Kepler Space Telescope: Exoplanet Hunter
The field of stars that the Kepler space telescope examined as it searched for exoplanets.
Credit: NASA/JPL

NASA's Kepler Space Telescope is an observatory in space dedicated to finding planets outside our solar system, particularly alien planets that are around the same size as Earth in the "habitable" regions of their parent star. In March 2018, NASA announced that Kepler is running low on fuel and is expected to cease operations within several months.

Since the launch of the observatory in 2009, astronomers have discovered thousands of extra-solar planets, or exoplanets, through this telescope alone. Most of them are planets that are ranging between the size of Earth and Neptune (which itself is four times the size of Earth). Most of these planets were discovered in a small region of the constellation Cygnus, at which Kepler was pointed for the first four years of its mission.

As of March 2018, Kepler had found 2,342 confirmed planets; add potential planets, and its find of exoworlds stands at 4,587. The mission continues to operate 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. 

A powerful new technique for hunting alien planets yields a major new crop of new worlds. [<a href="http://www.space.com/24827-kepler-space-telescope-exoplanet-bonanza-explained-infographic.html">See how Kepler made the planet discoveries in this Space.com infographic</a>]
A powerful new technique for hunting alien planets yields a major new crop of new worlds. [See how Kepler made the planet discoveries in this Space.com infographic]
Credit: By Karl Tate, Infographics Artist

The $600-million Kepler was originally launched in 2009 with the expectation that it would last a year. (It is 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 detected these exoplanets through watching for stars dimming as planets pass 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 well 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 perfect health otherwise, and investigated alternate mission ideas for the hardware.

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, said during the American Astronomical Society's annual meeting, at 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 that were 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.

This conception illustration depicts how solar pressure can be used to balance NASA's Kepler spacecraft, keeping the telescope stable enough to continue searching for transiting planets around distant stars.
This conception illustration depicts how solar pressure can be used to balance NASA's Kepler spacecraft, keeping the telescope stable enough to continue searching for transiting planets around distant stars.
Credit: NASA Ames/W Stenzel

Kepler's major achievement is showing the sheer variety of planetary systems that are available. Planet systems can exist in compact arrangements within the confines of the equivalent of Mercury's orbit. They can 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 has revealed that small, rocky planets similar to Earth are more common than larger gas giants such as Jupiter.

Kepler had a voluminous announcement in February 2014, when astronomers unveiled 715 new worlds confirmed in one go. The single release of information nearly doubled the number of known planets to that point to almost 1,700. Astronomers noted this find, using the verification by multiplicity technique, came out of the first two years of Kepler data. 

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. 

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 count stood at 4,034. Astronomers — unexpectedly — also found a clear division between super-Earths (rocky planets that are up to 1.75 times Earth size) and mini-Neptunes (gassy planets 2 to 3.5 times the size of Earth). 

Kepler was the first telescope to find a planet approximately the size of Earth in the habitable region of a star. Dubbed Kepler-69c, the exoplanet is about 2,700 light-years away and has a diameter about 1.5 times that of Earth.

The telescope also has the capability to find planets that are much smaller than Earth, such as Kepler-37b. The planet is considered to be close to Mercury's size and is likely rocky and airless, much like the planet in our own solar system.

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; the telescope also found no new exoplanets in the region. 

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 can only estimate when it is running out of fuel. In March 2018, NASA said it expects the spacecraft will run dry in several months. The agency is monitoring the spacecraft for signs of fuel running low, such as changes in pressure in the fuel tank and differences in the thruster performance.

Kepler, however, has an advantage over other spacecraft running low on fuel. Its orbit is nowhere near the Earth or any other major planet, so it is not posing a possible threat. That's a contrast from both the Galileo and Cassini missions, which were near Jupiter and Saturn respectively. NASA elected to steer Galileo into Jupiter in 2003, and Cassini into Saturn in 2017, to remove the small threat of these spacecraft crashing into an icy moon and potentially contaminating each surface.

"The Kepler team is planning to collect as much science data as possible in its remaining time and beam it back to Earth before the loss of the fuel-powered thrusters means that we can't aim the spacecraft for data transfer. We even have plans to take some final calibration data with the last bit of fuel, if the opportunity presents itself," read a statement from Charlie Sobeck, system engineer for the Kepler space telescope mission.

While Kepler is nearing its mission lifetime, another spacecraft is readying to take its place. A new exoplanet-hunting spacecraft called Transiting Exoplanet Survey Satellite (TESS) will launch no earlier than mid-April 2018 to do an all-sky survey from an orbit between the Earth and the moon. TESS is expected to uncover evidence of a few dozen rocky planets close to our planet, and many other planets of all types. It will monitor stars that are closer to Earth, providing more opportunities for ground observatories to follow up on the data collected.

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