Looking up at the night sky, it's hard not to wonder how many other Planets might be circling those pinpricks of light – and how many are home to beings gazing back at us.
Today, we are starting to get a handle on the number of roughly Earth-sized exoplanets that might be suitable for life.
How to spot a Planet
We can take a good guess at the number of alien Earths thanks to NASA's Kepler space telescope.
When a Planet passes in front of its parent star, it blocks some of the star's light. The Kepler telescope looked for distant worlds by measuring this dip in stars' glow.