Astrobiology is the interdisciplinary field of science that theorizes where life can be found in the Universe, and how to look for its signs. Life here is defined as anything that has a DNA-equivalent and is able to pass it on in some way.
This definition actually got me thinking... So does sentience or intelligence matter? If DNA can be reframed as a form of information, even a computer virus could be defined as alive in some sense? And does it necessarily mean life must be short-lived, cosmically speaking?
What are some assumptions that we can work with? Since carbon is the only atom that is capable the length and complexity required by DNA to encode biological information (that we know of), and water is the only compound that supports life as a result of its anamolous properties, we might expect that - using ourselves as the only case study - that any other lifeform might need the same chemicals to live as well. No other element has a proven working model.
Among Life's needs are the kind of chemicals available, engaged in geophysical chemical cycles to keep "cycling" these compounds so that life processes have something to work at, towards, or even against. Almost all energy in the universe originates from starlight and perhaps it can be assumed that oxygen plays a major role as well.
I remain unconvinced if these are valid or even valuable starting assumptions, or if they may be misguided. Consider the case of extremophiles, be it bacteria that thrive in the dark ocean floors on thermal vents, or the tardigrades that seem capable of surviving anything. In the same spirit of the Copernican principle, one could argue that if life can survive under such conditions on Earth, then they may survive under similar conditions elsewhere as well. Moreover, there is no requirement to assume that the same atmospheric conditions we humans find favourable must necessarily be the dominant atmospheric conditions on other planets. Consider also the case for shadow biospheres. It would be intellectually lazy to not consider the possibility of organisms and environments that may support life that we simply haven't observed yet.
So to summarise, the minimum requirements for life are:
C, H2O, O2, and other elements
the means to cycle the various elements
temperature stability for organic reactions [a buffer region of max 30º]
Now let's go to the planetary scale. The Circumstellar Habitable Zone (CHZ) is defined as the region around a star that can maintain water in liquid form. So within this region, the ratio of incoming energy from the star and radiated energy by the planet (surface temperature) must be just right to maintain the temperature range required by water to oscillate between its three states.
That's including the buffering effects of the atmosphere. If the planet were too close to the inner boundary (closer to star), there'd be a runaway greenhouse effect. And if it were too far away from the star, there'd be runaway glaciation. If the planet's core were not metallic enough or rotating fast enough, our magnetosphere would not function, bathing us in deadly solar radiation. If the planet's gravity weren't strong enough, thenn our atmosphere would leak away over time, suffocating the planet.
The planetary carbonate-silicate cycle can be roughly summarised into three chemical equations which I won't type out here. But it's a surprisingly "big" process, that takes place over eons.
Carbonated rain reacting with land masses (calcium silicate) gives rise to hydrogen-carbonate ions and SiO2. This process is extremely temperature-dependent and along with photosynthesis, help to remove CO2 from the atmosphere at variable rates.
In the seas, calcium ions react with hydrogen-carbonate releasing carbon dioxide.
Tectonic activity, and subsequent volcanic activity, also release CO2.
If the temperature in this system is increased, process 1 (including photosynthesis) works more efficiently removing more CO2, leaving less greenhouse gases in the atmosphere allowing more heat to be radiated away from the planet, thus cooling it. Wonderfully elegant equilibria and buffer conditions.
Having a moon also affects the planet's rotation. If there were no moon, there wouldn't be any tides at all, thus affecting life processes that depend on them. Considering life on Earth originated in the oceans, tides may have played a significant role that we may not be aware of.
On the other hand, if the moon were too close to the planet, then a phenomenon known as tidal lock would occur. The gravitational pull between planet and moon would slow each other down and eventually stop the planet from rotating, causing a cold trap! That's not to mention the eventual collision of moon with planet if they stop rotating and revolving.
Let's zoom out a bit further to the scale of the solar system and galaxy itself. Galactic Habitable Zones (GHZ) contain the building blocks for habitable planets, and essential for the survival of complex life. In cosmological terms, all elements other than hydrogen (H) and helium (He) - which collectively make up 98% of the Universe by mass - are known as "metals". Metallicity of the universe is increasing over time because the H and He are constantly being "used up" in the fusion processes that power the stars.
In general, solar systems furthest from the center of the galaxy were among the earliest, formed when the galaxy was still young. Consequently, they tend to be the least metallic, and not likely to host the complex chemistry that we would expect from living systems. Moreover, they are highly likely to be bombarded by the asteroids in the galactic equivalent of the Oort cloud.
On the other hand, galaxies closest to the center of the galaxy tend to be the most metallic. However, this region is bathed in deadly levels of cosmic radiation from the formation of the new stars and the many supernovae of failed stars. Metal-rich stars closer to the center of the galaxy are also likely to host giant planets. Giant planets happen to have eccentric orbits that greatly disturb the orbits of the all entities in that system. Without the regularity of a predictable orbit, even potential life-sustaining planets have no chance of staying within the CHZ.
So, the GHZ is again a Goldilocks zone that must necessarily be between two extreme regions.
Enough about placement, what about timing? The Cosmic Habitable Age (CHA) can be said to have begun when the Universe cooled down enough after the cataclysmic conditions following the Big Bang and the great expansion. However, we have seen how the Universe is slowly getting more metal-rich, and star formation is slowing down. The radio-isotopes that power planetary tectonics are also slowly but surely decaying. The stars will eventually run out of hydrogen and helium to power their nuclear fusion processes; at this point, only red dwarfs will continue to exist. All the free energy of the universe at some point would have been harvested fully by every permutation of entropic processes to unusable energy. This will inevitably signal the end of the CHA.
Having a planet (that happens to host the chemistry necessary for life processes) in the CHZ in a suitable GHZ during the CHA is so rare enough that even in a seemingly infinite universe, the chance for the conditions that spawn life remain cosmically low. These are in effect the astrobiological factors to consider when estimating just the first 4 terms of the Drake equation... No wonder it seems like we are alone in the Universe!
And yet, this realisation only inspires audacious hope in me, not nihilistic abandon. How lucky we are, like cosmic Goldilocks, that we get to be alive and sentient, capable of wondering and wandering about this magnificent Universe despite the odds that are stacked against that very possibility! If value is proportional to rarity, then look no further than the Drake equation to deem our existence itself priceless. On this verdant galactic oasis-planet we call home, our civilisation is like a cosmic flower, science and culture its fruit and nectar. The only resource that is truly scarce is our short time here, now... So spend it well my friends!