Honey, I Shrunk the Solar System or Pluto We Barely Knew Ye as a Planet Image credit JPL

The Way it Was… Image from JPL

And Then There Were Eight Image from JPL

From Where Did the Word Planet Come? The word “planet” is derived from the Greek word for “wanderer” and was traditionally applied to any heavenly body that moved with respect to the stars. In this sense the Sun and Moon were also planets. Dictionary says that a planet is any one of the nine large bodies that orbit the Sun. But some objects have been found that are larger than Pluto- so are they planets?

Who Discovered the First Planets? Ancient cultures knew that some objects were not fixed in the sky like the stars. The Greeks knew of five such objects: Mercury, Venus, Mars, Jupiter and Saturn By 800 B.C.E. Babylonian astronomers had records of planetary motion for Venus, Jupiter and Mars.

The Solar System Until 1781: Mercury, Venus, Earth, Mars, Jupiter and Saturn Images from NASA

The Solar System Grows: What to Name a New Planet? March 13, 1781 William Herschel discovers what he thinks is a comet, but he has discovered a new planet- the seventh in our Solar System. Herschel wanted to name the new planet George after King George III of England. It was decided to continue with the Roman god names that had been used for the other planets, thus it was named Uranus. This set the standard for the convention of using Roman god names for the planets.

Uranus- The First New Planet Distance: 19.1 AU Doubled the size of the Solar System Diameter: 4 Earth diameters Image courtesy of NASA

Another New World: Neptune The orbit of Uranus was not as expected. John Couch Davies, a 24 year old Cambridge grad, thought that this might be caused by another unknown planet In 1845 he submitted his calculations to the Astronomer Royal of England. English star charts not good enough. At nearly the same time French astronomer Urbain Jean Joseph de Verrier did the same calculations. The Berlin Observatory was given his data and the planet was found the first night due to better star maps.

Neptune Following the Roman god theme the planet was named Neptune, for the sea god since it was blue 30 AU from the Sun Diameter: 3.9 Earth diameters Image by Hubble Space Telescope

A Ninth Planet? Speculations about a ninth planet date back to the late 1800’s. Percival Lowell urged that a special camera be built to look for Planet X. In 1929 the camera was finished and installed at Lowell Observatory in Flagstaff, AZ Clyde Tombaugh discovered Pluto on February 18, 1930 after looking at over one million stars Name Pluto suggested by Venetia Burney, an 11 year old girl. Pluto was the Roman god of the underworld.

Clyde Tombaugh

Finding Pluto Pluto images by Nathan Twining Observatory

Here it is! Pluto images by Nathan Twining Observatory

Pluto 39.5 AU from Sun Diameter: about 0.18 Earth diameter (about 1400 miles) Pluto and Charon essentially a double system NASA New Horizons mission will reach Pluto in 2015 Image by Hubble Space Telescope

The Arguments for and Against Planethood for Pluto Pluto very small Doesn’t fit into any other categories of planets- terrestrial or gas giants Orbit strange- tilted 17° from plane of the solar system May be typical of thousands of icy objects found far from the Sun It is round like a planet and it orbits the Sun. Image by JPL

The Kuiper Belt John Hopkins University

Orbital Paths of Planets and Pluto

Orbit of UB 313 (Known as Xena) NASA

Kuiper Belt Object Sizes

The International Astronomical Union Resolution A planet is for the first time defined scientifically. A planet orbits a star, has sufficient gravity to become round, and has cleared the neighborhood around its orbit. A Dwarf Planet orbits a star, is not a satellite, has sufficient gravity to become round and has not cleared the neighborhood of its orbit. Pluto is the prototype of this class and currently includes Ceres and UB 313. Others will be decided upon later. A third class, Small Solar System Bodies, was defined as all other objects except satellites. This includes most asteroids, most comets and most trans Neptune objects.

So Long Planet Pluto and Hello Dwarf Planet Pluto! International Astronomical Union