Pluto – Planet or Not?

A few years ago Pluto was demoted from a planet to a dwarf planet. Many people were upset by this. I’m not sure why so many people cared so much but they clearly did. When I was young we learned a mnemonic for the planets - My Very Educated Mother Just Served Us Nine Pies for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Removing Pluto messes up the mnemonic. So why was Pluto demoted and just who was responsible for the demotion?

First of all, a few things about Pluto:

·        Pluto was discovered on February 18, 1930 by Clyde Tombaugh from the  Lowell Observatory in Flagstaff, Arizona

·        Pluto is smaller than the planets. It’s even smaller than our Moon

·        Pluto is mostly rock and ice

·        The name Pluto, who was the god of the underworld in Greek mythology, was suggested by an 11 year old girl from Oxford, England named Venetia Burney

·        Pluto circles the sun in about 248 years and is on average about 3.7 billion miles from the Sun (Earth is about 92 million miles from the Sun)

·        Pluto has 5 satellites (moons) – Charon, Nix, Hydra, Kerberos and Styx

·        Pluto has never been visited by a spacecraft (but that will change in 2015)

Who demoted Pluto?

The International Astronomical Union (IAU), an organization of professional astronomers, defined a planet and a dwarf planet, and designated Pluto as a dwarf planet on August 24, 2006.

Why did the IAU demote Pluto?

The IAU defined a planet as a celestial body that:

·        Is in orbit around the Sun

·        Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and

·        Has cleared the neighborhood around its orbit

Pluto is in orbit around the Sun and is relatively round but it passes inside the orbit of Neptune as it orbits the sun, so it doesn’t meet the last criteria above. Additionally, Pluto is different from the planets in several ways.

The four inner planets (Mercury, Venus, Earth and Mars) are known as terrestrial planets as they are dense and rocky. The four outer planets (Jupiter, Saturn, Uranus and Neptune) are known as gas giants as they are mostly made of gas (helium and hydrogen). Pluto, which is located beyond the gas giants, is more like the terrestrial planets than the gas giants. On the other hand, Pluto is more like the Kuiper Belt Objects than the terrestrial planets.

Kuiper Belt

The Kuiper Belt is an area beyond the planets made up of numerous relatively small objects orbiting the Sun. There may be hundreds of thousands of icy bodies larger than 62 miles and an estimated trillion or more comets within it. Pluto is now deemed to be a member of the Kuiper Belt. The Kuiper Belt was predicted by Gerald Kuiper in the early 1950s but objects in it were first seen in 1992. Kuiper Belt Objects (KBOs) are relatively small and very difficult to detect.

New Horizons

Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface. A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in July 2015. When New Horizons was launched in January 2006, Pluto was still considered a planet. We will learn a lot more about Pluto when New Horizons reaches it.

Pluto

http://en.wikipedia.org/wiki/Pluto

http://www.space.com/2791-pluto-demoted-longer-planet-highly-controversial-definition.html

http://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html

http://www.nasa.gov/audience/forstudents/5-8/features/what-is-pluto-58_prt.htm

Dwarf Planet

http://en.wikipedia.org/wiki/Dwarf_planet

https://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf

Kuiper Belt

https://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs

http://en.wikipedia.org/wiki/Kuiper_belt

New Horizons

http://www.nasa.gov/mission_pages/newhorizons/main/#.U3iQqyj-ITA

http://en.wikipedia.org/wiki/New_Horizons