E R I S : Part 2


Eris : The New Dwarf Planet

As stated in Part 1, Eriss name was announced on 13th September 2006 by the official naming body of the I.A.U. in their Circular No. 8747. This was only several weeks after the IAU Naming Committee decided on a new important definition of planets, placing Eris as the new catergory of so-called dwarf planets. This new sub-planet category presently includes the first minor planet discovered, Ceres, and the former planet of Pluto that was found by Charles Tombaugh in 1930. All dwarf planets are now given similar designations used for known asteroids, namely, (1) Ceres, (134340) Pluto and (136199) Eris. (The bracketed value being the ordered sequential asteroid number by discovery date.)


Discoverer : Brown et al.
Date : 05 January 2005
Satellites : 1
Diameter : 2400±100 km.
Polar Tilt : Unknown
Period (P) : 559.55±0.105 years
Synodic Period : 364.61 days
Orbital Velocity : 3.44 km.s-1
Distance (a) : 67.90 AU or 1.02×109 km
Perihelion (q) : 38.287 A.U. / 5.73×109 km
Aphelion (Q) : 97.519 A.U. / 1.43×1010 km
Eccentricity (e) : 0.4361
Inclination (i) : 44.023°
Mass : 1.66&215;1019 kg.
Mean Density : c.2 g.cm-3
Sidereal Rotation : > 8 hr.?
Mean Sidereal Rotation : > 8 hr. ?
Maximum Diameter : 0.103″ (arcsec)
Minimum Diameter : 0.040″ (arcsec)
Maximum Magnitude : 14.8
Minimum Magnitude : 18.8

Nature of Eris

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 Dysnomias 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).

Satellite Eris I : Dysnomia

Eris also has one known moon, named Eris I or asteroid Dysnomia (136199), and was discovered using the large 10-metre telescope at Keck observatory in Hawaii. Ay first the moon was amusingly named Gabrielle by the discoverers — after the long-term female companion to Xena — the Warrior Princess in the camp hit television series of the 1990s. Dysnomias name is taken from the mythological Greek demon goddess of lawlessness being one of Eris daughters.

We know little about Dysnomia itself, and in recent decades such asteroids with orbiting moons now seem an accepted common occurrence. The discovery has proved to be important as astronomers can use it to calculate the mass of Eris from the orbital parameters of the little moon. This includes the current estimates of mass, density and physical size. Preliminary results do suggest the moon is about 150 km in diameter, whose orbital period is 19 days in length. Dysnomia when observed from Earth remains very faint, averaging well below 21st magnitude.

Like the Earth-moon system, Eris-Dysnomia probably formed about 4.5 billion years ago following a massive planetary scale collision.

* NOTES: It should be mentioned that the search for new planets and other bodies in the Kuiper belt has been more recently made by Brown and Schaller is funded by Caltech and NASA. For more information on the program, see the Samuel Oschin Telescopes website.

1. For more information see Mike Browns research
2. To learn more about Eris, see http://www.planeteris.com.
3. An Eris image is available at; http://www.gps.caltech.edu/~mbrown/planetlila/moon/hst.jpg

Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.


Last Update : 2nd December 2012

Southern Astronomical Delights © (2012)

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