NASA's Kepler space telescope has been a hard worker since it was launched in 2009, having discovered 70 percent of all of the extrasolar planets to date -- those are planets that are orbiting stars other than our Sun. Today, October 30, 2018, Kepler's mission was officially ended after the spacecraft ran out of fuel about two weeks ago.The maneuvering fuel was necessary for Kepler to orient itself to observe objects around distant stars and then transmit that data back to Earth. Kepler mission managers had been watching the fuel situation closely for months, even putting the spacecraft in a "sleep mode" several times to stretch its operational life. Since Kepler is in a solar, not Earth, orbit, it was impossible to send a refueling spacecraft to "top it off" for extended life.Kepler searched for extrasolar planets using what's called the "transit method", basically looking for reduced brightness from a star caused when a planet crosses in front of it from the perspective of the spacecraft. The spacecraft was initially set up for staring at one small area of space to study 150,000 stars at the same time. That work resulted in 2,327 confirmed exoplanet discoveries.Kepler's science shows that planets outnumber stars in our galaxy and that Earth-like planets are quite common -- 20 percent of stars like our sun appear to have rocky planets that are in a distance from the star where water could exist as a liquid.Kepler's work is continuing through a new spacecraft (TESS) launched in April of 2018.
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