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.
Since the launch of the observatory in 2009, astronomers have discovered hundreds 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).
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.
The $600-million Kepler was originally launched in 2009 with the expectation that it would last a year. Gazing at a fixed spot in the constellation Cygnus, the 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 would move to a different field of view when the sun got too close to its sensors.
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's largest discovery in sheer volume occurred 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. Two years of data remain to be studied, and investigators expect hundreds more could come out of examining that information.
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.