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What's the Difference Between Planets and Dwarf Planets?
How did this debate get started? The 2005 discovery of Eris sparked the debate about the difference. At that time, the known Solar System consisted of nine planets with Pluto being the ninth planet. However, the discovery of a sphere with 27 percent more mass than Pluto raised this question: If Pluto was a planet, then wasn’t this new discovery a planet, also? Did the Solar System now have ten planets, rather than nine? To make matters more confusing, what would happen as new celestial bodies were discovered? How many planets could there be?
The matter was decided in 2006 by the International Astronomical Union (IAU.) Rather than elevating Eris to planet status, Pluto was downgraded to a dwarf planet, and Eris was classified as a dwarf planet. At the time of this writing, the IAU recognizes five dwarf planets: Eris, Pluto, Ceres, Makemake, and Haumea, with Eris being the largest of the group.
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Eris Fast Facts
Let's look at some fast facts about the dwarf planet now known as Eris:
- Discovery date: July, 2005
- Discovery Team: Mike Brown, Chad Trujillo, and David Rabinowitz
- IAU temporary designation: 2003 UB 313
- IAU name: Eris, named for the Greek war goddess
- Placeholder name/nickname: Xenia, named for the popular television show Xenia: Warrior Princess
- Satellites: One moon, Dysnomia
- Family: Kuiper Belt, Scattered Disk area
- Dwarf Planet Status: Largest of the IAU designated dwarf planets, and coldest - with a surface temperature of approximately -243 degrees Celsius.
- Reflectivity of Light: 86%
- Surface Appearance: Bright white (due to the high light reflectivity)
- Diameter: Approximately 2400 km (1490 miles, measured by the Hubble Space telescope)
- Mass: 1.66 by 1022 km (635 miles, determined by tracking the movement of its moon Dysnomia, 27% more mass than Pluto)
- Density: Estimated as approximately, half rock, half ice, with slightly more rock than ice
- Orbital Eccentricity: 0.44
- Orbital Incline: 44 degrees to the elliptic
- Orbital period: 560 years
At the time of this writing Eris is at aphelion, or as far away from the Sun as 97 astronomical units, with no surface atmosphere, but scientists believe the surface is covered with a highly reflective methane frost.
In March of 2227, it will be at perihelion, or as close to the Sun as 37 astronomical units, and scientists believe it will have a rudimentary atmosphere of methane gas due to slight warming of the surface.
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What does the Future Hold for Eris?
While much information has been gathered about Eris the dwarf planet, there is still much to be discovered. It will be interesting to see what additional data scientists gather as it continues on its long orbit toward the Sun, and becomes more accessible to scrutiny. If you would like more information on Eris or the other dwarf planets, a good place to start is at your local library or on the Internet.
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- California Institute of Technology, “The Discovery of 2003 UB 313 Eris, the 10th Planet,” Mike Brown
- Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, “The Outer Planets, Kuiper Belt Objects, Dwarf Planets”
- The Planetary Society, “Space Topics: Trans-Neptunian Objects Dwarf Planet Eris Formerly Known as 2003 UB 313”
- Solar Views, “Dwarf Planet Eris”
- International Astronomical Union, “IAU Names Fifth Dwarf Planet Haumea”