This is the Solar System we all learned about in school- nine planets and the Sun.
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.
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?
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.
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.
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.
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 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!
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.
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?
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.
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.
This points out the 17 degree tilt of Pluto’s orbit. It does not cross the orbit of Neptune. Origin of this diagram unknown.
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.
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!
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.
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.
What is a planet
Honey, I Shrunk the Solar
Our Solar System and the Three Dwarfs
Image credit JPL
From Where Did the Word “Planet”
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
• 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
• 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
• 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
Pluto images by Nathan Twining Observatory
Here it is!
Pluto images by Nathan Twining
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
• 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
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
• B) Must orbit a star, not be a star, and
not be a satellite of a planet.
The International Astronomical
August 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..