Eris

Eris (minor-planet designation 136199 Eris) is the most massive dwarf planet known in the Solar System, and the ninth most massive body known to directly orbit the Sun. It is estimated to be in diameter, and 27% more massive than Pluto, or 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 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 similarly sized objects 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 number as before Pluto’s discovery in 1930. Observations of a stellar occultation by Eris in 2010 showed that its diameter was only , not significantly different from that of Pluto. Given the uncertainties in their size estimates, there is a reasonable chance that Eris is smaller than Pluto.For r(Eris) = 1163±6 and r(Pluto) = 1153±10, and assuming normal distributions, the chance that Eris is smaller than Pluto is , where is the cumulative distribution function and is the error function. Erf(−0.6064) = −0.6088, and Φ = 0.1956, or an approximately 20% chance. Pluto’s atmosphere makes determining its diameter difficult, and until the New Horizons probe arrives at Pluto on July 14, 2015 the diameter is expected to remain unknown. Because of that, many scientists expect the question of the relative sizes of Eris and Pluto to remain unanswered until then.