Eris
Eris
Eris (as in a lifelike photo), officially designated (136199) Eris, is one of the main dwarf planets in the outer Solar System. Discovered in 2005, it played a central role in the International Astronomical Union (IAU)’s 2006 redefinition of the concept of a planet.
By mean diameter, Eris is the second-largest dwarf planet known, immediately after Pluto, but differs from Pluto by its approximately 27% greater mass. It is an icy body belonging to the population of trans-Neptunian objects (TNOs), more specifically the plutoid subclass, which are dwarf planets that orbit beyond Neptune.
Eris’s scientific importance lies not only in its size, but also in its extreme orbit, its physical properties, and the cultural and scientific debate it sparked, leading to Pluto’s reclassification and the modern definition of a dwarf planet.
Astronomical Observation –
Eris was discovered on January 5, 2005, by a team of astronomers composed of Michael E. Brown, Chad Trujillo, and David Rabinowitz, analyzing images acquired on October 21, 2003, with the 48-inch Samuel Oschin reflecting telescope at the Palomar Observatory.
The official announcement of the discovery took place on July 29, 2005, simultaneously with that of two other large Kuiper Belt objects: Haumea and Makemake. The delay between image acquisition and identification of the object was due to its very slow apparent motion and strong orbital inclination, which placed it far from the plane of the ecliptic, a region traditionally favored by observation campaigns.
With a mean apparent magnitude of approximately 18.8, Eris is theoretically observable from Earth using medium-diameter (≥ 20 cm) amateur telescopes equipped with CCD sensors, although its distance and location make observation challenging.
At the time of its discovery, Eris was the most distant known object in the Solar System, a distinction it held until 2018.
Physical characteristics –
The most accurate measurements of Eris’s diameter were obtained in 2010 through occultation by a star, a method that provides extremely precise results for minor Solar System bodies. The resulting diameter is 2326 ± 12 km.
This value makes Eris slightly smaller than Pluto, although its mass is greater, implying a high average density, estimated at 2.52 g/cm³.
This density suggests a mixed composition of ices (primarily methane and probably nitrogen) and rocky material.
Eris’s surface is characterized by an exceptionally high albedo: 0.97 ± 0.01.
This is one of the highest values in the entire Solar System, second only to that of Enceladus. Spectroscopic analysis, initially conducted with the Gemini North Telescope, revealed the presence of methane ice, making Eris compositionally very similar to Pluto.
The high reflectivity is likely due to the periodic condensation of atmospheric methane on the surface during the long aphelion phases, when temperatures drop to extremely low values. Unlike Pluto, however, Eris appears almost neutral or grayish, rather than reddish, an appearance whose origin is not yet fully understood.
Eris also has a single known satellite, Dysnomia, discovered on October 2, 2005. The satellite has an estimated diameter of between 250 and 350 km and orbits at an average distance of 30,000–36,000 km, with an orbital period of approximately 14 days.
Observations suggest a possible synchronous rotation between Eris and Dysnomia, similar to the Pluto–Charon system. The presence of the satellite has allowed a more accurate determination of the dwarf planet’s mass.
Eris orbits the Sun within the diffuse disk, following a highly eccentric and inclined orbit:
– Orbital period: ~557 years
– Perihelion distance: ~35 AU
– Aphelion distance: ~97 AU (approximately 14.6 billion km)
– Orbital inclination: ~44°
These characteristics make Eris’s orbit one of the most extreme of the known dwarf planets and explain its late discovery.
Space missions –
To date, there have been no dedicated space missions to Eris, nor have any probes flown by or studied it directly. Its extreme distance, long orbital period, and logistical complexity make a dedicated mission technologically and financially challenging.
However, Eris represents a target of great scientific interest for future missions to the outer Solar System, particularly for the comparative study of dwarf planets and the early evolution of the Kuiper Belt. Current knowledge comes exclusively from terrestrial and space-based telescopic observations (Hubble, Spitzer, and ground-based observatories).
Guido Bissanti