Eris (minor planet designation 136199 Eris) is the most massive and second-largest known dwarf planet in the Solar System. Eris is a trans-Neptunian object (TNO), has a high-eccentricity orbit, and is a member of the scattered disk. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its discovery was verified later that year. In September 2006 it was named 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 (including moons). It is also the largest object that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometers (1,445 ± 7 mi) in diameter. Its mass is 0.27 percent that of the Earth and 127 percent that of dwarf planet Pluto, though Pluto is slightly larger by volume. As Eris orbits the Sun, it completes one rotation every 25.9 hours, making its day length similar to Earth's. However, other sources disagree on the rotation period.
It has one large known moon, Dysnomia. In February 2016, its distance from the Sun was 96.3 astronomical units (1.441×1010 km; 8.95×109 mi), roughly three times that of Pluto. With the exception of some long-period comets, until 2018 VG18 was discovered on December 17, 2018, Eris and Dysnomia were the most distant known natural objects in the Solar System.
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 it was very slightly smaller than Pluto, which was measured by New Horizons as 2,377 ± 4 kilometers (1,477 ± 2 mi) in July 2015.
Size and Distance :
With a radius of about 722 miles (1,163 kilometers), Eris is about 1/5 the radius of Earth. Eris, like Pluto, is a little smaller than Earth's Moon. If the Earth were the size of a nickel, Eris would be about as big as a popcorn kernel.
From an average distance of 6,289,000,000 miles (10,125,000,000 kilometers), Eris is about 68 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight more than nine hours to travel from the Sun to the surface of Eris.
Orbit and Rotation :
Eris takes 557 Earth years to make one trip around the Sun. The plane of Eris' orbit is well out of the plane of the solar system's planets and extends far beyond the Kuiper Belt, a zone of icy debris beyond the orbit of Neptune.
As Eris orbits the Sun, it completes one rotation every 25.9 hours, making its day length similar to ours.
Physical Characteristics :
The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on January 25, 2005. Infrared light from the object revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune's moon Triton, which also has methane on its surface.
Due to Eris's distant eccentric orbit, its surface temperature is estimated to vary between about 30 and 56 K (−243.2 and −217.2 °C).
Unlike the somewhat reddish Pluto and Triton, Eris appears almost white. Pluto's reddish color is thought to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins.
Even though Eris can be up to three times farther from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublime. Because methane is highly volatile, its presence shows either that Eris has always resided in the distant reaches of the Solar System, where it is cold enough for methane ice to persist, or that the celestial body has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another discovered TNO, Haumea, which reveal the presence of water ice but not methane.
Eris was once considered for the position of tenth planet.
Eris is the most massive dwarf planet in the Solar System, exceeding Pluto’s mass by 28%. As such, it was a serious contender to be a tenth planet but failed to meet the criteria set out by the International Astronomical Union in 2006.
Eris was named after the Greek goddess of discord.
Fittingly, picking a name for the object took unusually long – more than 1.5 years after its discovery in 2005. Some of the rejected names were Xena, Lila, and Persephone (Pluto’s wife).
Eris is the second largest dwarf planet.
Eris is unquestionably the most massive of the known dwarf planets (the largest object that is not a planet that orbits the Sun) and was once thought to be the largest due to its relative brightness. After a stellar occultation in 2010 its diameter was calculated to be smaller than previously thought and more recent measurement suggested Pluto was larger than thought making it the largest.
All the objects in the Asteroid Belt could fit inside Eris.
Eris is still smaller than the Earth’s Moon, having about two thirds of the Moon’s diameter and one third of its volume.
Eris is not always the most distant dwarf planet.
At Eris’ furthest point from the Sun (Aphelion) it is so distant it is outside the Kuiper Belt, however at its closet point (Perihelion) it is closer the the than Pluto’s most distant point.