At the mean distance of 67.7 AU or 10.1 billion kilometres, Eris is almost twice the distance of Pluto. Its highly eccentric shape of the orbit (‘e’ or eccentricity) is very much unlike the eight other planets in the Solar System, being even more distorted than fellow dwarf planet, Pluto. Eris’ own orbit is also highly inclined to the ecliptic at some 44.2°, whose very high eccentricity causes solar distances to be possibly anywhere between 37.8 to 97.6 A.U or the range of 5.65 to 14.6 billion kilometres. Eris when discovered just happened at aphelion, near to the greatest maxima that occurred on 07th April 1977. Currently opposition distance is about 97.0 A.U. from the Sun (2008).
Using the orbital period of 557.769 years, the last closest approach to the Sun (perihelion) occurred back to the 19th May 1698 A.D., when Eris was in northern Virgo. This will not happen again until 24th February 2256 A.D. During each perihelion passage, maximum opposition brightness reaches 14.8 magnitude, though at aphelion, this may descend to as low as 18.8 magnitude.
Eris very small observed diameter only subtends between 0.040 and 0.103 arcsec, concluding it is about 2,800 kilometers across. Some difficult questions remain, which references ofen range between 2,400 to 3,000 km. — making it bigger than Pluto. Discoverer, Mike Brown first said; “Pluto and Eris are essentially twins — except that Eris is slightly the pudgier of the two,”, by cofounder Emily Schaller adds; “And a little colder”.
Using the Hubble telescope data in June 2007, Brown and Schaller found Eris’ had the lowish density of about 2 g.cm.-1, therfore suggesting Eris, like Pluto, is very possibly composed of ice and rock. When combining this with the earlier determined diameter of 2,400 km., Eris must be some 27% larger in mass than Pluto, or about 1.66×1019 kg. — 16.6 billion trillion kilograms — learnt from Dysnomia’s orbit. Brown and Schaller are the authors of the paper, “The Mass of Dwarf Planet Eris” that appeared on the 15th June 1997 issue of the international journal, Science. [* See Notes at the end of this page.]
Surface temperatures are very low at these vast distances, and are perhaps around -240°C or only about 20° above absolute zero. However Eris, even though is so distant from the Sun, appears as a pretty dark object. It is quite likely the surface has been slowly transformed by the effects of solar radiation and even internal volcanic flows produced by internal heat. Planetary astronomers suspect that its surface composition is probably frozen methane which has been gradually broken down by these enenergies to form either pale yellow or slighty reddish hydrocarbon-like organic compounds. Expected conditions on Eris should favour more yellowish than the reddish-yellow surface of Pluto, perhaps mainly because Eris is farther from the Sun. Evidence here mainly comes from the limited spectral information, and does suggest it is composed of water ice and methane — frozen rock-hard by the extreme cold temperatures.
Most obtained images show the dwarf planet as star-like. Even using the Hubble Space Telescope (HST) or large ground-based telescopes reveals a very tiny featureless disk, amd so we know little about the sidereal rotational period of this distant dwarf planet. Recent photometric observations suggest that it is less than 8 hours, and this will probably remain so until some better quality photometric observation have been made or by some future unplanned dedicated Eris space mission. Even if such a mission were proposed, the results would likely take more than twenty years before any details were obtained — not accounting the required time to construct the satellite. Now the best available images were made using the large Hawaiian 10-meter Keck Telescope. (See Eris image).
