When considering the likeliness of life arising repeatedly throughout our universe, the first three terms of the Drake Equation immediately come to mind. Since life needs a place to arise, the availability of such places becomes one of the most important considerations. Even though the rate of star formation is currently not determined exactly, based on evidence, it could be estimated to be relatively high. In our own galaxy, star formation can be observed in the frontal areas of the spiraling arms, where abundant quantities of gas and dust become more concentrated due to the rotation of the galaxy. There are constantly stars being born inside galaxies and considering the existence of about 100 billion galaxies in our universe, it is logical to assume that the rate of star formation is extremely high. Even if only 1000 stars form per galaxy per year, it would mean that 100 trillion stars form in our universe per year.
Not all areas around the formed stars are suitable for supporting life. For life as we know it to exist, liquid water has to be available. Based on the inverse square law, both the luminosity of the star and the distance away from it go into figuring out the "habitable zone" in a particular solar system. Also the fact that stars' luminosities change as the stars go through their main sequence has led to decreasing the "habitable zone" into a "continuously habitable zone". A planet located in the continuously habitable zone is considered able to maintain liquid water on its surface. Of course surface conditions like temperature, pressure, albedo, and the greenhouse effect also strongly contribute to the individual planet's ability to maintain surface water in liquid state.
Stars with spectral types of G and K are considered to have large enough habitable zones and long enough main-sequence times for the formation of life. Based on observations in our own galaxy, approximately 22.3% (15% are K, 7.3% are G) of all the stars forming in our universe fall in the G and K classification categories. Considering the calculation mentioned earlier, with which it was determined that around 100 trillion stars form in our universe per year, it could be derived that 22.3 trillion stars capable of supporting life form every year. The number could be increased even further if microscopic life, instead of complicated life is the one being considered. Since it only took approximately 1.5 billion years after the formation of our sun for microscopic life to form on Earth (3.5 million-year-old fossils), brighter stars with shorter main-sequence spans, but larger habitable zones could be considered as places where life can arise. Stars with main-sequence life spans of 1.5 billion and above would become possible sites of life.
If this approach is taken, an overwhelming majority of stars can be considered as capable of supporting life. Since life is incapable of surviving in the extreme environment on the surface of a star, planets orbiting around that star are the obvious place for life to be found. Two questions immediately arise when considering planets in other solar systems. Is it common for stars to have planets and if stars do have planets, how many are "earth-like" Planet formation is in many ways similar to star formation. And why should it be different The same laws of physics are in effect except on a smaller scale. Terrestrial planets are formed when gravity brings together small particles and dust left over in the accretion disk of the star until enough of them collide together to form planetesimals and then planets.
The formation of the Jovian planets is even more similar to star formation because when they form, aside from the collisions of icy planetesimals found in the further-out regions of the solar system, these planets are massive enough to form their own mini-accretion discs. These mini-accretion disks can then go one step further and form moons such as the ones orbiting Saturn and Jupiter. Since it has been observed that all stars have accretion disks when they form, it can be deduced that the overwhelming majority of stars does have planets orbiting around them. Also, other solar systems containing planets have already been detected (41 UMa, 51 Peg, 70 Vir, etc.) and in our own solar system we observe several levels of planet formation (sun has planets, Jovian planets have moons). Based on the observations of the facility of planet formation in our own solar system and having no reason to believe that other stars' accretion disks would be much different from our own, it could be assumed that planets form with relative ease and that nearly all stars have planets.
It is not enough to just have planets to have life. In order to support life, the planets have to be "earth-like". Meaning that they are located in a habitable zone and have the right surface conditions to have liquid water. If we only concentrate on the stars most likely to harbor life it would mean ignoring stars with spectral types other than G and K. These stars have either shorter main sequence times, enough for only microscopic life to form or too small of a habitable zone as in spectral type M. Considering the earlier calculation which gave us 22.3 trillion stars capable of supporting life forming every year and decreasing that number by a fraction to account for some stars that for whatever reason don't have planets, the result is 20 trillion stars forming every year that contain habitable zones, sufficiently long main-sequence periods and planets. The next question is how many of these 20 trillion stars have earth-like planets (planets located in the habitable zone) There is no current information based on which the exact percentage of stars containing earth-like planets can be determined. Our current technology, so far, allows us only to detect planets as small as several times the size of Jupiter.
Since the larger planets have been detected, there's no reason to believe that the smaller earth-like planets, closer to the stars don't exist. Just because they haven't been detected yet, doesn't mean they " re not there. Taking a relatively safe, pessimistic outlook on this issue, it can be said that half of the stars containing habitable zones actually have planets orbiting within those zones. This leaves us with 10 trillion stars with earth-like planets, where life has a chance to arise, forming every year. Our first consideration (places for life to arise) has been calculated. A truly astronomical number of 10 trillion new possible sites arising every year has been determined.
The next and most tricky consideration is the fourth term of the Drake equation. How many of these possible sites actually do foster life Due to the fact that currently there's no existing way of checking whether or not life develops on planets orbiting around other stars, all of the conclusions must be based on our own planet's experience with life. On Earth, life developed almost as soon as it could. The oldest micro fossils date back to the beginning of Archean (3.85 billion years ago) but scientists estimate that life on Earth might have arisen as early as the end of Hadean. It is highly improbable that formation of life on Earth is due to chance. To form the simplest living organism, containing 2000 proteins, which are 100 amino acids long, by a spontaneous chance assembly, would require 20200,000 tries.
If we assume that the oceans were full of the 20 amino acids necessary for life and that those reacted randomly and joined together once every minute in every volume region the size of a bacterium (this is a gross overestimate), there would have been only 2.631051 tries in the 5 billion years. If it were up to chance, a simplest prokaryote would not have came into existence on Earth for another 510199,950 years. The above calculation points to the conclusion that there must be some fundamental process acting in our universe, which has formation of life as its result. Having life arise by chance within 5 billion years is roughly equivalent to winning the lottery every week for several decades. One could attribute such an occurrence to chance or to tampering with the drawings. A rational mind would assume that some outside process is working behind such seemingly endless luck.
When calculating the amount of solar systems where life develops, even if an extremely pessimistic view is taken and only. 1% of the 10 trillion stars with earth-like planets suitable for life are determined to foster life, the conclusion is that 1,000,000,000 solar systems where life will arise, form every year! Another factor, which could add to the chances of life arising in other solar systems is the fact, that life on Earth is observed to survive in some of the most adverse conditions available on the planet. Bacteria has been found living on the ocean floor, under immense pressure, high temperature, with no light, deriving their energy from chemical reactions. Other bacteria live more than a mile below the Earth's surface, under searing heat, colossal pressure, no oxygen, consuming organic material found in sedimentary rock. One type of subterranean microbes identified as lithotrophs is still a mystery to modern science.
Even with all of our modern technology, scientists are still puzzled on the issue of how the microbe obtains its energy. Since we can't even completely identify the functions of bacteria living a mile beneath us, how can we say what is required for life billions of light years away The calculations presented in the earlier parts of the essay are very convincing of the fact that life as we know it exists on a grand scale, but these calculations don't go beyond the scope of life that needs moderate temperature, pressure and liquid water to survive. The above mentioned adverse conditions are not included in figuring out the habitable zones around stars. If they were, the zones would be much larger and the number of sites where life arises even higher than calculated. Not to mention the possibility of existence of life which is completely different from life on Earth and thrives under completely different conditions. Life is just another natural process in our universe.
Just like stars are born according to the laws of physics, life comes to exist according to its own set of laws and conditions. Just because we can't yet see life in other solar systems, it doesn't mean that it doesn't exist. Our universe is densely populated with life, and life is currently arising in other, far away worlds while we struggle to realize that it's doing so..