ORGANIZATION OF THE MATERIAL

Many textbooks, at this point in the material, go through the planets one by one. This turns planetary science into a series of facts about each planet rather than a comparative study of the properties and processes that are common or differ in a systematic way. Please use your text as a resource, but we will be organizing the structure a bit differently, grouping planets together by type. This is how we will do it, over several Sessions:

Solar system overview Solar system formation Terrestrial Planets: Moon and Mercury Earth, Venus and Mars interiors geology atmospheres Life in the solar system Giant Planets: Jupiter and Saturn Uranus and Neptune Moons Rings Pluto / Charon Cosmic Debris - 3 "swarms" The Kuiper Belt The Oort Cloud - Comets The Asteroid Belt

An Inventory of the Solar System

Our knowledge of the solar system, even basic quantities such as the number of objects, is constantly changing. The Sun and the five closest planets have been observed for millenia. The more distant planets, the moons (other than Earth's Moon) and smaller objects (such as asteroids) required telescopes for detection. About a dozen new objects in the solar system are being discovered per year.

(1) Distinguish between: (a) a star and a planet (b) a planet and a satellite (c) a moon and the Moon (2) Which planets were not detected until after the telescope was invented? When was each planet discovered? (3) Below is a table of days of the week in different languages. Note how they relate to the Sun, Moon and the 5 planets that can be seen with the naked eye. Do you know of any other languages where this relationship holds? Is this a coincidence?

Here is a site from The Nine Planets that goes into planetary linguistics.

Planetary Properties

The tables in the appendices are full of planetary facts. The objective of this section is for you to become familiar with the tables of information and with the basic properties of the planets.

 

(4) Look at the masses of planets. For comparison, mass of the Sun is 1.99 x 1030 kg. (a) which is the most massive planet? (b) How many of this heaviest planet's masses are equivalent to the Sun's mass? Msun ----------- = Mplanet

The table below compares the planets' properties to the Earth's. This really emphasizes the differences between the terrestrial and giant planets. (Note that if the diameter of a planet is, 0.5 x the diameter of Earth, then the radius of the planet is also be 0.5 Earth's radius).

The Overall Layout of the Solar System

Orbital Inclinations
All planets orbit the sun close to the same plane--the ecliptic plane--which is defined by the Earth's orbit. The angle of a planet's orbit from this plane is the orbital inclination. Find this in an appendix.

(7) (a) Which planet (not moon/satellite) has the largest inclination? (b) How many degrees? (c) Look at the diagram below of orbital inclinations (the angles are correct but the horizontal distances are not to scale). From our viewpoint standing on Earth, where in the sky are the planets located?

Orbital Distance and Eccentricity

It is difficult to show all of the solar system to scale in one diagram - the outer solar system is on a much larger scale than the inner solar system. The diagrams below are projections down onto the ecliptic plane - we are looking down on the Sun's north pole. The diagrams are drawn to scale so that you can see the true relative sizes of the orbits and the eccentricity of each orbit.

Solar System Live shows you where the planets are in their orbits right NOW. This is an interactive Solar System orrery that let's you view the positions of the planets for any time time and date.

(8) Between which 2 planets' orbits are the asteroids? (9) Which two planets appear to have intersecting orbits?

All planetary orbits are elliptical. The eccentricity of an orbit is how elongated it is. Eccentricity, e = 0 means a circle, e = 1 means a cigar-shaped orbit. Find orbital eccentricity in an appendix.

 

(10) Which planet has the greatest eccentricity? What is its value of e?

All planets orbit the Sun in the SAME direction--counter-clockwise looking down from way up above the north pole--this direction is called prograde. Look at the diagram of orbits above. This is the same direction that the Earth spins. Look at the Sidereal Rotation Period column of Appendix C ("sidereal" means relative to the stars). Look at the spin periods (in days) of the planets--if the number is negative it means the planet spins retrograde--backwards to the Earth.

 

(11) (a) Do all or most planets spin the same way as Earth? (b) Which planet(s) spin retrograde?

Terrestrial and Jovian Planets

The following figure shows the sizes of the planets and the Sun to scale (but not their distances, obviously!).

This diagram shows how the planets can be grouped into 2 main catagories - smaller, inner, terrestrial (="Earth-like") planets and larger, outer, Jovian (="Jupiter-like") or giant planets. The next table lists other ways in which the two types of planets can be compared. Check out Table 8.2 of the text. This is an important table.

(12) Look at the list of comparisons between Terrestrial and Jovian planets in the table. Which of the characteristics are related to the planet's size and which are related to the planet's distance from the Sun? (13) Why do you think there are only two types of planets - not 3 or four types or a continuous range of types? (The answer is not obvious - more about this issue at the end of the course).

Interplanetary Debris

There are 3 main populations or "swarms" of objects that populate the solar system in addition to planets (and their satellites). These are asteroids, comets and a newly-detected group called Kuiper Belt objects. This last catagory - which includes Pluto and its moon Charon - is so recent that many textbooks barely mention it. But new Kuiper Belt objects are being discovered very year, at a rate of about one per month. The objects that make up these 3 main swarms are often called interplanetary debris - even the larges of these objects are MUCH smaller than the planets and also because the sizes range down to the smallest chunks of material. Unlike the regular, well-ordered orbits of the planets, the objects in the 3 swarms of interplanetary debris can have irregular orbits. Frequently small pieces of debris strike the Earth - if they are very small they burn up as "shooting stars" in the atmosphere, if they are larger they may land as meteorites. Chemical analysis of meteorites allows us to study the interplanetary debris - without going into space.

(14) Have you seen a meteor shower? Or a meteorite? Describe what you saw.

Table 8.3 summarizes the Four Major Characteristics of the Solar System - make sure you know these!

1. Large bodies in the solar system have orderly motions
2. Planets fall into two main catagories
3. Swarms of asteroids and comets populate the solar system
4. Several notable exceptions to these general trends stand out

We have discussed 1,2 and 3 in this session.

Spacecraft Exploration of the Solar System

The Web has a vast amount of information about current and future missions to other planets. A good place to start is the SEDS site set up by University of Arizona students.

 

(16) Pick a planet - any planet - find at least one past or future mission to that planet. Use the SEDS site or any of the links below to find a picture of the spacecraft. What instruments did it have on board? Did it take any pictures? Can you find some of the pictures on the net?

Model answers to the comprehension questions.