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How Many Planets Are Out There?

Wikipedia Creative Commons

  In this episode of Our Island Universe, since the conversation this week has been about the discovery of two more planets around the star HD 7924, we decided to take a look at how many planets are in our galaxy.

Transcript:

Twenty years ago, we knew we had 9 planets in our Solar System and there 1 exo-planet, a planet orbiting another star, that was known. Today, we know of nearly 2000 planets around just the nearest stars out there. So, given that the Milky Way Galaxy is home to more than 100 billion stars, how many planets are out there? How many Earth-like planets are out there?

Its remarkable to me that not only can we ask questions like this in astrophysics today, but we can actually estimate the number of planets that live in the Milky Way Galaxy. The technological achievement that has ushered in this revolution in thought is the Kepler Space Telescope. Now it is not possible to detect and count every single star just like its not possible to count every single sand grain on Earth. So what astronomers do instead is a statistical census and then extrapolate their result to the entire galaxy. 

Astronomers using data collected from the Kepler Space Telescope estimate, conservatively, that there are at least 100 billion planets in our Galaxy. For their study, they only considered the number of planets orbiting one particular kind of low mass star. When you account for the fact that there are other types of stars, the number grows to 200 billion planets! To put that in context, we now think that for every star, there is on average, at least 1-2 planets orbiting that star in just our Galaxy! Around stars just like our Sun, we think there are at 20 billion Earth-like planets!

In just 20 years, we’ve gone from thinking that we’re the only solar system in the Galaxy to now realizing that planets around stars are ubiquitous! What will we find in the next 20 years? Is life ubiquitous too? Our ancestors looked up at the night sky and stars. We now look up at night sky and see planets orbiting those stars. I’m convinced that when my grandchildren look up, they will see billions of worlds inhabited by diverse life forms.