the proper term is circumstellar habitable zone. If there is other life out there, why are we assuming that it’ll have the same basic needs as our animals? The universe is seemingly infinite, and there’s endless possibilities of what’s out there, so why do we only consider planets that are the ‘perfect’ distance away from their star?
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It’s really just a matter of looking for what we know. There’s a high probability that life exists outside our habitable spectrum, but it makes sense from a resource perspective (ie, money) to first look for life in areas we know for certain can produce it.
While it is theoretically possible that there are other forms of life that don’t require water, we have no idea how to even begin looking for those sorts of theoretical alien life forms. We do know how to find signs of water, and that water is key to life on Earth. That gives us a place to start looking.
I think it’s based on the fact that we are carbon life. So all the planets we see that we deem “habitable” is solely based off of that idea. Carbon based life nurturing habitats. I’m sure scientist in that field probably dream of other elemental based life forms. It’s also safe to assume, that since were carbon based, there’s gotta be more similar to us as well.
It’s a bit like looking for a doctor in a hospital. Any person could be a doctor; people wearing “normal” clothes could be a doctor coming on/off their shift. But if you’re looking for a doctor, you’re going to run towards the person who’s dressed in a white or blue coat (maybe with a stethoscope on their neck) because that’s what you **know** doctors look like.
Likewise, any planet could harbor life. But we **know** that life can exist under the “Goldilocks” conditions, so we know that if we’re looking there, we at least have a chance of finding it. We don’t have the resources to look everywhere all the time, so we have to prioritize where we have the best chances.
Chemistry operates on the rules of physics which are universal regardless of the location. Life requires many complex chemical reactions to take place in order to exist. While it is possible, it is incredibly unlikely that life could develop somewhere without liquid water to slosh around and slam molecules together until they happen to make proteins and then slam those together until they make amino acids and so forth.
We have such incredibly finite resources to use, it would be a waste using them to look at places that can’t happen. The odds of life developing without liquid water is just infinitesimal.
We know for a fact that life can exist on planets in the Goldilocks Zone.
Source: You and I are alive.
We do not know if life can exist out side that zone.
We haven’t found any yet.
So it makes sense to look at the places we know for sure life can exists first, then look in places we aren’t sure about.
Because the only planet that we know of that has life is in the Goldilocks Zone around its parent star. That planet is of course the Earth. Without other examples we look for what we know.
However, the concept of the Goldilocks Zone around a star is considered a little lazy and possibly controversial even in the sciences that use it. The reason for this is that the Goldilocks Zone is entirely based on the planet Earth. A planet with a thicker atmosphere could support liquid water further from its parent star, for instance. In fact, we know of/suspect that bodies in the outer solar system have liquid water on their surface even though they are far outside of the Goldilocks Zone.
This all being said though, life is not going to develop on a world without some sort of liquid on its surface. This is because life requires chemistry to take place and chemistry requires some sort of mixing to happen. The best thing for this mixing to take place in is water as water is the universal solvent meaning that more chemicals dissolve in water than in any other liquid. So looking for at least liquid water is the way to not waste our time until we find life that developed in another liquid.
We have concrete evidence of exactly one form of life – the carbon and water based life found here on Earth. This is the only form of life we can point to as proven possible. Every other form is theoretical.
And by theoretical, we mean “might be possible, but we don’t have the means to prove it”. Biological systems are so incredibly complex that we barely have the means to fully model carbon based life, and we are absolutely surrounded by it. The prospect of modeling a realistic, alternative form of life is beyond daunting. We’re nowhere close to being able definitively prove if other forms of life are actually possible. In fact, the best proof of possibility might even be a real life specimen – that is, we might only know if other life is possible if we see it in the real world.
Because of this, we’re looking at a lot of planets which we are entirely uncertain of their ability to harbor life, and a select few planets which we know *can* harbor life, but might not. Taking a deeper look will be very expensive, so we’re playing the probability game. Goldilocks planets may have, say, a 4% chance of harboring life. While other planets might have anywhere from 0% to 2% chance. With the resources we have, we’re going to bet on the more likely option.
We aren’t assuming life will absolutely, definitely have the same properties as life on Earth, it’s just that we know what Earth-like life does, so we have a much better idea of what to look for. With a countless number of planets to search, it only makes sense to start with the ones where we have the highest chance of noticing any life that does exist.
That being said, chemistry works the same across the universe, so we can make some pretty good generalisations about what life needs, particularly in terms of temperature and radiation. Too much radiation anywhere makes any molecule that forms too unstable to be able to form life, because life, whatever molecules it uses, requires a degree of stability.
As for temperature – life is really just a bunch of chemical reactions happening in order. Too cold, and those reactions happen too slowly to be coordinated. If it could count as life at all, we couldn’t detect it because it would act over such long timespans that it didn’t look like life. Even colder and the solvent – the liquid that these molecules move in, which for Earth life is water – freezes solid. If molecules can’t move, they can’t react, so a cold planet can’t sustain life. Too hot and there’s so much energy that the complex molecules necessary for life break down into smaller things and just whizz about accomplishing nothing. This means there’s a minimum and maximum temperature that life can exist between, which is broader than the goldilocks zone we currently consider, but still exists.
We’re playing the odds. Life is essentially a complex symphony of chemistry, the easier and more common the processes life is based on, the more likely it is to come into existence.
Essentially life needs three things:
* A basic building block. The more complex the organism, the more numerous the complex molecules it needs. The ideal building block is very common and capable of forming many stable molecules.
* A means of generating energy. Without energy nothing happens.
* A medium of exchange. All that chemistry can’t happen if elements can’t come into contact with each other.
Earthlife uses carbon, oxidation and water for that. But that’s not because there’s something special about Earth. Those three solutions would be quite logical anywhere else in the universe.
Carbon is a fantastic building block. It’s the fourth most common element in the universe and it can enter into four covalent bonds. That means it can form strong and stable bonds with up to four other atoms or create double or triple bonds. Carbon is the basis for an incredible number of stable complex molecules.
Oxidation is a very simple reaction that produces a lot of energy. Oxygen is the third most common element in the universe.
And water is a fantastic medium of exchange for chemical reactions. It’s liquid at very reasonable temperatures where many other elements are solid or gaseous. It uses two of the most common elements in the universe. And an amazing number of elements happily dissolve in water.
Now there are alternatives, but none of them is quite as ideal. Silicon can enter into four covalent bonds just like carbon. But silicon loves to bond with oxygen over anything else. So much that silicon will break other molecules apart to bond to the oxygen instead. And when it does, it turns to silicate which is essentially rock.
Turning to rock in the presence of oxygen is not ideal for life. For one thing, it means oxidation is no longer a usable reaction nor is using water as a medium of exchange.
There’s alternatives for oxidation as well but they don’t generate as much energy so that’ll limit the complexity of life.
And that’s a key component as well, complexity. The overwhelming majority of all life on Earth is single-celled. Simple creatures with simple requirements. Finding alternatives for carbon-based, oxidation based, and water-based life will likely severely limit what’s possible.
Goldilocks planets are in the right place to have liquid water and a surface temperature that makes much of the chemistry for life possible. If you have to start the search for life anywhere in the universe, there are worse places than planets where all the conditions are right for the most common elements that make life possible to do their job.
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