When is a planet a dwarf planet?

by Joan Marie Galat

When visiting schools or delivering presentations via Skype, one of the most common questions I’m asked is “Do you think Pluto should still be called a planet?” If only my opinion mattered! My reply brings approving nods from my audience. “Deep in my heart, Pluto will always be a planet to me.” But the nodding stops when I go on to say "I understand why the International Astronomical Union (IAU) felt it was necessary to redefine what constitutes a planet."

It is no longer enough for a celestial object to orbit a star and be larger than an asteroid. A planet must be round or nearly-round, with gravity strong enough to disperse celestial objects from its orbit. The IAU defines Pluto as a dwarf planet.While it is the right shape and does orbit the Sun, it also travels through an area where asteroids exist. Pluto moves through the Kuiper Belt, a zone of icy objects beyond Neptune’s orbit. Other dwarf planets include Eris and the former asteroid Ceres. 

Here’s some fun space trivia about Pluto and the planets in our Solar System.

Photo courtesy of NASA

• Pluto was classified as a planet for 75 years.

• Venus, Uranus, and Pluto rotate in a direction opposite to that of the other planets.

• Mercury travels around the Sun at an average speed of 172,300 km an hour (107,000 miles an hour).

• Mercury’s core is made up of iron.

• The surface of Venus is a desert. About 64 km (40 miles) above the desert surface, winds blow at 426 km/hour (265 miles/hour). The upper atmosphere swirls 50 times faster than the planet rotates!

• Gently rolling hills, as well as thousands of volcanoes cover most of the surface of Venus. Some craters are produced by volcanic eruptions, while others are produced by impacts.
  
  
• When closest to Earth, Mars is only 56 million km (35 million miles) away, but its farthest point is 400 million km (250 million miles).

• The clouds surrounding Jupiter travel at different speeds, rotating faster at the equator than near the poles.
  
  
• Another moon of Jupiter, called J3, is only 40 km (25 miles) across. It is the fastest moon in the solar system, traveling 113,300 km/hour (70,400 miles/hour). J3 would only take a few minutes to travel over North America, from the Atlantic to the Pacific coast.
  
  
• There are always storms on Jupiter. It is not unusual for a Jovian storm to be 9600 km (6000 miles) across. Wind on Jupiter has been measured at 650 km/hour (404 miles/hour).
  
  
• Electricity flows back and forth between Jupiter and Io, which has active volcanoes that erupt with hot gas and molten sulphur.
  
  
• Wind speeds on Saturn can reach 1770 km/hour (1100 miles/hour).
  
  

• Uranus is tipped on its side, causing each pole to alternate facing the Sun. At times, this makes Uranus’s polar regions warmer than its equator.
  

• Neptune’s largest moon is called Triton. It is one of the coldest places in the solar system, with temperatures of –391° F (–235° C). Triton revolves in the opposite direction from the planet’s rotation and is the largest moon in the solar system to do so.

 

Explore more cool space facts in Dot to Dot in the Sky, Stories of the Planets, published by Whitecap Books. Look for it at your local bookstore, online, or order an autographed copy from my website. Discover the Greek myths associated with each planet's namesake, along with astronomy facts and tips for observing the planets.

8” x 9”64 pp. ISBN 1-55285-392-6
Color illustrationsAges 8-12

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“This is an excellent book. It’s very interesting to read about the mythological stories behind the names of the planets of our solar system, as well as the planetary facts. I appreciate that Joan Marie Galat is very careful in making the important distinction between the myths and the facts of what we know today. I believe that star gazers of all ages will be fascinated by these stories.”

Julie Payette, astronaut