# Presentation: 518361

## 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

**Notes:**
This is the Solar System we all learned about in school- nine planets and the Sun.  

## And Then There Were Eight

- Image from JPL

**Notes:**
After the IAU resolution on August 24, 2006 Pluto is longer considered a planet.  This presentation will explore how we got to the Solar System we knew before and how we have arrived at this latest version.  

## 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?

**Notes:**
There has never been a scientific definition for a planet before.  2003 UB 313 is actually larger than Pluto- based on these definitions from astronomy texts and the dictionary should it be a planet?

## 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

**Notes:**
Until 1781 astronomers only knew about six planets in our solar system.  Except the Earth, all the planets had names taken from Roman mythology by this time.

## 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.

**Notes:**
With the discovery of Uranus some basic issues had to be settled.  First of all, now that there was a new planet, the first found using a telescope, how would it be named?  Would the Roman mythology names be continued or would other names be used? In mythology Uranus was the Roman personification of the sky god and the father of the Titans.

## Uranus- The First New Planet

- Distance: 19.1 AU Doubled the size of the Solar System
- Diameter: 4 Earth diameters

- Image courtesy of NASA

**Notes:**
Note that the rings of Uranus were not seen by anyone until Voyager imaged them.  The size and make up of Uranus is similar to Jupiter and Saturn- lots of ices.  Methane makes it appear blue.  Note also that Uranus can be seen naked eye under the right conditions.  Use NSN OGOU kit info to explain the size of our solar system and our galaxy as you go along.  Most audience members don’t understand what an AU is or a light year so be sure to define these terms!

## 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.

**Notes:**
When astronomers tried to calculate and predict where Uranus should be in the sky, they ran into trouble.  The planet was not where they thought it should be.  Davies’ calculations were very accurate, but the star charts for that part of the sky that were available to the observatory in England were not detailed enough. When they looked for Neptune they saw many stars they could not identify and thus could not tell if one of them was the new planet or not.  The Berlin Observatory had access to much better charts and thus were able to see that there was something “extra”  the very first night they looked for the new planet.  A controversy broke out about who was to get credit for finding the planet and in the end both Davies and de Verrier were credited equally. 
NOTE:  Galileo had actually seen Neptune in the early 1600’s according to his logs.  But he thought it was something else and never went back to examine it more closely.  Can you imagine what might have happened to him had he done so and announced that he had found a new planet?  Several other astronomers’ logs also indicate that they had seen Neptune, but thought it was a comet.  It can also be seen naked eye under the right conditions.

##  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.

**Notes:**
Based on what were thought to be perturbations in the orbit of Neptune there was speculation about the existence of a ninth planet as far back as the late 1800’s.  Percival Lowell was a firm believer in Planet X and wanted to look for it, but he died before the special camera was built and installed.  Pluto’s moon Charon was found later and was named for the mythological boatman who was believed to carry the souls of the dead across the river Styx to be judged by Pluto.  One needed to have something with which to pay Charon- thus the need to bury some coins or something made of gold with the dead.

## Clyde Tombaugh 1906-1997 

- Clyde Tombaugh 1906-1997

**Notes:**
Clyde Tombaugh was a farm boy from KS who was interested in astronomy and was willing to do the job required by the search.  He had no degree in astronomy at the time.  The glass plates he used were very large and each required several hours of exposure time with him hand guiding the telescope.  
Clyde Tombaugh was a farm boy from KS who was interested in astronomy and was willing to do the job required by the search.  He had no degree in astronomy at the time.  At some time after he had found Pluto he enrolled at the University of Kansas where an astronomy professor refused to allow him to enroll in the beginning astronomy class saying he was not qualified.  Even though he had not taken any other courses the professor helped him to enroll in a more advanced course and he subsequently earned his degree from KU.  The plates he used were very large and each required several hours of exposure time with him hand guiding the telescope.  To his death he emphatically said that Pluto was a planet!  

## Finding Pluto

- Pluto images by Nathan Twining Observatory

**Notes:**
After being up all night exposing plates, Tombaugh would spend hours during the day using a blink comparator (picture on the left) looking back and forth between pairs of plates.  He once estimated that he had looked at over 1 million stars before he found Pluto. The images on the left are not Tombaugh’s. They were taken much more recently, but demonstrate what he had to do.  Both images have Pluto on them. The trick is to find something that has moved significantly in relation to the stars in the image. Can you see it?  Imagine spending day after day looking at plates like the one you can see on the blink comparator in the picture.

## Here it is!

- Pluto images by Nathan Twining Observatory

**Notes:**
Pluto is marked with an arrow in each image.   The discovery of Pluto set off a wave of Pluto mania! The media hailed Pluto as the tenth planet, the public was very excited and to this day Pluto has remained very popular with the public.  It is thought that the naming of the Disney dog character Pluto was for the “planet” Pluto.  Why else would one name a cartoon dog after the Roman god of the underworld?

## 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

**Notes:**
New Horizons was launched on January 19,2006 (Clyde Tombaugh’s 100th birthday would have been Feb 4, 2006) and is the speediest probe ever sent out by NASA.  It will do a gravity assist maneuver around Jupiter with its closest approach being on Feb 28, 2007, where it will gain more velocity and where its cameras will be tested.  Its closest approach to Pluto will be on July 14, 2015.  It will send back the first close up images of Pluto. Even Hubble can’t image Pluto with much detail.    Thus far all seems to be operating as planned.  The spacecraft which will also go on to explore objects in the Kuiper Belt carries a small vial filled with ashes of Clyde Tombaugh. 

## 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

**Notes:**
With this slide begin to emphasize how different Pluto is from the eight planets.  The only scientific argument for it being a planet is that it is round and orbits the Sun.  Emphasize how different Pluto is- it doesn’t fit with the terrestrial planets (top row) or the gas giants (bottom row).  It seems to be very different.

## The Kuiper Belt

- John Hopkins University

**Notes:**
This slide demonstrates how Pluto’s orbit reaches out into the Kuiper Belt. The Kuiper Belt is potentially filled with thousands of icy bodies.  Notice that the orbit of Neptune is inside the Kuiper Belt.

## Orbital Paths of Planets and Pluto

**Notes:**
This points out the 17 degree tilt of Pluto’s orbit.  It does not cross the orbit of Neptune.  Origin of this diagram unknown.

## Orbit of UB 313 (Known as Xena)

- NASA

**Notes:**
Now compare the orbit of UB 313 to that of Pluto.  UB 313 takes about 450 years to orbit the Sun.  Pluto takes about 250 year.  

## Kuiper Belt Object Sizes

**Notes:**
This slide compares the size of some of the better known Kuiper Belt objects, our own moon and Pluto. Be sure to point out that the Moon is there for size comparison only- it is not a KBO!

## 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.

**Notes:**
This briefly outlines the IAU resolution. This is a good chance to emphasize that science is not static, that terms and ideas change as we learn more, just as those in the past learned and revised their ideas.  Science changes as new data is discovered.   Review how the first proposal was to have a planet defined as having enough mass that gravity would cause it to be round (hydrostatic pressure) and that it orbits the Sun, but how that would have made it necessary to include many of the Kuiper Belt objects.   Emphasize that this is the first time that there has been a scientific definition of a planet.  Review briefly how the name planet came into use and what it meant.  This is an excellent time to talk about the search for extrasolar planets and how many have so far been found.  Explain that more and more are being found and the size of those found so far.  Talk about the Kepler mission and its goals to find earth size planets in the habitable zones of sun-like stars. Talk about how this will be done and where in the sky.  Then go on to say that eventually we may find other small bodies around other stars.  When this happens it will be very necessary to have definitions in place.  Talk about how the Spitzer Space Telescope has already found 2300 possible developing solar systems in the Orion Nebula.  We could be up to our eyeballs in planets!!!  Emphasize that Pluto will be the object to which all other Dwarf Planet candidates will be compared, thus it is not really demoted, but has been given a new role not only for our own solar system, but possibly for our future exploration of other planetary systems.

## So Long Planet Pluto and  Hello Dwarf Planet Pluto!

- International Astronomical Union

**Notes:**
This image released by the IAU shows our Solar System as defined by the new scientific definition of a planet.   Again emphasize that this is an example of how science is done, that it changes as new discoveries are made.