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On February 27, the team of astronomers involved with NASA’s spacecraft Kepler published their most recent catalog of exoplanets (short for extrasolar planets, which are planets beyond our solar system) that Kepler has detected.
Data from the newest catalog is cumulative and includes information from the two catalogues created in June 2010 and February 2011. As of now, the total number of exoplanets Kepler has detected is 2,321, which orbit 1,790 stars. A full 93% are smaller than Neptune, the smallest of the gas giants in the solar system. Over 200 are Earth-sized and more than 900 are smaller than twice the size of the Earth’s diameter. There are 46 exoplanets located in the habitable zone, 10 of which are Earth-sized.
“With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging,” Natalie Batalha, Kepler deputy science team lead at San Jose State University in California, states in NASA’s press release. “This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant.”
Furthermore, the percent for more than one planet orbiting a star has increased to 20 percent from last year’s 17 percent (many other planets are rogue, unattached to a parent star, twirling alone in space). More and detailed statistics can be found here.
Three methods can be utilized to find exoplanets: gravitational lensing, radial-velocity, and transiting. Kepler largely uses the latter method, using the software Transiting Planet Search (TPS) pipeline module, because it has proven to produce more results compared to the former two. Transiting works as thus: one measures a star’s periodic drop in brightness due to an object – in the most hopeful scenario, a planet – passing in front of the star.
Sifting through 150,000 stars, Kepler detected around 5,000 transit signals, through which the spacecraft had more to sort. One can easily misidentify an object to be an exoplanet when using the transiting method; one may instead find a binary star system, which contains two stars that orbit and eclipse one another. To confirm its detection of a planet, Kepler has to record the transit at least three times.
Kepler was launched in mid-2009 to find Earth-like exoplanets that are able to sustain water and life. These planets would have to be located in the habitable zone, an area in which a planet must orbit a star in order for liquid water to exist on its surface. Kepler goes about attempting to detect exoplanets by looking at their parent stars first, largely searching for G-type stars, or Sun-like stars (or at least stars a part of the Main Sequence), which astronomers believe to be ideal parent stars.
For much of the time Kepler began exploring, it mostly detected gas giants tens of times larger than Jupiter. As the spacecraft endured, it began, recently, to find numerous smaller rocky planets. Soon after, astronomers working with Kepler have calculated that there are more of these kinds of planets than there are gas giants.
Kepler’s latest milestone includes Kepler-20e and Kepler-20f, which were detected this January and are the first Earth-sized planets known to exist. Another milestone occurred in December 2011, when Kepler discovered super-Earth Kepler-22b, the first known exoplanet in the habitable zone.