From 1930 to 2006, students were taught that there are 9 planets in our solar system. Mercury, Venus, Earth, and Mars are the small, rocky planets closest to the sun. Farther out are the gas giants Jupiter, Saturn, Uranus, and Neptune. Beyond Neptune is the small, icy world of Pluto. Pluto was our 9th planet.
But in 2006, we were told that Pluto is not a planet. There are only 8 planets in our solar system. What happened? Why isn't Pluto a planet? Uranus, Neptune, and Pluto were discovered with telescopes. Mercury, Venus, Mars, Jupiter and Saturn can be seen with the naked eye.
In the early 20th Century, most astronomers believed that Neptune was the 8th and final planet in our solar system. A few disagreed. They thought there was a 9th planet. They believed that another planet was causing the orbits of Uranus and Neptune to change very slightly. An American astronomer named Percival Lowell used the name "Planet X" for this mysterious planet theory.
In 1930, astronomer Clyde Tombaugh made a discovery while working at Percival Lowell's observatory. He discovered Planet X. One of the best telescopes ever made is the Hubble Space Telescope. It has greatly improved our knowledge of Pluto. However, Pluto is so far away that even the best images of Pluto are still blurry.
Pluto is over 3.6 billion miles from the sun. Since it is so far from the sun, Pluto is one of the coldest places in the solar system. Temperatures can fall to -387 degrees Fahrenheit. Pictures from the Hubble Space Telescope showed that Pluto's surface has dark and light areas.
For many years, astronomers weren't able to measure Pluto's mass. In 1978, Pluto's largest moon, Charon, was discovered. Astronomers were able to compare the two objects to find out Pluto's mass. Until then, people thought Pluto was much bigger.
Astronomers now realize that the discovery of Pluto was a lucky accident. Scientists had been incorrect about Neptune's mass and therefore hadn't figured out its and Uranus's orbits correctly. Percival Lowell's Planet X never existed. Even if there had been a planet affecting Uranus's and Neptune's orbits, Pluto is too small to be responsible.
Pluto is smaller than Earth’s moon. Pluto's diameter is about 1500 miles. The smallest planet, Mercury, has a diameter of 3032 miles. Pluto has 5 moons that we know of: Charon, Hydra, Nix, Kerberos, and Styx. Pluto's largest moon, Charon, is about half the size of Pluto. Pluto and Charon are only about 12,200 miles apart. Because Pluto is so small, it has very little gravity. A person who weighs 100 pounds on Earth would only weigh 7 pounds on Pluto.
All planets and moons spin, or rotate, around an imaginary line called an axis. Usually, a planet's axis runs from the top to bottom. This means that it rotates like a spinning top. Pluto is different. Pluto rotates on its side. Uranus rotates on its side too.
Days are measured by the amount of time it takes for a planet to make one rotation on its axis. Pluto rotates slowly compared to Earth. It takes Earth 24 hours to make one rotation on its axis. Pluto takes 6.4 Earth days to make one rotation.
The planets in our solar system fit neatly into two groups: small rocky planets and gas giants. Pluto doesn't fit into either group. Pluto is made mostly of ice, but probably has a rocky core. Pluto has a very strange orbit. It's not on a flat geometric plane. So Pluto is above the other planets at times and below them at other times. Also, instead of having a circular orbit, Pluto's orbit is shaped like an oval. Sometimes, Pluto's orbit brings it closer to the sun than Neptune is. It takes Pluto 248 Earth years to orbit the sun.
In 2001, a newspaper reporter visited New York City's Hayden Planetarium. He happened to overhear a girl asking where Pluto was in a display of the planets. It wasn't there. After the New York Times published the reporter's story about the missing planet, letters from around the world poured in to the planetarium. Neil deGrasse Tyson explained that Pluto wasn't included because only the gas giants and rocky planets are shown in the display.
Astronomers once thought that Pluto was the only object in that part of the solar system. As telescopes got stronger, astronomers learned there are many objects near Pluto. In fact, there are over 70,000 objects in Pluto's "neighborhood". Astronomers have named that region the Kuiper Belt. For years, astronomers have been finding larger and larger objects in the Kuiper Belt.
In 2003, astronomers found a large object that was farther away than Pluto. In 2005, they confirmed its existence and thought it was bigger than Pluto. The object, Eris, is believed to be a mix of ice and rock like Pluto. The discovery of Eris caused a lot of astronomers to question what a planet really is. Was Eris the 10th planet? At the time, there was no clear definition of a planet. Astronomers from around the world agreed that a decision had to be made. (It takes Eris 557 Earth years to orbit the sun. Some scientists now think Eris is smaller than Pluto.)
In 2006, the International Astronomical Union (IAU) held a conference. They voted on different definitions of a planet. One possible definition would have given us a lot more planets. Another definition would have kept the number at 9. A third possible definition would reduce the number to eight.
The conference voted for the third definition. The number of planets in our solar system was reduced to eight. Pluto was no longer a planet. Even after the IAU vote, there are still some astronomers, like Alan Stern, who insist that Pluto is a planet.
The IAU agreed that in order for an object to be called a planet, it must pass three tests. First, the object must orbit the sun. Next, the object must have become round due to the force of its own gravity. Finally, it must have cleared out all the objects (other than moons) in its path.
Pluto only met the first two parts of the definition. It was too small to clear out all the objects in its neighborhood. Pluto was called something new: a dwarf planet. In order to clear its neighborhood, a planet must either draw in other objects or fling them out into space. Pluto can’t do either of these.
Since its discovery in 1930, Pluto has remained a dark and mysterious world. In 2006, NASA launched a spacecraft made to study Pluto. The New Horizons will fly past Pluto in 2015. New Horizons will take the first close-up images of Pluto and its moon Charon. Afterwards, it will move on to study other objects in the Kuiper Belt. New Horizons will begin taking pictures of Pluto a few months before it reaches the dwarf planet. New Horizons will be close to Pluto for only about 30 minutes.
Our solar system has 8 planets and 5 dwarf planets. Five dwarf planets: Ceres was discovered on January 1, 1801 by Giuseppe Piazzi. Pluto was discovered in 1930. Haumea was discovered in 2004. Makemake was discovered on March 31, 2005. Eris was discovered in July 2005. Before the Copernicus-Galileo heliocentric theory was proven, Christians thought the Earth was stationary (Joshua 10:12-14). Aristarchus of Samos was rejected until Copernicus invoked him.