In 1930, astronomer Clyde Tombaugh discovered the first dwarf planet we now refer to as Pluto. For decades, our solar system contained nine planets. Teachers taught elementary school students various tricks and tips to remember the names and order of the planets that make up our solar system.
During that same time, scientists and astronomers continued to explore our universe. As they learned more about the planets, they developed specific definitions of what constitutes a planet.
First, it has to orbit the sun. Second, it has to have enough mass in order for gravity to condense it into a round ball. And finally, it must clear other objects out of the way in its orbital neighborhood.
That’s where the issue with Pluto comes into play.
According to the International Astronomical Unions, Pluto does not meet the third requirement of what constitutes a planet and was thus classified as a dwarf planet in 2006.
The declassification of Pluto as a planet sparked vigorous debate in both the scientific community and among the public. However, if the three rules of defining a planet were not established, we would have thousands of “planets” in our solar system because of the numerous icy bodies that exist alongside Pluto in the Kuiper Belt.
In fact, according to NASA, there are 43 other known Kuiper Belt objects and possibly billions that have not been cataloged yet.
Even though it was “downsized” to a dwarf planet, Pluto has some fascinating qualities that makes it ripe for scientific investigation. It is the largest and brightest known member of the Kuiper Belt, which stretches almost 2 billion miles beyond Neptune’s orbit.
Pluto has five known moons.
Charon, the largest of the five, was discovered in 1978 by James Christy, a United States Naval Observatory astronomer. The surface of Charon is known to be mostly water and ice and its density indicates that it, like Pluto, is composed of a mixture of 50 percent ice and 50 percent rocky material.
Charon is half the size of Pluto and roughly the same diameter as Texas. Because of its size, the Pluto-Charon system makes up what scientists call a double-planet because they orbit around a common center of gravity in the space between them.
It is the only one known in our solar system.
Scientists have observed the other four moons of Pluto and determined the following:
Hydra, discovered in 2005, orbits Pluto every 38 days at a distance of approximately 40,200 miles.
Nix, discovered that same year, orbits Pluto every 25 days at a distance of approximately 30,300 miles.
Both Hydra and Nix are estimated to be between 25-95 miles in diameter.
Styx, discovered in 2011 and named for the goddess of the river Styx in the underworld, circles Pluto every 20 days and is only 4 to 13 miles in diameter.
Kerberos, discovered in 2012 and determined to be the fifth moon of Pluto, orbits between two other moons – Nix and Hydra – every 32 days. This moon is thought to be approximately 6 to 20 miles in diameter.
Not much else is known yet about these other moons of Pluto because they are difficult to study in too much detail from Earth.
However, NASA’s New Horizons mission to the Kuiper Belt and close flyby of Pluto provides new information for scientists to learn much more about the moons of the dwarf plant Pluto and of Pluto itself.