I believe in aliens, I'll say that. However, not the 'little green men' or Area 51 sort. That is, and I quote Nina, a load of 'hooey'. The chances that other intelligent species live near our little dust mote in the universe is on par with the LHC spawning black holes and devouring the Earth. That is slim to none. As an
extremely conservative estimate, if we have been sending radio signals into space for roughly one hundred years, our presence has only been made aware to .02% of the Milky Way galaxy. By conservative, I mean that that figure is a result of only two dimensional distances, not factoring in three dimensional ones or stellar densities.
That aside, I do have reasonings that support the existence of other species. Life on Earth is, while an anomaly, extremely robust. When the planet was only a hellish place of roiling hydrocarbonic seas and volcanic landmasses, life was killed off multiple times. However, it (obviously) recovered. Later, when it had diversified to macroscopic scales, it was almost nearly obliterated five times. But again, it persevered. Quoting Jurassic Park:
If there is one thing the history of evolution has taught us it's that life will not be contained. Life breaks free, expands to new territory, and crashes through barriers, painfully, maybe even dangerously.
Which brings me to my next point: life can exist in the most bizarre circumstances. Until the past few decades, it was believed that all life required oxygen to live. Then it was discovered that various forms of bacteria that live deep under the Earth's surface get their energy from the chemical makeup of the rocks on which they live. Life has also evolved to survive the incredible pressures of the deep sea, and that's only the parts we've explored.
Despite all this,
intelligent life is much rarer than any life at all. By intelligent, I am referring to the ability to construct edifices, grasp mortality, comprehend the structure of the universe, look for something besides itself etc... In the 4.5 billion year history of the Earth, humanity is the only form of life to fit that description. It took .8 billion years for life to first arise, and in the time of life's existence of modern humanity only covers .000053%. While life itself may be relatively quick to exist, intelligent life is hard to come by.
The most important factor for life on Earth is liquid water. No life could exist without it. Thus, we are putting a lot of interest into exploring Mars and Jupiter's moon Europa. If they contain liquid water, they might have life. Someone asked about this before, and I'll give a reasoning about the importance of water in the search for life. As I said, no life on earth would exist if there was no water. I'm not going to say that this means no other forms of life exist, but if we have seen nothing else, what's the reason to spend billions of dollars on a limb? Additionally, what would we look for? We have no idea what the signature of silicon-based life would be, so how would we know it if we found it? That said, there is a possibility that non-carbon-based life exists and should be explored when technological advancement and scientific knowledge make it more feasible.
To summarize, I think that life exists beyond our atmosphere, but it's not anything film-worthy. At least not near our small ball of rock. As a finality, I leave you with the
Drake Equation: