Eris (dwarf planet)
Eris (minor-planet designation: 136199 Eris) is the most massive and second-largest known dwarf planet in the Solar System.[22] It is a trans-Neptunian object (TNO) in the scattered disk and has a high-eccentricity orbit. Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown and verified later that year. It was named in September 2006 after the Greco–Roman goddess of strife and discord. Eris is the ninth-most massive known object orbiting the Sun and the sixteenth-most massive overall in the Solar System (counting moons). It is also the largest known object in the solar system that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometers (1,445 ± 7 mi) in diameter;[12] its mass is 0.28% that of the Earth and 27% greater than that of Pluto,[23][24] although Pluto is slightly larger by volume.[25] Both Eris and Pluto have a surface area that is comparable to the area of Russia or South America.
This article is about the dwarf planet. For the asteroid with a similar name, see 433 Eros. For other uses, see Eris.Discovery
January 5, 2005[2]
(136199) Eris
Ἔρις Eris
2003 UB313[6]
Xena (nickname)
Eridian /ɛˈrɪdiən/[9][10]
.svg){kind=small}
(mostly astrological)
September 3, 1954
38.271 AU (5.725 Tm)
67.864 AU (10.152 Tm)
0.43607
3.434 km/s
205.989°
0° 0m 6.307s / day
44.040°
35.951°
≈ December 1, 2257[11]
±11 days
151.639°
1 (Dysnomia)
2326±12 km
(1.70±0.02)×107 km2[a]
(6.59±0.10)×109 km3[a]
2.43±0.05 g/cm3[14]
1.38 ± 0.01 km/s[c]
15.786 d (synchronous)[15]
18.7[20]
–1.21[6]
34.4±1.4 milli-arcsec[21]
Eris has one large known moon, Dysnomia. In February 2016, Eris's distance from the Sun was 96.3 AU (14.41 billion km; 8.95 billion mi),[20] more than three times that of Neptune or Pluto. With the exception of long-period comets, Eris and Dysnomia were the most distant known natural objects in the Solar System until the discovery of 2018 AG37 and 2018 VG18 in 2018.[20]
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, Pluto and Ceres are "dwarf planets",[26] 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 it was slightly smaller than Pluto,[27][28] which was measured by New Horizons as having a mean diameter of 2,377 ± 4 kilometers (1,477 ± 2 mi) in July 2015.[29][30]
Rotation[edit]
Eris displays very little variation in brightness as it rotates due to its uniform surface, making measurement of its rotation period difficult.[77][15] Precise long-term monitoring of Eris's brightness indicates that it is tidally locked to its moon Dysnomia, with a rotation period synchronous with the moon's orbital period of 15.78 Earth days.[15] Dysnomia is also tidally locked to Eris, which makes the Eris–Dysnomia system the second known case of double-synchronous rotation, after Pluto and Charon. Previous measurements of Eris's rotation period obtained highly uncertain values ranging tens of hours to several days due to insufficient long-term coverage of Eris's rotation.[77][78][79] The axial tilt of Eris has not been measured,[14] but it can be reasonably assumed that it is the same as Dysnomia's orbital inclination, which would be about 78 degrees with respect to the ecliptic.[16] If this were the case, most of Eris's northern hemisphere would be illuminated by sunlight, with 30% of the hemisphere experiencing constant illumination in 2018.[16]
Exploration[edit]
Eris was observed from afar by the outbound New Horizons spacecraft in May 2020, as part of its extended mission following its successful Pluto flyby in 2015.[17] Although Eris was farther from New Horizons (112 AU) than it was from Earth (96 AU), the spacecraft's unique vantage point inside the Kuiper belt permitted observations of Eris at high phase angles that are otherwise unobtainable from Earth, enabling the determination of the light scattering properties and phase curve behavior of the Eridian surface.[17]
In the 2010s, there were multiple studies for follow-on missions to explore the Kuiper belt, among which Eris was evaluated as a candidate.[83] It was calculated that a flyby mission to Eris would take 24.66 years using a Jupiter gravity-assist, based on launch dates of April 3, 2032, or April 7, 2044. Eris would be 92.03 or 90.19 AU from the Sun when the spacecraft arrives.[84]