Kepler: The Mission

The Kepler Mission is to find new planets that are close to Earth in size and composition. The scientific equipment is designed to detect planets as they pass in front of their stars, which causes a tiny dip in the stars’ light.

Think of standing way out beyond the edge of our solar system and just staring at the tiny face of the Sun for a decade or more. You would see regular patterns of tiny dots moving across the face of the Sun. Those dots would be Earth, Mars, and the other planets of our solar system. Every time one of those dots moved across the sun, there would be a change in the light output from the Sun.

Now, think of the Sun as just another star in the Milky Way galaxy. Kepler is simultaneously watching over 100,000 stars, every 30 minutes, waiting for the tiny little winks of light that happen when a planet crosses in front of its sun and changes the light. The change can last an hour or a half-day, depending on the planet’s orbit and the star.

100,000 stars! Think about that for a minute. Kepler is watching 100,000 stars, searching for those stars that have planets circling them. Even more precisely, Kepler is looking for habitable planets with approximately 1-year orbits (more on that below). What are the odds of finding those planets? Pretty good, actually! The Kepler team hopes to find

  • About 50 planets the same size as Earth.
  • About 185 planets that are 1.0 to 1.3 times larger than Earth.
  • About 12% of star/planet systems will contain 2 or more planets.

So why is Kepler focusing on planets with approximately 1-year orbits? It’s practicality, actually. Kepler is staring at the same section of space for 3 or 4 years (the longer the spacecraft can last, the better for observation purposes). It is looking for repeatable “winks” of light (same dimness and position every time) and it has to see those “winks” at least 2 or 3 times to determine there is a pattern – an orbit. Any wink that is seen less than 3 times is discounted. It may be a planet with a really long orbit but Kepler won’t be around long enough to ascertain if it is or not. Any wink that is seen more than 2 or 3 times (a shorter orbit than Earth’s) probably indicates a planet too close to its star to support our kind of life. So those get discounted too. What’s left are planets that are most similar to ours in size and in distance from their stars (their Suns). And Kepler is hoping to find over 200 planets in that category. 200 planets in a tiny section of a single galaxy.

And here’s what’s really amazing. Kepler is only looking at those stars from one angle. There may be planets that don’t show up in that angle of view. So the “200+ planets” figure is even more amazing because it’s a tiny bit of space viewed from a single viewpoint. Imagine what else is out there, beyond what we can see or measure now? It’s truly mind-boggling.


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