Dear Derrick,

This is a question that scientist have long been pondering. Water is essential for life to exist, so a planet needs to be at a distance from its star that will allow it to have water, so it can't be too close or too far. The Earth seems to be at a very lucky distance to support all types of life, from the very simple forms (bacteria) to the very complex (us!). Planets that are located at a similar distance from their star (if their star is like the Sun) as the Earth is from the Sun will probably have a better chance of containing life. Planets at a distance similar to the orbit of Mars may also sustain some sort of life. I hope we will be able to determine whether there is other life in the Universe during my lifetime!

Holly

Holly is right about liquid water being critical. If a planet is orbiting around a bigger star than the Sun, it would be hotter, so a planet with liquid water could be farther away. However, those stars put our more UV light so it would be difficult for life to survive in that radiation. If a planet is orbiting a star that is smaller than the Sun, the planet would need to be closer to have liquid water. But smaller stars give our more red light than blue, and blue light is necesssary for photosynthesis. So you not only have to have the right distance, you have to have the right size of the central star! Also, a planet with a magnetic field gives more protection to its life forms than one without one. So in many ways we are very fortunate to live on a planet that is perfect for us!