Honey, I Shrunk the Solar System

or

Our Solar System and the Three Dwarfs

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.

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. 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.

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. His star maps were 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.

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 1906-1997

Finding Pluto

Pluto images by Nathan Twining Observatory

Here it is!

Pluto images by Nathan Twining Observatory

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

2003 UB313

Orbit of UB 313 (now named Eris)

NASA

August 16, 2006 Proposal to the IAU

• A) Must be of sufficient mass to be spherical in shape. (Usually at least 500 miles in diameter and a mass of 5 x 1020 Kg.)

• B) Must orbit a star, not be a star, and not be a satellite of a planet.

The International Astronomical Union ResolutionAugust 24, 2006

• 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. This applies to only the first 8 “Classical Planets” – Mercury through Neptune.

• 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 Eris (formally UB313). 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-Neptunian objects..

The Deciding Vote August 24, 2006

So Long Planet Pluto and Hello “Dwarf Planet” Pluto!

International Astronomical Union

New “Dwarf Planets”

Ceres

Pluto

2003 UB 313Eris & Dysnomia

Newest Dwarf Planets

Haumea

Makemake

How to Remember the Planets

My Mercury

Very Venus

Educated Earth

Mother Mars

Just Jupiter

Served Saturn

Us Uranus

Nine Neptune

Pizzas

X N = ?Pluto

To make matters worse . . .

The Minor Planets Center –

on September 7, 2006 –

gave PLUTO an Asteroid number!

PLUTO is now number

1 3 4 3 4 0