Categories: Solar System

Dwarf Planet Facts: Eris

Image Credit: Hubble Space Telescope /NASA

The dwarf planet designated 136199 Eris is named after the Greek goddess of strife, while its satellite, designated Dysnomia, is named after the goddess of lawlessness, who incidentally was Eris’ daughter. Eris is the most massive and second-biggest dwarf planet known to exist in the solar system, the tenth biggest object that is directly orbiting the Sun, and the sixteenth-most massive solar system object, after seven solar system moons that are more massive than all known dwarf planets.

Quick Facts

• Aphelion: 97.651 AU (14.602 × 109 kilometres)
• Perihelion: 37.911 AU (5.723 × 109 kilometres)
• Eccentricity: 0.44068
• Orbital period: 558.04 years (203,830 days)
• Equatorial rotation velocity: 3.4338 km/sec
• Diameter: 2,326 kilometres
• Sidereal rotation period: 25.9 hours
• Mass: 1.66 ± 0.02 × 1022 kilograms (0.0028 Earths)
• Surface area: 1.70 ± 0.02 × 107 square kilometres
• Mean density: 2.52 ± 0.07 gram/cubic centimetre
• Apparent magnitude: 18.7
• Satellites: Dysnomia


Although Eris has an apparent visual magnitude of 18.7, it is currently below the horizon for most northern hemisphere observers. However, southern observers that are up for the challenge can find Eris at coordinates RA: 01h 41m 15s |Dec: -02° 28’ 07” in the constellation Cetus, but note that at least an 8-inch (200mm) telescope and a CCD device are required to see the object, and then only under dark skies.

In the year 2036, Eris will move out of Cetus and into Pisces where it will remain until 2065 when it enters the constellation of Aries. However, it will only be visible in the northern sky in the year 2128 when it will enter the constellation Perseus, after which it will move into Camelopardalis, after which it will reach its highest northern declination in the year 2173.

Physical Properties

Although Eris’s mass is accepted to be about 27% higher than that of the dwarf planet Pluto, not much is known about Eris’ internal structure mainly because it is more than three times further away from us than Pluto. In February 2016, Eris was estimated to be 96.3 AU from the Sun, and apart for some long-period comets, this makes Eris and its satellite Dysnomia the second-most distant natural objects in the solar system, the most distant being the object designated V774104, which was discovered in November of 2015 at a distance of 103 AU.

Kuiper Belt Object

Eris is a member of a population of TNO’s (trans-Neptunian Objects) that inhabit the “scattered disc” region of the Kuiper belt, where all objects have orbits that are highly eccentric, and steeply inclined with respect to the ecliptic. In Eris’ case, its orbital inclination is about 44 degrees, and its orbital period is 558 years. However, due to its high orbital eccentricity, in about 800 years or so Eris will be somewhat closer to the Sun than Pluto will be for a while.


Although no direct observations of Eris have been made in visible light, spectroscopic studies made with the Gemini North Telescope in Hawaii in infrared light suggest that like on Pluto and Triton, there might be some methane ice on Eris’ surface. However, due to its vast distance from Earth, no surface detail on Eris can be resolved with any ground based or orbiting telescope currently available.

Nevertheless, Eris’ high albedo (reflectivity) of 0.96 suggests that the dwarf planet’s surface is likely white, unlike the surfaces of Pluto and Triton that have distinctly red hues due to the presence of tholins and various other compounds on their surfaces. Moreover, since Eris is far enough from the Sun to allow methane to condense onto the surface, any low albedo areas on the surface would be covered up by the condensing methane, thereby increasing the overall albedo of the surface.


While various methods of estimating Eris’ true diameter have yielded slightly different results, an occultation of the dwarf planet by a star in 2010 has yielded a size estimate of 2,326 km (1,467 miles). If this value is used in conjunction with the fact that Eris’ moon orbits it in 15.774 days, Eris’ mass can be calculated to within a few percent, which yields a density value of 2.52 gram per cubic centimetre, which means that Eris is significantly denser than Pluto is. Based on these values, most investigators have concluded that Eris must therefore be composed mainly of rocky materials, although the exact ratio between the rock and ice fractions remains somewhat uncertain.

Nevertheless, based on complex modelling of radioactive decay rates, these preliminary findings suggest to most investigators that Eris is fully differentiated, and that like some other major bodies in the outer solar system, Eris might host a liquid water ocean at the core-mantle boundary.

Peter Christoforou :