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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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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