Liquid water is the most common source of an item that is necessary for life. Other things that we need to look for are nutrients from which evolution has built our living cells on. Another big item we need is energy to fuel all of our daily activities of life. Energy could come from sunlight, chemical reactions or even the heat from the planet itself. DNA is the molecule that provides the genetic material of our life, it allows scientist to understand evolution that has occurred here on Earth over 4 billion years ago. Living organisms are the ones that copy DNA when reproducing on a molecular level. Living organisms pass their DNA onto their descendants, along the way a mutation might have occurred. A mutation is a change in an organism's DNA, it could be a fatal error or it could be an improvement. Oxygen was created about 2.5 million years ago; it was originally released through photosynthesis by single-celled organisms. These single-celled organisms are also called cyanobacteria. For millions and millions of years chemical reactions with surface rocks pulled oxygen right back out of the atmosphere. Over time surface rocks had become 100% saturated with oxygen, it then finally began to accumulate into the atmosphere. It may have only been a few hundred million years ago that it had reached the levels needed in the atmosphere to be able to breath. Oxygen is not needed in all organisms, about 2 million years ago it was deadly to the organisms living before oxygen was even created (Bennett, Donahue, Schneider & Voit, 2012, p.505-510).
Mars may have had oxygen one point in time from the water molecules in the atmosphere but something happened about 3 billion years ago when it went through a dramatic climate change. The oxygen in water molecules would have remained in the atmosphere but over time it was lost from either being stripped away by solar wind or through being drawn out of the atmosphere by chemical reactions with surface rock. Venus may have been a brother or sister to Earth, but it is located a bit to close to the Sun. If you would move Earth into Venus's orbit today, over time Earth would become just like Venus, mainly due to the runaway greenhouse effect. Carbon dioxide dissolves in liquid water, Venus or Mars does not have liquid water that carbon dioxide can break down into therefore it builds into the atmosphere (Bennett, Donahue, Schneider & Voit, 2012, p.212-217). To be able to guide us to determine the number of civilizations in our galaxy that we could possibly make contact with scientist have come up with the Drake equation. It looks like this Nhp×Flife×Fciv×Fnow. The number of planets that could potentially harbor life is calculated in the Nhp. Flife is the fraction of the planets that actually harbor life. For example, Flife=1 would mean that all planets that are habitable currently have life. 1/1,000,000 would mean that only 1 in a million planets that are habitable actually have life. Nhp×Flife can tell us how many planets in this galaxy are life-bearing. Fciv will tell you the number of life bearing planets that have had the capacity to communicate at one time or another. This will tell you the number of planets whose species are intelligent enough to accomplish some type of communication. So if we take Nhp×Flife×Fciv we will get the number of planets that have at one point on time harbored life capable of interstellar communication. Fnow is the fraction at which these life bearing planets currently have life present as we speak, compared to billions of years in the past or even what could become in the future. Scientist do not know the values of the drake equation but if ever needed in the future it will be a helpful was to organize their thoughts. Fermi's paradox say galactic civilizations must exists but yet there is no evidence of this being so. But yet how can we say no other civilization exists if we can't travel to other solar systems and scope them out We can't base a definite a to other life existing if we do not have the current technology to have a very clear view of the billions of star clusters out there. If there is one civilization for every 1 millions stars then our universe should have about 100,000 civilizations and new ones being created every 50,000 years. This still may not be so! Maybe we can't see anyone else yet, or maybe it just is not possible to bring technology so far forward that we will never be able to detect anyone or no one will be able to detect us (Bennett, Donahue, Schneider & Voit, 2012, p.519).
Interstellar travel is not as easy as it seems on TV. There are currently five spacecrafts that will be leaving our solar system to travel among the stars, but because they travel at less than 1/10,000 the speed of light it will take those 1000,000 years to reach the next star system. Einstein's theory of relativity tells us that real interstellar travel must be slower than the speed of light. If we would want to make a trip within our lifetime we would need a spacecraft that could travel very close to the speed of light we would need an energy source that is 2000 times more than what our whole world uses in one years time. Let's say we did achieve all of this someday; the spacecraft, the energy source, and traveling at speeds of light, the crew of the spacecraft would have extreme social and mental situation to deal with. Again according to Einstein's theory of relatively an object traveling at the speed of light will experience "time" passing much slower on the spacecraft that it would on Earth. For example, is a spacecraft traveling near the speed of light to a star system called Vega that is 50 light years away, the crew on board would have aged only two years, but on Earth 50 years would have passed. All the people and family you may have been connected to will either be dead or very aged. We have no idea what a "living" in every nook and cranny of our universe. It is very possible that alien life at any minute could come crashing into our atmosphere, it is possible that this has already happened and we do not even know it. It could have happened one billion years ago and we missed it.
Just because we have not seen it does not mean it is not there. We, as human beings need to remember we do not know everything and there are endless possibilities out there that go way over our heads. SETI stands for search for extraterrestrial intelligence. They are a funded organization that listens for signals that are being sent through interstellar space. They use large radio telescopes to search for alien radio signals. SETI have several projects under way that are capable of detecting signals. SETI scan millions of radio frequency bands simultaneously, if it is out there they will be able to pick it up. Of course there are always other thinks that we could be spending money on but the SETI is also providing jobs to people who are conducting the scanning. They are also providing work to the manufactures that are making the products that help with the radio transmission. It is one of those issues that will never have a right or wrong answer (Bennett, Donahue, Schneider & Voit, 2012, p.519-525).
References:
Bennett, J., Donahue, M., Schneider, N. & Voit, M. (2012) The essential cosmic perspective. (6th ed.) San Francisco, CA: Pearson Addison-Wesley
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