Copy pasta from the last thread on this (in addition to below, however, this scientific proof of life being able to exist in completely different forms (arsenic based), makes it even more likely to be life out there).
Okay, just a little bit of math here for you. Nothing complex, just basic multiplication and division.
Okay, so we have like 100 billion stars in a galaxy on average (the Milky Way is predicted to have between 100 billion and 400 billion stars, and some galaxies have less than this (and some have much more) so we'll be conservative and just for the sake of argument assume the average is just 100,000,000,000 stars).
Now, with our current technology we're pretty certain there are at least 50 billion galaxies out there, although estimates are that there are FAR more. Just for the sake of argument let's say there are only 50 billion galaxies (even though the real number is likely to be more than double this amount). Now, by multiplying these two numbers together (100 billion stars per galaxy where there are 50 billion galaxies), we get a total of 5 x 10^21 stars (or 5,000,000,000,000,000,000,000, that's 5 sextillion stars).
Of course, not every star system has planets on it, however it's thought to be very common for stars to have planets in orbit around them. Let us assume that only... one in a million stars has a planetary system (that's being very conservative). And to keep things simple, let's assume that every one of these solar systems has only one planet, even though they're likely to have more (remember our Solar System has eight). So divide the number of stars by the number of stars with a planet (1 in a million, so by a million) and we have 5 x 10^15 planets in the universe (or 5,000,000,000,000,000, that's 5 quadrillion planets).
Also not every planet is capable of supporting life, so let's say 1 in a million planets is capable of supporting life (so only one planet in every million is capable of supporting life, that's a low estimate). So all up we have 5 x 10^9 planets that can support life (or 5,000,000,000, that's 5 billion planets).
So get this - 5 billion planets in the universe are capable of supporting life according to this little math thingy. Let's remember that we've assumed a lower number of galaxies than there actually is, a lot lower number of planets than there is thought to be and a lower amount of planets capable of supporting life than there probably is. 5 billion planets is a much smaller estimate than probable, but even then - five billion planets! You can't look at that and say that there is NO life out there.
There's a few problems though. How common is life, exactly? The most accepted theory to the beginning of life was that it was an accident, that amino acids that had been moving around for a long, long period of time happened to collide in the right way and that started a chain reaction that gave birth to life. How likely would this be to happen? And then on top of this, we need to figure out if life would even evolve past bacteria and single-cell organisms. I don't know exactly, but I think it was like 2 and a half billion years for us to evolve from cells. Stars don't last forever and that 2 and a half billion years has taken up a lot of our sun's lifetime. What if life hadn't evolved far enough before its local star died? Also, what if life hadn't evolved on a particular planet enough to be considered intelligent or self aware?
Also, if there are intelligent aliens somewhere else in the universe, it's likely that they are far more advanced than we are. I mean, what are the chances that all advanced civilisations are progressing at the same rate we are and at the same time (in the same tens of thousands of years or whatever)? Our medical advancements have been amazing, what if these other life forms have found the secret to practically being immortal? The point I'm making is, if there is advanced life, why haven't we come in contact with them yet? Surely if they're that advanced they would have figured out a way to travel great distances in a short amount of time by now. Are they avoiding contact with us? Is it inevitable that intelligent life destroys itself before it is advanced enough to contact extra-terrestrial life?
Despite the points against there being intelligent life, I do believe there is intelligent life out there. Think of how big the universe is, how many stars, how many galaxies... it'd just be weird if we were the only race.
(Now I expect to get *shot* by Mizan or another science person if I got something wrong)
