by Steven Mars

Created on: July 07, 2011   Last Updated: July 11, 2011

Because Pluto has been considered to be a planet since it was discovered by the young astronomer, Clyde Tombaugh, at the Lowell Observatory in Arizona on February 18, 1930, it should be considered a planet today. Three moons have been discovered revolving Pluto. The largest moon Charon was discovered in 1978, Nix in 2005, and Hydra also in 2005. Nix and Hydra were discovered using the Hubble telescope. Pluto was thought to be bigger than Mercury for several years.    

The dwarf planet, Eris, was discovered by Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005. It was first thought to be a planet much farther away and a little bigger than Pluto. In 800 years, Eris will move in closer than Pluto (Neptune and Pluto did the same thing from February 7, 1979 to February 11, 1999). The fact that Eris and Pluto might have diameters equal to about 2330 kilometers is one reason Pluto is being considered for reclassification. Eris was first thought to be bigger, but Eris and Pluto have appeared to be about the same size in recent calculations. Jupiter’s moon Ganymede and Saturn’s moon Titan are both considered to be bigger than Mercury, Pluto, and Eris.

Eris has an aphelion of 97.5 astronomical units and a perihelion of 37.9 astronomical units. It was 96.7 astronomical units from the sun in 2009, very close to its aphelion. It will be close to its perihelion again in about 2257. An astronomical unit is the average distance from the earth to the sun (about 149,597,870.691 kilometers or about 93 million miles). The perihelion is the closest a planet's orbit comes to the sun and the aphelion is the farthest a planet's orbit gets from the sun. Eris is probably the tenth planet of the solar system.

The perihelion of Pluto is 29.7 astronomical units and the aphelion is 49.5 astronomical units. The fact that Pluto’s probable aphelion (49.5 astronomical units) is greater than Eris’ probable perihelion (37.9 astronomical units) is why Pluto and Eris are expected to change places with respect to their distance to the sun.

Even the Hubble space telescope’s pictures can only take pictures of the largest objects on Pluto. Maybe the spacecraft named New Horizons launched in January 2006 and scheduled to arrive at Pluto in 2015 will find out more about the planet.

In 2006, it was thought that there might be twelve planets in the solar system. The planets included the usual nine planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto. It also included the asteroid Ceres that lies in the asteroid belt between Mars and Jupiter, the moon of Pluto Charon, and a newly discovered object 2003 UB313. The newly discovered object 2003 UB313 is the aforementioned Eris.

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