Scientists have gone back to the drawing board, looking to find worlds capable of supporting life, small rocky planets like earth.

 

"We don’t know that life has to evolve on the surface of small rocket planets. But we know it did once here on earth. And so it’s a reasonable place to start the search."

 

Earth is important, because we know that this little planet had everything it needed to create life. And if a distant world has similar conditions, there’s at least a possibility life could emerge there too.

 

"If you were to give a biologist a piece of paper and say write down on every line something you need for life. He’ll say you need energy source, you need oxygen, you need water, you need carbon, you need organic compounds, you need amino acids, you need all the stuff."

 

There is a long list of factors that make our earth special and perfectly suited for life. But the most important is its distance from our energy source, the sun. At 93 million miles away the earth is at just the right place for water to exist as a liquid. So why is liquid water important?

 

Imagine a world, so much hotter than the earth, there are no oceans, no liquid water all on its surface. Such a place would be like a desert. Without water, there is nothing to let the grains in sand interact. And in the same way without water, there is nothing to allow the atoms, the carbon and oxygen, and trace elements to form the molecules that would give rise to the chemistry of life. 

 

On the other hand, on a world much colder than the earth, water can only exist as ice, and we have another problem. This frozen block of sand has all the grains in sand locked together. They can't move around and interact. Just the same way that on a frozen world, the atoms in molecules can't move around and interact to from the chemistry necessary for life. But if temperatures and conditions are just right like here on earth, you get liquid water.