Kepler: The Original Planet Hunter
The Kepler Space Telescope was the first telescope launched with its primary mission to search for exoplanets. Kepler was launched in 2009 and:
Artist concept of exoplanet Kepler 452b side by side with Earth. Credit: NASA
Kepler Field of View. Credit: NASA
Are we alone in the universe? Are there many planets that look like earth? Or is the rocky planet that we call home a rare occurrence? Kepler's mission was to find earth-sized planets in or near the habitable zone of a star. In order to do so it pointed its photometer at a portion of Earth's region of the Milky Way.
Before Kepler became operation only a couple dozen exoplanets had been discovered. But after Kepler found its first 5 exoplanets in 2010 the flood gates were opened. As of November 2019 Kepler has found over 4000 exoplanets with more than 2000 planet candidates yet to be confirmed. Check out NASA's Exoplanet Archive for the most recent data.
In order to find these exoplanets Kepler used its 95MP image array to look at approximately 150.000 main sequence stars. These types of stars are in a similar point of their lifespan as our own sun. By looking at stars similar to ours this increases the chances of finding rocky exoplanets like earth in the inhabitable zone where liquid water can be present and maybe even life.
The main method employed by Kepler to detect exoplanets is know as Transit Photometry. Whenever a planet passes in front of its parent star a small portion of starlight is blocked the same way the moon blocks the sun during an eclipse. Only on a much smaller scale. By constantly being pointed at the same point in the sky, monitoring the same stars, Kepler is able to measure starlight emission over time and detect these minute dips.
Transit Method. Credit: NASA
Kepler's original mission was designed for 3.5 years. However it was built with a larger than necessary fuel tank which in the end was completely filled up because the lift vehicle (the rocket) was not at maximum capacity. This gave Kepler about 10 years of fuel when launched in 2009. Even when reaction wheels used to keep the space craft pointed started failing in 2013 clever tricks were able to keep the telescope operational until the end of 2018 when Kepler was retired on October 30th.