Earth is the only planet in the solar system that has life on it what qualities make life possible on Earth?
More appropriately: Earth is the only planet in the solar system that is known to have life (as we currently understand it and/or define it) on it.
Two basic qualities that are required for life (again, as we understand it and/or define it) would be water and a temperate climate. Earth, abundant with water, sits in the 'Goldilocks Zone' of the solar system - a place where the temperature is not too not, nor too cold.
Consider that if the 'seeds of life' were scattered through our solar system & the sun was either a bit cooler or a bit warmer, there is a chance that life as we know it could have begun on a different planet... and, in fact, may have at some point earlier in time.
We are still scratching the surface of the origins of life here on our own planet - only a small portion of our oceans have been explored & new species are found all the time. Since we have yet to truly explore another planet, we cannot say with certainty that life is exclusive to the third rock from the sun. Only time will tell.