Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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Chapter 7

Read Chapter 7 from Discovering Astronomy.


WebAssign -- Chapter 7. Answer homework questions covering Chapter 7.

What is a planet?

In 2006, Pluto officially lost its status as a planet. It is now officially called a dwarf planet.

The debate regarding Pluto's status has been going on since astronmers began discovering many rocky, icy objects at similar or further distances than Pluto.

There are two interesting articles about the planet debate at space.com. (1) What is a Planet? Debate Forces New Definition (Nov. 2000) and (2) Controversial Proposal Would Boost Solar System's Planet Tally to 12.

The above articles on space.com predate the designation of Pluto as a dwarf planet. In fact, these articles predate the discovery of the so-called Tenth Planet (originally code-named Xena after the television Princess Warrior) that is now called Eris.

Announced July 30, 2005, the discovery of Eris, which is larger than Pluto, forced the hand of IAU to make a decision. Either add more planets to the list of planets in our solar system or distinguish between the much larger 8 planets and all of those rocky, icy bodies out beyond Neptune.

August 24, 2006, the IAU officially defined the term planet for the first time. They also created a new term dwarf planet. The news release by the IAU that describes resolutions 5A and 6A, passed August 24, 2006, says:

Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a "Planet" in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects , except satellites, orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

1The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

IAU Resolution: Pluto


The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.1


(Picture of Pluto and Charon taken by Hubble Space Telescope in 1990)

Ok, so Pluto is NOT a planet, but a dwarf planet. Nonetheless, it's still interesting!

The eccentricity of its orbit is 0.25 which is greater than any other planet. Also, its orbital plane is tilted 17° with respect to the ecliptic. This is much greater than the other planets which for the most part orbit in the ecliptic planet.

Its density is 2 g/cm3 which is larger than that of the gas giants but much smaller than the terrestrial planets. This indicates that its mostly rock and methane ice.

Astronomers studying the orbits of Uranus and Neptune suggested that a more distant planet existed. Subsequently, in 1930 Pluto was discovered by Clyde Tombaugh, though it turns out that the calculations that suggested Pluto's existence were imprecise and Pluto's mass is too small to cause the discrepancies.


To discover Pluto, Tombaugh used a procedure called blinking. In the animated gif shown below, you will see separate images of the night sky, taken a number of hours apart.

Do you see the object that's moving relative to the background stars?

In 1930, Tombaugh used a device called a blink comparator. The mechanical device would literally move photographic plates back and forth under a microscope.

Today, computers are used to compare images and look for objects that move. Astronomers have written computer programs that operate telescopes with digital cameras. The telescopes take multiple pictures of a certain part of the night sky. Computers then automatically compare the images, find any moving objects, and compare them with known objects to determine if the moving objects are newly discovered or not.

This is the method that astronomers use to find asteroids and Kuiper Belt Objects (KBOs). In fact, a Pluto-sized object has recently been found using this technique. Could it be the tenth planet?

Pluto's Moons

Why is it useful to find Pluto's largest moon, Charon?

The principal method we have for measuring the mass of a planet is to measure the period and semimajor axis of its companion moon. This allows us to calculate the sum of the masses of the planet and moon. By measuring the radii of Pluto and Charon and by assuming similar composition, we can infer the mass of Pluto.

Charon was not discovered until 1978 when James Christy noticed that some pictures of Pluto (taken with ground-based telescopes) showed a buldge. The bulge would show up at different places in the picture or would not show up at all, as the moon orbited Pluto.

Until Charon was discovered, Pluto's mass was merely an estimate. However, since Charon was 1990 clearly imaged by Hubble Space Telescope, we can more precisely calculate the mass of Pluto.

Recently (2005), the Hubble Space Telescope found two additional moons, temporarily called S/2005 P1 and S/2005 P2, around Pluto, thus giving Pluto a total of three moons (see the press release).

New Horizons Mission

The New Horizons Mission is the first flyby of Pluto and Charon. The probe was launched Jan. 19, 2006. It will be closest to Pluto on Tuesday, July 14.

July 3, 2015

Documentary about Pluto's demotion

Watch Bye Bye Pluto.






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