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The mysterious Eris and moons (NASA)
How come it’s more massive than Pluto but not considered a full-grown planet? Apparently, the conditions set forth by the International Astronomical Union (IAU) for a celestial body to be considered a planet is now more stringent.
For a celestial body to be a planet, it has to be massive enough to be rounded due to its own gravity and should have cleared the neighborhood of its orbit. Eris is rounded enough but has not cleared its neighborhood, hence is considered as a dwarf planet.
This kind of classification came about when scientists started to discover an increasing number of celestial bodies that had about the same size as Pluto. The discovery of Eris was the deciding factor since Eris is larger and more massive than Pluto.
Eris has a mean radius of approximately 1,300 km. Pluto’s mean radius is only estimated to be at 1,153 km. Furthermore, Eris has a mass of approximately 1.67×1022 km, while Pluto’s mass is only approximately equal to 1.305×1022 kg.
Eris was only discovered in January 5, 2005, by Michael Brown, Chad Trujillo, and David Rabinowitz, of the Palomar Observatory in San Diego County in California. When Eris was first discovered, it was immediately adopted as the 10th planet.
However, since a number of celestial objects had comparable sizes to Eris and Pluto, the IAU decided to define the word ‘planet’ once and for all. As of this day, the IAU considers both Eris and Pluto as dwarf planets.
Due to its distance from the Sun, Eris is extremely cold, at about 30 to 56 kelvins. That’s about -243 to -217 degrees Celsius. Infrared readings indicate the presence of methane ice on its surface.
Scientists have spotted a natural satellite orbiting Eris. That moon is now known as Dysnomia.
The current distance of Eris from the Sun is estimated to be about 96.7 AU. It is so far that it’s orbital period lasts about 557 years.
This distance can however change substantially as Eris proceeds along its orbital path. This is because of the high eccentricity of its orbit. That means, it is very elliptical in shape. Right now, its distance of 96.7 AU is believed to be its aphelion distance. At its perihelion, Eris would be only 37.77 AU away.
Universe Today has some articles that talk about dwarf planets. Here are two:
Dwarf Planets
Dwarf Planet Pluto
Here’s more from NASA:
Astronomers Measure Mass of Largest Dwarf Planet
Hotly-debated Solar System Object Gets A Name
How about letting your ears do the work for a change? Check out this podcast at Astronomy Cast:
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