A simple answer to the question would be we don't know. There are certain problems when asking a question like this. First of all we need to know how we define life. You may say that some if alive if it uses energy in an orderly way, it has a boundary and it can reproduce. However this doesn't work if you think about it. This could mean that a flame is alive because it has a boundary, uses energy and reproduces. However a mule - a sterile cross between a horse and an ass - is not alive because it cannot reproduce.
There is also the term 'life, as we know it'. There is the possibility of life forms that can survive in ways we can't. In an episode of Star Trek their Tricorders could not detect a life form until they altered them to detect a silicon life from. This means that there could be life forms out there that we don't even realise are alive.
Another thing we have to look at is if a life form can survive on a planet. Earth is perfect planet. Is has a suitable gravity, it is just the right distance from the Sun to be not too hot and not too cold. It as an oxygen / nitrogen atmosphere and had the elements of oxygen, nitrogen, carbon and hydrogen, It has hydrogen oxide (water) and a magnetic field protecting us from the Suns deadly radiation.
This may be what we call necessary to survive but Andean Indians can survive whilst living in mountain villages as an altitude of 17,000 feet (5,200 meters). They have larger hearts and lungs than us so they can breath normally in air that is too thin for us to breath and there is nothing to say that an extra terrestrial life form can't do the same on a planet with a thin atmosphere. Also Captain Jacques-Yves Cousteau showed us that there is life at the bottom of the Mariana Trench, south-west of Guam in the Pacific Ocean, at the depth of 36,198 feet (11,033 meters). Although it is very cold down here the life gets it heat from volcanic vents (called Black Smokers) but it still has to survive a pressure of 16,000 pounds per square inch (1,000 kilograms per square centimetre). To take this to the extreme there are life forms on this planet than can only survive in sulphuric acid.
However there is a possibility of there being life on other planets. The Galileo probe reported there is life on Earth. This however was a test to see if it could detect life anywhere else in to Solar System and it also reported life on the Jovian moon Europa. It is believed that there is an ocean below its ice-covered surface.
As for life out side our Solar System, we have no evidence on way or another. We have detected planets round other stars and it maybe that these planets or their moons can support life. These stars are much further than our stellar neighbourhood and so sending a probe to these stars could take centuries and we have no guarantee that there is life on the planets. The only extra solar life we have a real chance of detecting is intelligent life.
The term intelligent life means life forms that we can communicate with. In the 1950s, when a young American, Frank Drake, was working in a brand new field of radio astronomy, aliens were something no respectable scientist thought about. But Drake took a much broader view. He knew that radio dishes could pick up naturally produced signals from halfway across the Universe.
They could also be used in reverse to transmit a signal. He wondered if there were other inteligences out there, using telescopes to transmit messages which his instruments should be able to pick up. There is a SETI (Search for Extra Terrestrial Intelligence) project taking place using the Arecibo Radio Observatory in Puerto Rico and there are a few others are being conducted, like Project Pheonix, and others have taken place in past all in hope that we can pick up a radio signal from another planet. One than you can take part in is known as SETI@Home. It runs as a screen saver. Your computer downloads data from the Arecibo Radio Observatory and when ever your computer is in screen saver mode, it will check the data.
We have had thousands of messages but we have to verify them to make sure they are not from Earth satellites, hoaxes, or naturally produced but we haven't got that far yet so we really don't know if there is life out there yet. We'll just have to wait and see.
Frank Drake devised the "Drake equation" - a formula to calculate how many civilisations might be broadcasting radio signals. This is the Drake equation.
N = R* x fp x ne x fl x fi x fc x L
R* is the rate starbirth.
The number of new stars formed in the Galaxy. This is widely accepted by astronomers to be 10 per year.
fp is stars with planets.
The percentage of those new stars that form planets.
ne is habitable planets.
The number of those planets that are the right distance from their star, and of the right size to be suitable for life.
fl is planets with life.
The percentage of those planets on which life has actually evolved.
fi is intelligent life.
Percentage of planets where life forms have evolved sufficiently for "intelligent" creature to exist.
fc is communicable life.
The percentage of those planets on which intelligent lifeforms have developed technologically so they can communicate with other civilisations.
L is life span of civilisation.
The length of time that a technologically advanced civilisation exists and in which it can send a potentially receive signals.
Here are two examples for outcomes that can come out. One from an optimist and one from a pessimist.
The first person is relatively optimistic. She chooses 100% for the first two answers. 10% for the third, and then 100% again. The next answer is a more cautious 1% for the number of intelligent civilizations that want to communicate, and an optimistic lifetime of 1,000 million years. She ends up with 10 million communicating civilisations in the Galaxy.
The second person is a pessimist. He give answers of 10% to the first two questions, but is more hopeful 100% of life evolving on a suitable planet. He is pessimistic again for the following questions, with answers of 1%, 10%, and a lifetime of only 1 million years. His number of communicating civilisations is just 100.
I have included here a table of how far away the nearest civilization is likely to be depending upon what answer you come up with.
|Number of civilisations in Milky Way.||Distance from Earth to the neatest civilization in light years (ly).|
|10,000 million civilizations.||10 ly away.|
|1,000 million civilizations.||22 ly away.|
|100 million civilizations.||46 ly away.|
|10 million civilizations.||100 ly away.|
|1 million civilizations.||220 ly away.|
|100,000 civilizations.||460 ly away.|
|10,000 civilizations.||1,000 ly away.|
|1,000 civilizations.||2,200 ly away.|
|100 civilizations.||4,600 ly away.|
|10 civilizations.||10,000 ly away.|
|1 civilization.||200,000 ly away.|
|0.1 civilizations.||3 million ly away.|
|0.01 civilizations.||10 million ly away.|
|0.001 civilizations.||50 million ly away.|
The number or civilizations is less than one in some cases.
For 0.1 civilizations, it means our civilization is the only one within ten galaxies of here. For 0.01, our civilization is the only one within 100 galaxies of here, and within 1,000 galaxies of here for 0.001.
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