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For centuries, human beings have wondered about the possibility of other Earths orbiting distant stars. Perhaps some of these alien worlds would harbor strange forms of life or future stories. But it was only in 1995 that astronomers spotted the first planets orbiting sunlike stars outside of our solar system.
In the last decade, in particular, the number of planets known to orbit distant stars grew to 2,000, with another 2,000 likely planets awaiting confirmation. Most of these new discoveries are due to a single endeavor – NASA's Kepler mission.
Kepler is a spacecraft housing a 1-meter telescope that illuminates a 95 megapixel digital camera the size of a cookie sheet. The instrument detected tiny variations in the brightness of 150,000 distant stars, looking for the telltale sign of a planet blocking a portion of the starlight as it transits across the telescope's line of sight. It's so sensitive that it could detect a single streetlight in Chicago from an orbit above the Earth. It can see stars shake and vibrate; it can see starspots and flares; and, in favorable situations, it can be seen as small as the moon.
Kepler's thousands of discoveries revolutionized our understanding of planets and planetary systems. Now, however, the spacecraft has run out of its hydrazine fuel. Luckily for planet hunters, NASA's TESS mission launched in April and will take over the exoplanet search.
Kepler's history
The Kepler mission was conceived in the early 1980s by NASA scientist Bill Borucki, with later help from David Koch. At the time, there were no known planets outside the solar system. Kepler was eventually assembled in the 2000s and launched in March 2009. Joined the Kepler Science Team in 2008 (as a wide-eyed rookie), eventually co-chairing the group studying the motions of the planets with Jack Lissauer.
Originally, the mission was planned to last for a few years with possible extensions for the fuel, or the camera, or the spacecraft lasted. As time passed, the end of the mission has failed. However, in 2013 when two of its stabilizing gyros (technically "reaction wheels") stopped, the original Kepler mission ended.
Even then, with some ingenuity, NASA was able to use the light of the Sun to help steer the spacecraft. The mission was rechristened as K2 and continued finding planets for another half decade. Now, with the fuel gauge near empty, the business of planet hunting is winding down and the spacecraft will be left adrift in the solar system. The final catalog of planet candidates is the last mission and the last observations of K2 are wrapping up.
Kepler's science
Squeezing what knowledge we are going to have, but what we've seen so far has amazed scientists across the globe.
We have seen some planets that orbit their stars and are so hot that the surface rock vaporizes and trails behind the planet like a comet tail. Other systems have planets so close together that if you were to stand on the surface of one, the second planet would appear larger than 10 full moons. One system is so packed with planets that the Earth is to the Sun. Many have planets, and sometimes multiple planets, orbiting within the habitable zone of their star, where liquid water may exist on their surfaces.
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As with any mission, the Kepler package cam with trade-offs. It needed to be a part of the sky, blinking every 30 minutes, for four straight years. The stars had to be quite distant – just as when you stand in the middle of a forest, there are more trees than you right here. Distant stars are dim, and their planets are hard to study. Indeed, one challenge for astronomers who wants to study the properties of Kepler planets is that Kepler itself is often the best instrument to use. High quality data from ground-based telescopes requires long observations on the largest telescopes that can be observed.
We now know that there are many planets in the galaxy, and many of the planets are quite unlike what we have here in the solar system. Telescopes can be brought to bear by telescopes orbiting brighter and closer.
NASA's Transiting Exoplanet Satellite Survey mission, led by MIT's George Ricker, is searching for planets using the same technical detection that Kepler used. TESS 'orbit, rather than being around the Sun, has a close relationship with the Moon: TESS orbits the Earth twice for each lunar orbit. TESS 'observing pattern, rather than staring at a single part of the sky, will scan the whole sky with overlapping fields of view (much like the petals on a flower).
Given what we learned from Kepler, astronomers expect to find more planetary systems. Kepler – opening up new possibilities for measuring planet and densities, studying their atmospheres, characterizing their host stars, and establishing the full in which the planets reside. This information, in turn, will tell us more about our own planet's history, how life may have started, what we were avoiding and what other paths we could have followed.
The quest to find our place in the universe continues as Kepler finishes its leg of the journey and TESS takes the baton.
Jason Steffen is an Assistant Professor of Physics and Astronomy at the University of Nevada, Las Vegas. This article was originally featured on The Conversation.
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