(Sen) - NASA's Kepler telescope, which is searching the galaxy for Earth-like planets, has completed its primary mission and is now beginning its extended mission through to 2016. The space telescope uses the transit method of planet hunting, monitoring over 150,000 stars to search for dips in light that could be caused by a planet crossing in front of its star.
During its first three and a half years Kepler has identified more than 2,300 potential planets. More than 100 have been confirmed as planets. Finding the dip in a light curve which indicates that a planet might be orbiting a star is only the first step in the process. These dips could also be caused by binary stars, and so follow up Doppler spectroscopy must be performed to confirm the mass of the transiting object, and thus if it is actually a planet. This is why the Kepler team have announced so many candidates compared to confirmed planets.
Kepler's mission now is to focus on finding Earth sized planets in the habitable zone of a star - where the temperature of the orbiting planet is just right for liquid water and potentially life to exist - and which have an orbital period of one year like Earth.
Principal investigator at NASA's Ames Research Center, William Borucki, said: "The initial discoveries of the Kepler mission indicate that at least a third of the stars have planets and that the number of planets in our galaxy must number in the billions. The planets of greatest interest are other Earths and these could already be in the data awaiting analysis. Kepler’s most exciting results are yet to come!"
The space telescope, which launched on March 6, 2009, was granted a four year extention earlier this year. As a space telescope Kepler sits above the atmosphere and has a uninhibited view of the light from distant stars. Just as the transit of Venus was observed from Earth earlier this year, other planets can also be seen crossing the face of their parent star. Different sized planets block different amounts of starlight and the amount of starlight blocked by a planet's transit reveals its size relative to its star.
Kepler's field of view is a 105 square degree star field located within the constellations of Cygnus and Lyra. This viewpoint was chosen due to both the amount of stars and because it is above the ecliptic plane.
Artist's illustration of Kepler-22b which orbits its star in the habitable zone. Credit: NASA/Ames/JPL-Caltech
The most Earth-like planet discovered so far is Kepler-22b. This planet is 2.4 times larger than Earth and is the first Kepler planet found to be orbiting its star in the habitable zone.
There are many highlights from Kepler's primary mission. In August 2010 Kepler discovered the Kepler-9 star system, the first multi-planetary system outside our own solar system.
In January 2011 Kepler announced the discovery of the first rocky planet outside the solar system, Kepler-10b. This planet, 1.4 times larger than Earth, was the first small rocky world to be confirmed. Other potentnial rocky worlds have subsequently been found suggesting that as well as numerous 'hot Jupiters' - large gas giants orbiting close to their parent star - there may be many smaller rocky worlds like Earth and Mars too.
Another discovery made by Kepler is the multi-planetary system designated Kepler-11 which has six planets larger than Earth all orbiting their star at a very close distance - closer than Venus orbits the Sun.
Kepler-16b is like the fictional planet Tatooine in Star Wars, a planet orbiting two stars. Observers on the surface of Kepler-16b would see two sunsets like those observed by Luke Skywalker and other Tatooine inhabitants. Another double star system was announced in September 2012 when Kepler revealed it had discovered its first circumbinary planetary system - multiple planets orbiting two stars. The binary star system, designated Kepler-47, is 4,900 light-years from Earth in the constellation Cygnus.
Artist illustration of Kepler-16b, a planet with two stars. Credit: NASA/JPL-Caltech/R. Hurt
Kepler's extended mission will continue analysing the data it has captured and focus on identifying smaller planets with orbital periods similar to Earth. To help analyse the vast amounts of data from its scanning of over 150,000 stars Kepler has teamed up with citizen science organisation Zooniverse to create Planet Hunters, enabling the public to access the data searching for dips in light. In January 2012 Planet Hunters confirmed that participants had found their first planet.
NASA's Kepler mission is one of a number of instruments looking for exoplanets. The European Southern Observatory's HARPS (High Accuracy Radial velocity Planet Searcher) instrument on its 3.6-metre telescope at the La Silla Observatory in Chile is also used to gather data to detect planets. NASA's Spitzer telescope is also used to perform follow up observations of exoplanets in the infrared region of the spectrum.
The Kepler mission development is managed by NASA's Jet Propulsion Laboratory (JPL) in Pasedena, California, whilst data analysis is managed by NASA's Ames Research Center.