Alien world. Extrasolar planet. Exoplanet. Whatever name you like to use for the planets we've been discovering around other stars, the catalogues of these finds are starting to fill up, and several of them are drawing close to a milestone, as we have almost reached a total of 1,000 confirmed planets!
With all the planets astronomers have found so far, or at least found evidence for, there are a total of five different catalogues kept to log these discoveries. Each has their own method of keeping track of all the planets, and their own 'criteria' for whether or not a planet has been confirmed. All planets are considered 'unconfirmed' when evidence for them has first been found, and from all the various missions currently searching, scientists have logged around 4,700 potential planets. However, of that total, several hundred of them have been confirmed, either by scientists carefully going over the data or by other telescopes also seeing the same evidence.
The Extrasolar Planet Encyclopaedia, run by the Paris Observatory, in France, and the PHL Exoplanets Catalog, of the University of Puerto Rico at Arecibo, are both closest to the milestone with 990 confirmed planets. The Open Exoplanet Catalogue is next in line, with 948 confirmed worlds, and the NASA Exoplanet Archive follows with 906 confirmed planets. The Exoplanet Orbit Database, which specifically takes its info from peer-reviewed works, and only includes 'confidently detected exoplanets', lists 734 confirmed worlds.
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Along with all of these confirmed worlds, each database lists unconfirmed as well, with the greatest number coming from NASA's Kepler telescope. Even though mechanical problems forced the Kepler team to end its primary mission, there are still roughly 3,500 candidates on the list as of now, and scientists (professional and citizen) have only gone through a tiny percentage of the data so far. Of all the different missions up until now, Kepler's database has the best chances of making the list of confirmed worlds jump above the 1,000 mark, and it could propel it far beyond to set even more milestones.
If you'd like to help find, you can join the Planet Hunters team as a citizen scientist. With the incredible amount of data Kepler has gathered during its four years of searching, they need all the help they can get to sift through it all to find new worlds. There are tutorials on the website to get you started, and this video gives a great introduction to the project:
(Image courtesy: PHL/University of Puerto Rico at Arecibo)
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