17. Pluto,Charon & the Kuiper Belt

 Reading: Chapter 12 pages 353-356


Poor little Pluto - the ninth planet, the misfit planet. Some people do not even regard it as a planet. Yet, recent discoveries in the outermost part of the solar system are showing us that Pluto is just one of probably thousands of objects that reside beyond the giant planets - in the Kuiper Belt, named after a Dutch planetary astronomer. These frozen worlds likely hold key clues to our understanding of the formation of the solar system. Pluto is given little attention in the textbook and the Kuiper Belt is barely mentioned. In this session we will summarize their basic properties and incorporate some of the very latest discoveries.

Pluto the Misfit Planet

As we shall see, Pluto and its large moon Charon (pronounced shehr-on) do not fit into our simple picture of 2 types of planets: terrestrial and giant. But problems with our simple planet classification is less important than what Pluto and Charon have to tell us about the outer solar system.

Discoveries of Pluto and Charon

Both Pluto and its moon Charon were discovered by US astronomers - Pluto was discovered by Clyde Tombaugh at Lowell Observatory in Flagstaff, Arizona, in 1930, as shown below, by comparing the motion of a faint object relative to the stars (just as, more recently, the Kuiper Belt objects).

After Clyde Tombaugh died in 1997, a website was created in his memory.

Charon was discovered by James Christy of the US Naval Observatory in 1978, in the photograph shown in Figure 12.15a


Even with a good telescope and clear skies Pluto and its moon Charon are fuzzy objects. Figure 12.15b is a Hubble Space Telescope picture of Pluto and Charon taken in 1994 (go here for more information about this image).

(1) What prompted the searches for a planet beyond Neptune? Why did astronomers think there might be additional planets? Were their suspicions justified in the end?

Characteristics of Pluto and Charon

There is no doubt that Pluto is small compared with the other planets. At the same time, compared with its parent planet, Charon is the largest moon in the solar system.

Here is a comparison of the sizes of Pluto and Charon with the United States (click on the picture to get a larger version):

With such fuzzy images, even with the best telescopes, how we able to measure the sizes of Pluto and Charon? The opportunity was provided by a fortunate quirk of Pluto and Charon's orbits. The rotation axis of Pluto as well as Charon's orbit around Pluto are, like Uranus, aligned almost with the plane of its orbit:

During the period from 1985 through 1990, Pluto and Charon would eclipse each other on a daily basis - a Pluto day, that is. These eclipses turned out to be very important, since observations of the eclipses (called "mutual events") led to the first accurate determination of Pluto' and Charon's sizes, and subsequently to surprising albedo maps of the surface. Here are albedo maps of Pluto based on these mutual eclipses of Pluto and Charon. Below are maps based on Hubble Space Telescope data.

This synthetic image of Pluto is created by processing many Hubble Space Telescope observations of the planet (typical images are shown in the small insets on the upper left). The dark and light regions could be due to different types of ices on the surface, or could be topological features.

This surface map of Pluto's albedo is reconstructed by imaging the planet at different times during its day.

(2) Compare the albedo maps derived from the mutual eclipses with the maps derived from Hubble data. Did the mutual eclipse researchers get roughly the same features as shown by Hubble? (e.g., compare where the light and dark regions are on Pluto in each case).

The mutual eclipses of Pluto and Charon provide their sizes. Measurements of Charon's orbit gives us Pluto's mass. With these pieces of information models have been constructed of Pluto's interior (the interior of Charon is really little more than guesswork).

(3) (a) HOW does Charon's orbit allow us to determine Pluto's density?
(b) What are the densities of rock and of pure ice?
(c) The density of Pluto is about 2 g/cm3--corresponding to about 70% rock and only 30% ice. By comparing the density of Pluto with the densities of Charon and of the moons of Uranus and Neptune, would you judge Pluto to have more or less ice that is typical for small bodies in the outer solar system?
(d) Compare the density of Pluto with that of Triton.

(4) Compare the properties of Pluto and Charon listed above with the table of properties of terrestrial and giant planets, Table 8.2 of the textbook. Why Pluto is a "misfit" planet? Consider Pluto's size, orbit, moon, density, etc;

Impact Origin

Pluto and Charon are thought to have come together and collided--in a similar way that we believe the earth's Moon collided with the Earth and re-formed to form the Moon. This impact theory for the origin of Charon explains various 'strange' things about Pluto and Charon. One clue is the density of Pluto and Charon.

(5) (a) Say Pluto had a lot more ice before it suffered a big impact, what would have happened to the ice during the impact? Would the density of Pluto increase or decrease due to the impact?
(b) What is believed to have happened to Triton that might similarly have led to Triton loosing ice and having a similar, more rocky composition as Pluto?

Is Pluto a Planet?

This topic has led to a major public debate. You have just listed why Pluto is a misfit - but some want to re-classify Pluto as an asteroid or "minor planet" instead of a planet.

Is Pluto a Planet?

(6) (a) What are some of the criteria that people use for deciding whether to call Pluto a planet or not?
(b) Is this a scientific issue?
(c) So, what do YOU think? Should Pluto be called a planet?

The Kuiper Belt

Pluto & Charon are not alone. Over the past 5 years over a hundred new objects have been discovered beyond the orbit of Pluto. These objects are listed and plotted as they are discovered. Lots of information can be found on the Kuiper Belt page of Dave Jewitt, the co-discoverer of the first Kuiper Belt object, including a movie sequence of its detection.

The issue of why the outer solar system dramatically changes at 30 A.U. from 4 large giant planets to lots of small icy objects is hotly debated. At stake is the understanding of how our solar system formed and evolved to be the way it is now--and whether we should expect other solar systems to be the same.

(7) (a) Look at the plot above. The masses of outer solar system objects are plotted vs. distance from the Sun. The graph has scales that increase in powers of ten--with MASS (relative to earth) on the Y-axis vs. Distance from the Sun (in A.U.) on the X-axis. How many powers of 10 are there on each axis?
(b) Note that the giant planets form a group that are 10-300 times the mass of the earth. Pluto, Charon and Triton form a group that is 0.001-0.01 times the mass of the earth--that is 0.1 to 1% of the earth's mass. The new "QB1" objects are about 10 to 100 times smaller than Pluto/Charon/Triton. The smallest objects, comets, are found outside Pluto's orbit, greater than 50 A.U.--how many Kuiper Belt objects (such as QB1) does it take to add up to the mass of the Earth?
(c) How many comets does it take to add up to the mass of the Earth?

It is now thought that there are probably 1000s of small objects--with masses between the those of Pluto and comets--that orbit the Sun at distances between 50 and 100 A.U., forming a "fuzzy" disk in the ecliptic plane. This disk is named after the Dutch astronomer, Gerald Kuiper who proposed such a disk existed in the 1950s. These Kuiper objects are just now being discovered.

The following SKETCH shows the outer planets and the location of the objects that we believe make up the Kuiper Belt. Pluto's orbit is shown in red. The four outer giant planets (Jupiter, Saturn, Uranus, and Neptune) have their orbits outlined in blue.

(8) Using this picture and what you know about the sizes of the orbits of the outer planets (e.g. Neptune's orbit is 30 AU from the Sun) how far away from the Sun are the objects at the inner edge of the Kuiper Belt? What about objects at the outer edge?

Here is a MAP showing the actual location of the Kuiper Belt Objects discovered to date. The map is from the Minor Planets Center at the Harvard/Smithsonian Center for Astrophysics and is updated as new objects are found.

Pluto Links - ( Here is a Pluto scientist's ranking of Pluto sites):

Model Answers for the study session.