|
From the total of known exoplanets orbiting around different stars (as of ), only a small fraction are located in the vicinity of the Solar System. At the beginning of 2015, the nearest 71 confirmed exoplanets were located within , with some 38 other unconfirmed candidates. Among the estimated 1,400 stars located within 50 light-years, only 37 have confirmed planetary systems and 13 have only unconfirmed exoplanets. Of the 133 stars within 50 light-years that are bright enough to be visible with the naked eye,〔 and only 18 have confirmed planetary systems. Among the nearest 100 known star systems that are less than 20 light-years away, only three (Gliese 674, Gliese 876, and Gliese 832) have confirmed exoplanets, although proposals have been made for another ten systems. Reports of planetary systems first came in 1996 for three stars located over 40 light-years away: 55 Cancri, Upsilon Andromedae,〔 and 47 Ursae Majoris. Since 1999, more planets have been reported, including a total of five planets revolving around 55 Cancri, and four planets around Gliese 876 and Upsilon Andromedae. Eight star systems have three confirmed planetary partners, four others have only two planets, while the remaining 20 systems have only one confirmed planet. A total of seven planets have been suggested for Gliese 667 C (only two have been confirmed), six around HD 40307 (three confirmed), and a total of five planets revolving around Tau Ceti (not yet confirmed). The closest exoplanet widely accepted as confirmed is Gliese 674 b, 14.8 light-years away from our Solar System. Among the other 42 closer star systems, some have been suggested to contain planetary companions, including the closest system, Alpha Centauri (4.1 light-years away), with a companion that may be similar in size to Earth. Of the known planetary systems, 21 are located within 40 light-years, eleven are within 30 light-years, and only four are within 20 light-years. Among of the 71 planets within 50 light-years which had been confirmed by the beginning of 2015, more than half were found to revolve around their star closer and complete an orbit faster than Mercury does around the Sun, and 18 have a more eccentric orbit than that of Mercury. Considering their minimum masses, 50 exoplanets are expected to be gas giants, and only 21 under ten times more massive than Earth. The smallest of these has an estimated minimum mass of 2.4 times that of Earth's and revolves around 82 G. Eridani, 20 light-years away. A few exoplanets within 50 light-years have been hypothesized to be potentially habitable, including Gliese 667 Cc and Gliese 163 c,〔 but only three have been directly imaged: Fomalhaut b, Ross 458(AB) c, and VHS 1256-1257 b. In 2015, the International Astronomical Union announced intention allow the public to vote on renaming some exoplanetary systems, among which: Epsilon Eridani, Fomalhaut, Pollux, 55 Cancri, Upsilon Andromedae, Gamma Cephei, 47 Ursae Majoris, and Mu Arae. ==Inclusion criteria== There is no official organization that acknowledges reports for the existence of exoplanets, but the Working Group on Extrasolar Planets of the International Astronomical Union adopted in 2003 a working definition limiting the upper masses of exoplanets below that where thermonuclear fusion of deuterium occurs. This limit is calculated to be around 13 times the mass of Jupiter, and above it, an object is classified as a brown dwarf. Usually, nearby exoplanets have been discovered by measuring the changes in the radial velocity of the host star, which allows for a precise estimate of their orbital parameters, but does not allow for an estimate of their orbital inclination (''i''). Because of this challenge, generally only their minimum masses (Mreal *sin(''i'')) are known, as only 17 of the 71 exoplanets there have been any estimate on their inclination. It is generally expected that most of these masses to be close to their true masses, as there is only about 13% chance for the mass of an exoplanet to be more than double its minimum mass. However, there have been a few cases where planetary candidates were suggested but later refinement showed that these were more than 13 times more massive than Jupiter, and thus are likely brown dwarfs. This has been the case for WISE 0458+6434 B, WISE 1217+1626 B, 2M 0746+20 B,〔(【引用サイトリンク】title=2M 0746+20 )〕 SDSS J1416+1348 B, HD 104304 B, SCR 1845-6357 B, and Gliese 22 B. For the purpose of this list, an exoplanet is regarded as unconfirmed when there is only a single (primary) report which presents its discovery, but there are no follow-up papers discussing their existence. International Astronomical Union has mentioned that "a period of at least five years since the discovery has been considered as a simple and satisfactory criterion to include exoplanets which can be considered as confirmed". There are a few examples where the existence of an exoplanet has been discussed in multiple papers, but there exists a minority of skeptics. For example, Alpha Centauri Bb has received criticism on the methods used to propose the existence of the planet. Several proposals of other nearby exoplanets have been made, but because their existence has been widely disproven since, they are not included in this list, as was the case for proposed candidates around Teegarden's star, VB 10, Gliese 581, Gliese 876, and CM Draconis. There are known examples of potential free-floating sub-brown dwarfs, sometimes referred as "rogue planets", that are excluded from this list, such as WISE 0855–0714, UGPS 0722-05, WISE 1541−2250, and WISE 1828+2650. Notable uncertainties exist for the distances measured for some stars. Because of this, some sources regard some stars as being within the 50 light-years (ly) threshold, but these stars were not included in the current list: 51 Pegasi (50.1 ly), Tau Boötis (50.9 ly), Gliese 758 (51.4 ly). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「List of nearest exoplanets」の詳細全文を読む スポンサード リンク
|