Why is earth the only planet as far as you know that has life on it?

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The operative part there is "so far as (we) know". Although dozens of extra-solar planets have been discovered, as well as the other seven or eight in our solar system, our science so far is unable to determine if any of them have "life", as least "life" as we know it. Our definition of life is 'earth-based', in other words we can only define life by the kinds of lifeforms we observe here on earth. So if we can't imagine any life forms from earth existing on other planets, we say they are "lifeless". This may or may not be true, but we don't have any other way do define life. Certain moons of the outer planets have what may be the proper environment for some kinds of life - Europa, or Io for example. We believe there may be liquid water there, in some areas, and that is one of the major requirements for life. We don't believe that intelligent life forms can exist on frozen planets, or those with barren rocky surfaces and high temperatures - Mercury and Venus, for example. Jupiter and Saturn are huge balls of gas, and probably don't have any solid surface - so if any life forms exist there, they'd have to be wildly different from anything we know here. Neptune and Uranus are similar gas balls, but we're not sure if they have any life. They are so far from the Sun, and so cold, that life as we know it could not exist there. Many people believe that Mars once had life of some sort, because there is evidence that Mars had liquid water, and some atmosphere. Our probes there now are searching for any thing that would support that hypothesis. We just don't know enough about the planets we've discovered around other stars yet. However, given the number of stars, and the number of planets we've discovered, it would be foolishly earth-centric to believe that we are the ONLY life in the universe. The cosmos is probably teeming with life, we just haven't recognized it yet.
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