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| perihelion = | semimajor = | eccentricity = | period = | avg_speed = | mean_anomaly = | inclination = | asc_node = | arg_peri = | satellites = Dysnomia | physical_characteristics = yes | mean_radius = 〔〔 | mass = | surface_area = | volume = | density = | surface_grav = ''g'' | escape_velocity = ±0.01 km/s | sidereal_day = 〔 | albedo = 〔 | temperatures = yes | temp_name1 = (approx) | min_temp_1 = 30 K | mean_temp_1 = 42.5 K | max_temp_1 = 55 K | spectral_type = B−V=0.78, V−R=0.45〔 | magnitude = 18.7〔 | abs_magnitude = | angular_size = 40 milli-arcsec〔 | pronounced = or | adjectives = Eridian }} Eris (minor-planet designation 136199 Eris) is the most massive and second-largest dwarf planet〔 known in the Solar System. It is also the ninth-most-massive known body directly orbiting the Sun,〔Eris is more massive than Pluto even though Pluto has a larger diameter. It is only the 16th-most-massive known body in the Solar System, because seven moons are more massive than any known dwarf planet.〕 and the largest known body in the Solar System not visited by a spacecraft. It is measured to be in diameter.〔 Eris is 27% more massive than dwarf planet Pluto, though Pluto is slightly larger by volume.〔 Eris's mass is about 0.27% of the Earth's mass.〔〔 Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown, and its identity was verified later that year. It is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disk. It has one known moon, Dysnomia. , its distance from the Sun is ,〔 roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the second most distant known natural objects in the Solar System (following the November 2015 discovery of V774104 at 103 AU).〔(AstDyS-2 list of minor planets more than 57.0 AU from the Sun )〕 Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union (IAU) to define the term ''planet'' for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea and Makemake,〔 thereby reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellar occultation by Eris in 2010 showed that its diameter was , not significantly different from that of Pluto.〔〔 After ''New Horizons'' measured Pluto's diameter as in July 2015, it was determined that Eris is slightly smaller in diameter than Pluto. ==History== 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「Eris (dwarf planet)」の詳細全文を読む スポンサード リンク
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