The Dwarf Planets: Eris, Pluto, and Ceres
Eris, Pluto, and Ceres
Most people know of the existence of Pluto, and how it was moved into the category of dwarf planet, but did you know there are 5 recognized dwarf planets? We will focus on three of them here: Eris, Pluto, and Ceres.
Comparison of Dwarfs, their Satellites, and Small Solar-System bodies
What is a Dwarf Planet?
A dwarf planet is an object that orbits the Sun, has enough mass for it's own gravity to give it a rounded shape, and is not a satellite.
In 2006, scientists coined the term "dwarf planet", because of the increase in discoveries of celestial bodies that were nearly the size of Pluto, and the discovery of Eris, which is larger than Pluto. Basically, if a celestial body is large enough to have cleared their orbit, it is a planet, if it isn't massive enough to be rounded in shape by it's own gravity, it is a small solar-system body, and if it's somewhere in-between, it's a dwarf planet. Some scientists do have problems with this classification, but it is accepted for now.
There is popular belief amongst the scientific society that there could be anywhere from 200 to 2000 dwarf planets in our solar system, they just need to be discovered.
The image above shows the accepted size, albedo (reflective properties), and color of some of the known dwarf planets and satellites.
The image is protected under the creative commons license and is courtesy of Wikipedia and Chesnok.
Eris is one of the dwarf planets in our solar system. Not only is it the largest one we have discovered so far, but it is also the 9th largest known body to orbit the Sun, making it larger than Pluto. Eris was nicknamed Xena, after the character Xena, on the popular television show, Xena, Warrior Princess. Not much is really known about Eris because it is so far away. As a matter of fact, it is 3 times farther from the Sun than Pluto. Eris has one moon called Dysnomia.
Because of the discovery of Eris, and then finding that it was actually larger than Pluto, which was considered the 9th planet at the time, scientists had to define what a planet was, and this was when Pluto was moved into the dwarf planet category along with Eris.
Eris was first seen in 2003 but not identified until 2005. It is located past the Kuiper belt in a region we currently call scattered disc.
The NASA image above is of Eris and Dysomia.
Below is an image showing how Eris was discovered, and consists of 3 frames shown over a 3 hour period. It is courtesy of LifeFantastic via wikipedia.
Below, you can see just how far out Eris can be from the rest of the known bodies in our solar system, which is why not much is known about the dwarf planet.
The image above is courtesy of Orionist, and is protected under a creative commons liscense.
Dwarf Planet Pluto
Pluto, once our 9th planet, was re-classified as a dwarf planet in 2006 because of the discovery of Eris, which was larger than Pluto. It is the second largest known dwarf planet, but smaller than many of the larger moons, which include Ganymede, Titan, Callisto, Io, Europa, Triton and even our own Moon. Pluto is located in the area of our solar system known as the Kuiper belt.
Hubble Images of Pluto
Pluto has three moons: Charon, Nix, and Hydra. Pluto and Charon are sometimes considered a binary system because of their differing orbit. Charon may actually be one day considered another dwarf planet when the International Astronomical Union (IAU) defines what a binary dwarf planet system is, and how it differs from a dwarf planet and its wayward or eccentric moon.
Pluto is rather small when compared to the Earth, and is made up of mostly rock and ice. Pluto also has a very eccentric orbit, which causes Pluto to actually come closer to the Sun than Neptune at a certain point in it's orbit.
Being so far away, only a few facts are known about Pluto. We know that the surface of Pluto is made up of mostly nitrogen ice, with small amounts of methane and carbon monoxide. Pluto and its largest moon, Charon, always have the same face towards each other, meaning if you were in the side of Pluto which faces Charon, you would always see the moon, but if you were on the other side of Pluto, you would never see the moon. We also know that the side of Pluto facing Charon has more methane ice than the opposite side of Pluto, which contains more nitrogen and carbon monoxide ice. Pluto has a very thin atmosphere which consists of nitrogen, methane, and carbon monoxide. When Pluto moves closer to the Sun, the ice melts and forms the gases that become its atmosphere, and as Pluto moves away from the Sun, these same gases freeze and fall to the ground as ice.
Pluto orbits the Sun differently than the planets. While the planets have an ecliptic and relatively flat orbit, Pluto has an elliptical and inclined orbit. When one looks at the orbits of Neptune and Pluto, it looks as if the two celestial bodies would be in danger of colliding, but calculations show that considering their stable orbits, and barring some extremely unusual catastrophic celestial event, the two will never be in the same area at the same time.
On January 19, 2006, NASA launched the spacecraft New Horizons, with some of the ashes of Pluto's discoverer, Clyde Tombaugh, aboard. New Horizons is making its way to Pluto, and is expected to arrive July 14, 2015.
The Pluto and Neptune orbit image above is courtesy of Eurocommuter and protected under a creative commons license.
By-passing all the names proposed by some of the leading men of science, a committee unanimously voted on the name Pluto, proposed by an 11 year old school girl, Venetia Burney.
The Launching of New Horizons on January 19, 2006
Ceres is currently the smallest dwarf planet in our solar system, and is sometimes also classified as an asteroid. Ceres was discovered on New Years Day in 1801, and is the only dwarf planet located in the asteroid belt. Ceres is much closer to the Earth when compared to the other dwarf planets, which makes it easier to study. While Ceres is the smallest of the dwarf planets known to us today, it is by far the largest object in the asteroid belt.
In the photo below, you can see how small Ceres is compared to the Earth and Earth's Moon.
Ceres is believed to be made up of a mix of water ice and hydrated minerals such as carbonates and clay, which are also common on some meteorites. Scientists speculate that Ceres may have a liquid ocean beneath the surface, and a tenuous atmosphere of water vapor. NASA launched the Dawn Mission on September 27, 2007 to explore Ceres and Vesta.
Ceres seems to be a left over planet in the making, or protoplanet. Scientists believe that while most of the protoplanets were ejected from the solar system by Jupiter, or merged with other protoplanets to form the larger planets, Ceres escaped this melee and is still quietly orbiting the Sun.
Image of the orbit of Ceres courtesy of Orionist and protected under a creative commons license.