Kepler was a space-based telescope originally designed to search for Earth-sized planets orbiting sunlike stars. Scientists refer to planets outside of the solar system as extrasolar planets or exoplanets. The telescope began making observations in May 2009. The telescope watched many stars simultaneously for small changes in brightness that might be caused by a passing planet. Kepler’s main goal was to find small, rocky planets—called terrestrial (Earthlike) planets—that orbit within their star’s habitable zone. In this region, temperatures allow for the existence of liquid water, which many scientists think is essential for life. The mission also helped scientists understand the variety of planetary systems that exist around sunlike stars. The United States National Aeronautics and Space Administration (NASA) launched Kepler in March 2009. The telescope trailed behind Earth in its orbit around the sun. Kepler is named for Johannes Kepler, the German astronomer who discovered the laws of planetary motion.
Kepler spacecraft
Kepler looked for the decrease in brightness that occurs when a planet passes in front of its star, an event called a transit. During a transit, a terrestrial planet blocks a small fraction of the star’s light from reaching Earth—roughly 1 part in 10,000. The tiny dimming for such planets typically lasts from a few hours to as long as 16 hours. Kepler hunted for transits that repeat at regular intervals ranging from a few days to two years. Planets that orbit their stars in such periods could transit at least three times over the possible duration of the mission, enabling scientists to confirm their presence.
For Kepler to see a transit, the planet’s orbit had to be angled so that it appears nearly edge-on as viewed from Earth. How nearly edge-on depends on the size of the star and the size of the planet’s orbit. For an Earth-sized planet that orbits a sun-sized star at the same distance that Earth orbits the sun, scientists calculate the chances of seeing a transit at about 0.5 percent. To increase its chances of finding such planets, Kepler continuously observed approximately 100,000 stars. Even if few stars have Earth-sized planets, Kepler would detect some of them.
Loading the player...Kepler field of view
Kepler collected light with a primary (main) mirror measuring about 4.6 feet (1.4 meters) in diameter. The craft measured the light using 42 sensors called charge-coupled devices or CCD’s (see Charge-coupled device ). Scientists expected Kepler to detect hundreds of giant, gaseous planets similar to Jupiter orbiting close to their stars. Astronomers had previously found many such planets using ground-based telescopes. Because Kepler’s observations were free of the distorting effects of Earth’s atmosphere, Kepler could detect smaller planets orbiting farther from their stars. The mission specifically looked for terrestrial planets that circle their stars in about a year. Kepler identified thousands of exoplanets . A number of these are thought to be small, terrestrial planets orbiting within their star’s habitable zone.
The Kepler mission was originally planned for 31/2 years of observation, but in April 2012, it was extended through 2016. However, the telescope ceased planet-hunting operations in May 2013. A malfunction left Kepler with only two usable reaction wheels, devices used to stabilize the telescope. Without the third axis of control provided by three functioning wheels, the telescope could not permanently hold steady in one spot in the sky, ending its primary mission. In 2014, scientists came up with a solution to the lack of a third functioning wheel. Using the pressure created by the solar wind, a constant stream of particles from the sun, the scientists could keep the telescope pointed in a particular direction. As the telescope orbited the sun, however, the direction of the solar wind changes relative to the objects in the sky. The telescope could thus no longer look at one spot indefinitely, but instead would rotate every few weeks to a new position.
NASA officially retired Kepler on Oct. 30, 2018, after it had run out of fuel needed for further science operations. Kepler will remain in its current orbit behind Earth indefinitely, but all mission functions and observations have ended. The space telescope discovered more than 2,600 new planets outside our solar system over the course of its mission. In April 2018, NASA launched the Transiting Exoplanet Sky Survey (TESS) telescope to continue the mission of searching for new planets using space-based telescopes that began with Kepler.
