Why is Pluto not a planet?
Which sentence best describes Pluto?
- Pluto is actually closer to the Sun than Neptune for about 8% of its orbit.
- Pluto is just one of many icy objects in a distant area of our solar system.
- Pluto and its large, orbiting companion object Charon, are tipped on their sides.
- All of the above.
Well, just pick the answer you like best, because they are all true!
Pluto is a dwarf planet that lies in the Kuiper Belt. It's an area full of icy bodies and other dwarf planets at the edge of our solar system. Because Pluto is the biggest known object in this region, some call it "King of the Kuiper Belt."
One thing is certain. Pluto and its neighborhood are very peculiar. If scientists could unravel some of their mysteries, we would know more about how our solar system formed.
More Fun Facts About Pluto:
- Pluto is only about half the width of the United States. Charon is about half the size of Pluto. Charon is the largest moon compared to the body it orbits (whether planet or dwarf planet) of any moon in the solar system.
- Almost all the planets travel around the Sun in nearly perfect circles. But Pluto does not. It takes an oval-shaped path with the Sun nowhere near its center. What's more, its path is quite tilted from the nice, orderly plane where all the planets orbit. (Mercury has a slightly lop-sided orbit, although not nearly so much as Pluto's.)
- Compared to most of the planets and their moons, the whole Pluto-Charon system is tipped on its side. Like the planets, Pluto's spin axis stays pointed in the same direction as it orbits the Sun. But unlike all planets except Uranus, Pluto is tipped on its side. The planets' axes of rotation stand more or less upright from the plane of their orbits.
- If you lived on Pluto, you'd have to live 248 Earth years to celebrate your first birthday in Pluto-years.
- If you lived on Pluto, you would see Charon from only one side of the planet. Charon's orbit around Pluto takes about six and one-half Earth days. Pluto's day (that is, one complete rotation) takes exactly the same amount of time. So, Charon always "hovers" over the same spot on Pluto's surface, and the same side of Charon always faces Pluto.
- At Pluto's current distance from the Sun, the temperature on its surface is about 400 degrees below zero Fahrenheit! It will get even colder as it moves farther from the Sun. From Pluto, the Sun looks like just a bright dot in the sky, the brightest star visible. The light from the Sun is as bright on Pluto as the light from the full Moon is on Earth.
- If you weigh 100 pounds on Earth, you would weigh only 7 pounds on Pluto!
- Pluto orbits in a far-out region of the solar system called the Kuiper (rhymes with viper) Belt. There are lots of icy, rocky objects out there. But they are so far from the Sun they are really hard to see, even with powerful telescopes.
Why Pluto is *not* a planet
A while back, Pluto was the ninth planet from the Sun. It was also the smallest planet.
But not anymore. Poor Pluto. Just how did it get "kicked out" of our family of planets? And who are its "real" family members?
Astronomers have already named three other objects in the solar system that are about the same small size as Pluto. They are Ceres [SEAR-ees], Makemake (MAH-kee-MAH-kee], and Eris (AIR-iss]. These objects, along with Pluto, are much smaller than the "other" planets.
Ceres orbits in the asteroid belt between Mars and Jupiter. Makemake, like Pluto, is part of the Kuiper [KI-per] Belt, which is a region of trillions of icy objects orbiting beyond Neptune. Eris' orbit is even farther out.
Astronomers have put these objects into a new family called dwarf planets.
Let's go there!
We finally got to visit Pluto, Charon, and the Kuiper Belt! On January 19, 2006, NASA launched a robot spacecraft on the long journey. This mission is called New Horizons. The spacecraft arrived at Pluto in July 2015, and will continue to study other objects in the Kuiper Belt from about 2018 to 2022.
With New Horizons, we are visiting and learning about the objects at the very edge of our solar system. They may help us understand how our solar system formed.