In Favor of Life
One key to finding life on other planets are phenomenon known as extremophiles, microscopic organisms that can live in extreme environments. These life forms can be found in the most  inhospitable environments, places that are extremely hot, cold, salty or even deep inside solid rock. Given that such organisms exist here on Earth, it follows that similar organisms might live in extreme conditions on other planets.
 Based on our current understanding, liquid water is required for life to exist. This is why life on Earth developed in the oceans long before it developed on land. So to maximize the chance of discovering life forms elsewhere, scientists eliminate the least likely places by looking for celestial bodies that evidence the presence of water.
For example, Mercury has no air, so any water on that planet would boil up into space. Similarly, Venus is much too hot for liquid water to remain on its surface, so both these planets can be eliminated as unlikely to sustain life.
Theoretically, the Jovian planets— Jupiter, Saturn, Uranus, and Neptune—might have isolated droplets of liquid water in their atmospheres, but because these planets do not have solid surfaces  for oceans to form. They too are not likely to host carbon-based life forms. This is because large bodies of liquid water are probably necessary for nutrients, chemicals and other molecules to interact sufficiently to form and sustain life.
Europa, one of the moons orbiting Jupiter, seems to have a liquid-water ocean below an icy crust. However, the problem is that this moon might have been frozen solid at some point in the past, killing off any organisms living there.  Of course, one can always hope there are extremophiles that survived nonetheless.
Though both Saturn and Jupiter have additional moons that hold a degree of promise, Mars is probably the most likely place we know of where other life might exist in our solar system. In fact, there is evidence that liquid water once flowed on the surface of Mars. Yet, recent soil samples show little evidence of life there now or in the past. Consequently, the best chance for life to exist on Mars might be underground, where there is a possibility liquid water still runs and where life would be shielded from the Sun’s U.V. radiation.
When it comes to planets outside our solar system, the possibility of discovering life does exist, but the conditions would have to be nearly perfect.