Here's a neat little article about what factors make life on earth such an unlikely thing and how there are a bunch of other planets outside our solar system that might be better suited for life to develop and thrive upon.

From the yahoo article:

This greater gravity means a Super Earth can easily hold onto an atmosphere, so it would not end up with a tenuous atmosphere like Mars. But the role of a planet's atmosphere in creating prime conditions for life can be tricky. Venus has a surface temperature of nearly 900 F (480 C) due to the thick greenhouse atmosphere that doesn't let heat escape.

One of the biggest influences on a planet's climate is the star it orbits. Earth has a circular orbit 150 million kilometers away from the Sun, a yellow dwarf star. This helps keep conditions warm enough so that our oceans don't freeze over, but cool enough so that we don't lose all our water through evaporation.

The Super Earths discovered so far orbit a variety of stars. The first Earth-like extrasolar planets ever found orbit a pulsar, a rotating neutron star that emits high energy radiation. The other Super Earths orbit stars that are smaller and cooler than our Sun.

Most of the known Super Earths are very close to their stars, closer than the planet Mercury is to the Sun. Even though these stars don't burn as brightly as our Sun, the planets are so close they are like burnt cinders flickering close to a fire.

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Read the full article here.