NASA's Kepler Space Telescope was an observatory in space dedicated to finding planets outside our solar system with a particular focus on finding planets that might resemble Earth. The observatory was in commission for just under 9 years, from its launch in March 2009 to its decommission on Nov. 15, 2018.

Since the launch of the observatory, astronomers have discovered thousands of extra-solar planets, or exoplanets, through this telescope alone. Most of them are planets that are between the size of Earth and Neptune (which is four times the size of Earth). Many of these planets were discovered in a small region of the constellation Cygnus, where Kepler was pointed for the first four years of its mission.

Kepler discovered 2,682 exoplanets during its tenure and there are more than 2,900 candidate planets awaiting confirmation — history suggests most of those are the real deal. The mission continued well beyond its scheduled end date, although problems with pointing in 2013 forced mission managers to create a K2 mission in which Kepler swings its view to different spots of the sky.

In the early years of exoplanet hunting, astronomers were best able to find huge gas giants — Jupiter's size and larger — that were lurking close to their parent star. The addition of Kepler (as well as more sophisticated planet-hunting from the ground) means that more "super-Earths" have been found, or planets that are just slightly larger than Earth but have a rocky surface. Kepler's finds also allow astronomers to begin grouping exoplanets into types, which helps with understanding their origins.

Primary mission

The $600-million Kepler was launched in 2009 with the expectation that it would last a year. It was part of NASA's Discovery program, which targets lower-cost spacecraft for exploration of the solar system; Kepler was selected in 2001 at the same time as Dawn, a spacecraft that visited the small worlds Vesta and Ceres.

Gazing at a fixed spot in the constellation Cygnus, the Kepler telescope continually monitored 100,000 main-sequence stars for planets. The telescope found exoplanets by watching for stars dimming as planets passed in front of them.

Because star dimming can also take place through other means (for example, another star slightly grazing the surface), in the early days these planets were confirmed through other telescopes, generally by measuring the gravitational "wobble" the planet has on the star.

In February 2014, however, astronomers pioneered a new technique called "verification by multiplicity," which works in multiple-planet systems. A star with multiple planets around it is gravitationally stable, according to the theory, while a star that is part of a close-knit system of stars would have a more unstable system because of each star's massive gravity. Through this technique, the team unveiled 715 confirmed planets in one release, which was then the largest single announcement. [Gallery: A World of Kepler Planets]

Kepler was approved far beyond its original mission length and was operating just fine until May 2013, when a second of its four reaction wheels or gyroscopes failed. The telescope needs at least three of these devices to stay pointed in the right direction. At the time, NASA said the telescope was still in good condition otherwise, and investigated alternate mission ideas for the hardware.

An artist's illustration of NASA's Kepler space telescope hunting for exoplanets. (Image credit: NASA)

New mission

Within a few months, the agency came up with a mission that it dubbed "K2." The mission would essentially use the sun's solar wind to stabilize the telescope's pointing for several months at a time. Then, about four times a year, the telescope, which is about 15 feet (4.7 meters) long and 9 feet (2.7 m) in diameter, would move to a different field of view when the sun got too close to its sensors.

While the pace of planetary discovery is less with the new mission, new finds continue to be announced. By January 2016, more than 100 new planets were discovered with the K2 method. "This is a validation of the whole K2 program's ability to find large numbers of true, bona fide planets," said Ian Crossfield, an astronomer at University of Arizona, during the American Astronomical Society's annual meeting, during which the find was announced.

Kepler examined the TRAPPIST-1 system — which likely has multiple Earth-sized planets in it — between December 2016 and March 2017. That February, another team of astronomers announced more Earth-sized planets had been found. Kepler scientists then released the raw data from their TRAPPIST-1 observations for other teams to analyze, if they were interested.

In February 2018, NASA put out another release of Kepler data with 95 new planets found during the K2 mission. One of those planets was orbiting a bright star, making it an easy candidate for follow-up by a ground observatory.

Major discoveries

Kepler's major achievement was discovering the sheer variety of planetary systems that are out there. Planet systems can exist in compact arrangements within the confines of the equivalent of Mercury's orbit. They can even orbit around two stars, much like Tatooine in the Star Wars universe. And in an exciting find for those seeking life beyond Earth, the telescope revealed that small, rocky planets similar to Earth are more common than larger gas giants such as Jupiter.

NASA had a voluminous announcement in February 2014, when the first two years of Kepler observations allowed astronomers to confirm 715 new worlds in one haul. The single bulk release of information nearly doubled the number of known planets to almost 1,700.

Another huge release of data came in May 2016, with 1,284 new planets announced. Kepler's finds at that time totaled 2,235 planets, with the number of overall exoplanets discovered (by all observatories) totalling about 3,200.

An artist's depiction of the long-lived Kepler space telescope and some of the planets it discovered during its near-decade in space. (Image credit: NASA)

The following year, in June 2017, came the final release of data from Kepler's primary mission. Kepler's confirmed planetary finds were boosted to 2,335. Including potential planets, the total count stood at 4,034.

Kepler also came through on its mission to find a planet approximately the size of Earth in the habitable region of a star. The exoplanet, dubbed Kepler-69c, is about 2,700 light-years away and has a diameter about 1.5 times that of Earth.

Other weird worlds discovered by the telescope include Kepler-62e and Kepler-62f, two water worlds that likely have a global ocean — as opposed to Earth, which has a significant fraction of dry land. The planets are about 1,200 light-years away in the constellation Lyra and are close to the size of Earth.

Long-term Kepler observations of the star KIC 8462852 revealed a bizarre pattern of dimming and brightening. Astronomers are still trying to figure out the nature of the brightness changes, which has been attributed to anything from comets to an uneven ring of dust to the less likely explanation that it is an alien megastructure.

Kepler's ability to look at the changing brightness of stars was exploited for the Pleiades, a well-known cluster of stars that is only 400 light-years away and visible to the naked eye. Kepler's observations provided the best tracking of their variability yet.

Mission's end

Kepler was launched with 3 gallons (12 kilograms) of hydrazine in its fuel tank. The fuel powers the thrusters that help correct drift and perform big maneuvers, including pointing to new fields of view and orienting its transmitters to Earth to downlink science data and receive commands. Since Kepler does not have a precise gauge on its fuel tank, engineers could only estimate when it was running out of fuel. In March 2018, NASA announced it expected the spacecraft's fuel tank to run dry in the following months. A little over seven months later, on Oct. 30, NASA confirmed that Kepler was out of gas, and the spacecraft was officially decommissioned on Nov. 15.

While Kepler has reached the end of its mission lifetime, another spacecraft is in position to take its place. A new exoplanet-hunting spacecraft called the Transiting Exoplanet Survey Satellite (TESS) launched on April 18, 2018 and began collecting data on July 25. Unlike Kepler, TESS isn't pointed in just one direction; instead, the spacecraft is scheduled to scan about 85 percent of the sky during its initial two-year mission. Mission officials expect TESS to uncover evidence of a few dozen rocky planets close to our planet, and many other planets of all types, building on Kepler's legacy.

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This article was updated on Dec. 7, 2018 by Space.com Reference Editor, Kimberly Hickok.