Let's imagine you are a hypothetical being looking for randomly occurring intelligent life within our Galaxy. You contact your local galactic real estate broker, and together you begin the search. There are an estimated 100 billion stars in the Milky Way Galaxy, so there should be plenty from which you can choose. However, there are many things you must consider on your search.
Galactic Survivable belt
You've got to pick out a star in a place where life can survive. Galaxies have a belt where life can theoretically exist. If a star system is too close to the center of its galaxy it receives too much radiation from supernovas, and x-ray from the supermassive black hole at its heart. These worlds would be sterile. However, closer to the outer edge of the galaxy, the stars are older and don't have the heavy elements needed for terrestrial planets, and advanced life. Too close to the galactic center and the star system is sterile, too far away and life is impossible. Based on their location and age, 99.7% of stars in the Milky Way Galaxy couldn't support life. You've narrowed your search substantially. You now only have 300,000,000 stars from which you may choose.Astrosphere
You need a star with the proper Astrosphere. The universe is full of cosmic radiation, which is deadly. A star emits stellar wind that pushes that radiation back. Each star creates a shell of protection. If the star doesn’t emit enough stellar wind, life on the surface of local planets can’t survive the cosmic radiation. However, if the star emits too much, then it can impede or even bring life to extinction. About 96% of Stars do not have the right mass to create a life sustaining astrosphere.Ultraviolet Balance
Now you have to consider what type of lighting is available because life is very sensitive in some parts of the spectrum. Too much Ultra Violet destroys DNA. If a star is visible, then it is sending out UV. However, for life to develop, It can’t be too much. Only a small number of stars have both the right amount of UV at a distance that can sustain liquid water on a planet. Stars below 4600Kelvin are off the list. Also stars above 7100Kelvin can't support life either. About 92% of Main Sequence Stars won't support life because of issues pertaining to its UV band.Star Stability
Now to complicate this, the habitable zone of a star system moves. That’s right it moves. That’s because the star changes in luminosity throughout its cycle, and throughout its life. If the habitable zone moves out of range, all life that does exist on a planet will die. By this estimate, 96% of stars are not stable enough for prolonged life.Light quality, astrosphere, and stability are all related so we will lump them together. Even if we are being conservative, these three aspects exclude about 96% of all available stars. So you've narrowed your search once again. You now have 12,000,000 stars from which you may choose.
Planet Presence
Life can't live on the star, so there needs to be some terrestrial real estate. You're looking for stars with planets. No problem, most stars have at least some planets. Scientists think the average number of planets per star is around one, although even this is still being researched. Let's be generous and say that every star in the Galaxy has at least one planet. We now have 12,000,000 planets from which we may choose. Those are pretty good odds, right?Water Habitable Zone
Surface temperature must be in a narrow range for liquid water to exist on a planet. Too cold and it's all ice. Too hot, and it's all vapor. Outside of our solar system, it's believed, 1 in five planets reside in this orbital range. That means that 80% of planets discovered cannot support life because any water they do have would be ice or vapor. Even for those in the right range, it does not ensure the presence and abundance of water. We have narrowed our search to 2.4 million planets.Tidal Locking
When an orbiting body is tidally locked, it means that only one side ever faces the object it’s orbiting. Our Moon is an example. If this happens with a planet, the sunny side becomes blazing hot and the dark side incredibly cold. Life is very unlikely in a world that doesn’t rotate properly. Of around 218 orbiting objects in our solar system, around 61 of them are or believed to be tidally locked. If this proportion were average across the whole galaxy, this would mean another 28% of planets are too lethally hot, or cold for life to survive. We've narrowed our search to 1.7 million planets.Planet Size
If the planet is too small plate tectonics, which is vital for planetary life, would not occur. It would be geologically dead as Venus is. Secondly, there is a minimum gravitation threshold for maintaining an adequate atmosphere. If the planet is too massive, life as we know it couldn't work. 83% of star systems do not have an Earth-sized planet. We've narrowed our search to about 293,760 star systems.Planetary Obliquity
Obliquity is the tilt of the axis at which the planet rotates. Without an obliquity, a planet has no seasons. With too much obliquity the seasons will be too violent to sustain complex life. Neptune, Saturn, Mars, and Earth all have a similar obliquity, and probably would be survivable if all other factors were equal. The rest of the planets have an obliquity that would either destroy or make complex life extremely unlikely. So that's 4 out of 11 (I'm including Pluto, the Moon, and the Sun). So if that was average for the Galaxy, about 64% of planets have an obliquity that, likely, would not be life-sustaining. You have narrowed your search to 105,753 planets. The pool is getting smaller.Orbital Eccentricity
Elliptical orbits result in variations in the planet's average temperature. The more elongated the orbit is, the more violent the climate change through the year. The orbit has to be close to round to sustain a consistent average temperature. Nothing orbits in a perfect circle, but Earth's is close. Mars' orbit is more elliptical than Earth's. Mars is about 43 million Km farther away from the sun on the long end of its orbit than it is on the short end. If Earth's orbit did that, we'd be dead, because we would spend a lot of our year outside the habitable zone. Even if we had the average solar system eccentricity, it would still be a difference of 19.4 million Km. Even with that average elliptical orbit, life would be decimated if not destroyed. Out of a list of 65 orbiting objects in the Solar System, Earth has the 3rd most circular orbit and is not far from 2nd place. Earth's orbit is unusually round, statistically uncanny, and magnificent for life. Around 90% of known orbits within our solar system are deadly, even if everything else was properly life sustaining. We're down to 10,575 planets. The pickings are looking slim.Electric Wind
Electric wind Is an electric flow that can carry water vapor and atmosphere away from a planet. If it's slightly too high, the planet loses atmosphere, higher still and it loses it’s water as well. Venus, which measures at around 10 volts, is parched because of this phenomenon. Earth, on the other hand, possess an atmospheric electric field weaker than 2 volts. Scientists are currently studying whether this could have occurred on Mars as well. There is not much data on this subject yet, but of the two well measured planetary electric fields, 1 out of 2 have been too high to sustain life. Although, it’s likely that this number will increase as more data comes in let’s just stick with the numbers. Of the planets measured 50% have a known electric fields too high to sustain life. We've narrowed the search to 5,287 planets.Planet Rotation Rate
A planet with an iron core must rotate fast enough to create a dynamo effect. This results in a magnetic field around the planet that protects its atmosphere from being carried off by stellar wind and offers further protection from the radiation in space. However, if it rotates too fast, it will stress the crust of the planet and induce more frequent and violent volcanos. The Climate would become hostile. Beyond this, if the days are too long, the day-time heat build up is too severe for life. If the nights are too long, life dies in the freeze. In our solar system, six our of eight planets have a rotation rate that would probably be survivable if all other aspects were perfect. If this represents the average throughout the galaxy, that means that 25% of planets are hostile to life because of their rotation rate. I know it's getting dire, but we still have 3965 planets available.Orbital Length
If the year is too long, the seasons will cycle too slowly to be of benefit. If the year is too short, then the seasons pass too quickly to offer their replenishing effect. Neptune, for instance, takes 165 years to make one orbit. If Earth had that orbit length, winter would last the equivalent of 41 years. Provided that everything else was life-sustaining, long orbits would either limit life to regions or exclude it all together. Only three out of the eight planets in our solar system have year lengths that would work for life as we know it. If that were the galactic average, it would narrow our search to 1487Atmosphere Composition
12 of 13 bodies in our solar system that we've been able to measure have either no atmosphere or one that is toxic to life. If that is average for the galaxy, it will narrow our search to 114 planets.Plate Tectonics
Long term life requires plate tectonics. Earth and, it's believed that Mars as well have tectonic activity. If that ratio of 2 out of 8 is average for the galaxy, it narrows our search by 75%. So we're down to 28 planets.Gravitational Fence
Any star system that is going to sustain life long-term needs a few large bodies in outer orbits. Meteorite impacts are dangerous for life. Jupiter, Saturn, Uranus, and Neptune act as Earth's barrier from flying space junk. Let's say that we need a system with at least four planets, three of which are large and in outer orbits. So since each star has an average of 1 planet, that means that this cuts our search by another 75%. We're down to 7 available planetary systems, each of which must have at least one planet in the habitable zone. That means we're down to 7 planets. Only about 7% of discovered star systems have a Jupiter or larger sized planet. 7% of our remaining 7 star systems shows that we've met the end of our search. We are out of possible locations where life could randomly occur.We've run out of planets even though we haven't considered these other life requirements.
Presence and abundance of waterAtmospheric pressure
Nutrients
Planet's rocky composition
A moon
Binary Star Systems
The rarity of an ozone layer
The unknowns concerning abiogenesis.
And about a million other things.
So, whether life could randomly exist is still up for debate. However, it seems that our Galaxy is not big enough. It just doesn't have a large enough sample size.
However, we are faced with this powerful fact. Life does exist. Life exists in a corner of the galaxy, solar system, and on a planet that is perfectly suited for it. It'd be easy to think when we look at the number of stars; that life is a fore drawn conclusion. However, we know enough now to be sure that life requires help. The galaxy is too hostile and too small for us to be statistically confident that life could occur randomly.
There is another explanation, however. What if the placement of life wasn't random. What if we are here on purpose.
When you consider what we currently know about the galaxy, intelligent design makes much more sense than randomly occurring life.
Note: Much of the above estimates are based on articles and sources that you can find below.
Sources:
https://en.wikipedia.org/wiki/List_of_Solar_System_objects
https://en.wikipedia.org/wiki/Tidal_locking
http://www.keckobservatory.org/recent/entry/one_in_five_stars_has_earth_sized_planet_in_habitable_zone
http://lasp.colorado.edu/home/maven/files/2015/11/Electric-Mars_The-first-direct-measurement-of-an-upper-limit-for-the-Martian-“polar-wind”-electric-potential.pdf
https://en.wikipedia.org/wiki/Solar_analog
https://en.wikipedia.org/wiki/Planetary_habitability
uniqueness: http://www.reasons.org/articles/rare-sun
https://en.wikipedia.org/wiki/Galactic_habitable_zone
http://newsroom.ucla.edu/releases/ucla-scientist-discovers-plate-237303
http://www.popsci.com/science/article/2012-01/new-exoplanet-analysis-determines-planets-are-more-common-stars-milky-way
https://www.nasa.gov/mission_pages/kepler/news/17-percent-of-stars-have-earth-size-planets.html#.V5zTQ1cw38s
https://lcogt.net/spacebook/what-are-requirements-life-arise-and-survive/
http://www.nasa.gov/image-feature/kepler-planet-candidates-january-2015
http://www.learnastronomyhq.com/articles/15-goldilocks-factors-that-allow-life-on-earth-to-exist.html
