There are a few key ingredients that are necessary for life to exist. Without these, life (as we know it) cannot exist:

• Liquid water
• Basic chemicals (carbon, nitrogen, and oxygen)
• Energy source

Earth supports life for several reasons:

• The distance Earth is located from the Sun makes liquid water available on the surface of our planet and allows the sun's energy to be utilized by organisms on Earth.
• The tilt of our planet gives us the seasons that makes our planet habitable
• The composition of our atmosphere (including greenhouse gases) provides a suitable environment for life to exist

While no life has yet been found on other planets in our solar system, there are a few possibilities for life to exist:

• Mars: there is evidence that liquid water once existed on the surface of this planet. Also, meteorites from Mars contain have been shown to contain carbon compounds, which are possibly biological in origin.
• Europa (one of Jupiter's moons): may contain a liquid ocean beneath its icy crust. Also, Europa's rocks and ice could contain some of the chemicals necessary for life. Europa has two possible sources of energy - its own internal heat and tidal heat caused by its gravitational attraction to Jupiter.
• Titan (one of Saturn's moons): contains water ice on its surface and some of the basic chemicals necessary for life.

Scientists are actively searching for life beyond our solar system. They are searching for other planets orbiting stars similar to our sun, and they are using radiotelescopes to listen for a signal from life on other planets in our universe.