Eris

Eris
Date of discovery	January 5, 2005
Name of discoverer	Mike Brown, Chad Trujillo, and David Rabinowitz
Name origin	Greek goddess of discord
Orbital characteristics
Primary	Sun
Order from primary	11
Perihelion	37.77 AU
Aphelion	97.56 AU
Semi-major axis	67.6681 AU
Titius-Bode prediction	154 AU
Orbital eccentricity	0.44177
Sidereal year	577 a
Avg. orbital speed	3.436 km/s
Inclination	44.187° to the ecliptic
Rotational characteristics
Sidereal day	8 h (0.333 da)
Physical characteristics
Mass	1.66 * 1022 kg (0.278% earth)
Density	2,300 kg/m³
Mean radius	1,300 km
Surface gravity	0.8 m/s² (0.0816 g)
Escape speed	1.442 km/s
Surface area	21,237,000 km² (4.164% earth)
Minimum temperature	30 K
Mean temperature	42.5 K
Maximum temperature	55 K
Number of moons	1

Eris, the largest of all dwarf planets, is named for the Greek goddess of discord and strife. Considering the debate that the discovery of this object provoked, the name is probably quite apt.

Discovery

Eris was first photographed on October 21, 2003. Then on January 5, 2005 Mike Brown, Chad Trujillo, and David Rabinowitz at the Mount Palomar Observatory analyzed the images and realized that they depicted a new object.

With the announcement of Eris' discovery on July 29, 2005, the International Astronomical Union faced an embarrassing problem. Eris weighs in at 1.66 * 10²² kg, which makes it 27% more massive than Pluto and in fact more massive than all the asteroids combined. And yet Eris could not be said to have cleared its path of all other objects. So if Eris should not be called a planet, then Pluto had no right to that designation, either.

In 2006, the International Astronomical Union decided the issue: they redefined a planet as a satellite of the sun that had cleared its path of all other objects, and by that standard, Pluto did not qualify as a planet any longer.

At 67.7 AU from the Sun, it is the most distant object yet discovered that has the Sun for a primary. At aphelion, it is far beyond the Kuiper Belt and in what is known as the Scatter Disk of the solar system. Because Eris is so distant from Earth (currently near aphelion and hence three times more distant than is Pluto as of 2008), it has a very long sidereal year of 577 Julian years. Its synodic year is very nearly the same as an Earth year.

Surface and atmosphere

Near infrared spectrum of dwarf planet Eris, taken with the Gemini 8 m telescope.

Infrared spectroscopic scans of Eris reveal an infrared reflectance spectrum remarkably like that of Pluto, which is known to have a layer of methane on its surface. From this, the discovery team concludes that Eris is surfaced with solid frozen methane, with rock and ice beneath.

Yet Eris is the second most reflective body in the entire solar system, reflecting about 86% of the incident sunlight. Eris is also uniformly white on its surface, whereas Pluto is a mottled brown. The discoverers believe that this is due entirely to Eris' present far-flung position (near aphelion), and point out that Eris' orbit is the most eccentric orbit of any satellite of the Sun, except for comets.

Naming of Eris

The first name given to the object, oddly enough, was Xena, the name of a fictional character in an American television show set in the supposed "classical past" of Argive and/or Mycenean civilizations. Mike Brown, the lead discoverer, stated in an interview that the name Xena came partly from the name of the fabled "Planet X," the subject of repeated searches since the discovery of Pluto.

Then in 2006, after the IAU made its determination and definition of dwarf planets, the discovery team proposed the name Eris to symbolize the discord in the astronomical community that its discovery had caused. The IAU, for whatever reason, accepted that recommendation.

Satellites

Eris has one known satellite, a tiny moon called Dysnomia. The name Dysnomia means "lawlessness," and some still regard that name as an oblique reference to the name of the actress who portrays the aforementioned character of Xena on American television. However, in classical mythology, Dysnomia is the name given to Eris' daughter, who is a symbol of actual lawlessness.

References

• Reitan, Kari. "Astronomers Measure Mass of Largest Dwarf Planet." Space Telescope Science Institute, National Aeronautics and Space Administration, June 14, 2007. Accessed January 21, 2008.
• Hamilton, Calvin J. "Entry for 'Eris'." Views of the Solar System, 2007. Accessed January 21, 2008.
• Brown, Mike. "The discovery of Eris, the largest known dwarf planet." Accessed January 22, 2008.