There are many, many lectures that can be given about various facets of the Solar System. One can easily give a lecture or two about each planet. Instead of doing this, I will focus on some of the common features of the Solar System and attempt to address questions as to why one planet (or moon) is like/unlike the others. To do this, I will start with a brief tour of the Solar System, and present some odd factoids on the way. I will not give explanations for every phenomena; these will be revealed (I hope) in the next few lectures. Below is a table summarizing some of the properties we will focus on:

 

Name	Distance	Period	Mass	Density	Surf. Temp	Rotation	Tilt	Comments
	(A.U.)	(year)	(earth)	(1=water)	min/max  (C)	Time	(degrees)
Mercury	0.387	0.24	0.056	5.4	-170 /+430	58 d	7	Many craters
Venus	0.723	0.61	0.82	4.2	472	-243 d	2	Thick atmosphere
Earth	1.0	1.0	1.0	5.55	-50/+50	24 h	23	Oxygen atmosphere
Moon			0.012	3.35	-170/+130	29 d	6	Binary companion
Mars	1.52	1.88	0.11	3.3	-140/+20	24 h 37 m	24	Polar ice
								2 small moons
Jupiter	5.2	11.87	318	1.34	-130	9 h 50 m	1	Many moons/rings
								H/He rich
Saturn	9.5	29.4	95	0.69	-180	10 h 39 m	2	Many moons/rings
								H/He rich; Moon with atmosphere
Uranus	19.2	84	14.5	1.29	-220	17 h 14 m	98	Many moons/rings
								H/He rich
Neptune	30.1	165	17.2	1.66	-216	16 h 3 m	2	Many moons/rings
								H/He rich
Pluto	39.4	248	0.002	2.0	-230	6d 9 h	122	Binary with Charon
								Highly eccentric orbit

Two other features of the Solar System must be noted. First, there is a collection of rocky asteroids in the Solar System, mostly at a distance of 2.8 A.U. from the Sun. Most of these asteroids are extremely small (almost too small to stand on), but a few have sizes of up to 500 kilometers. If all these asteroids were added up, their combined mass would be about 1/1000 that of the Earth

The final constituent of the Solar System are comets. Some comets orbit in the Kuiper belt, a region of space beyond the orbit of Neptune that should contain many icy bodies (including Pluto). Most comets, however, are in highly elliptical orbits around the Sun. At their closest approach, these comets can get much closer to the Sun than the earth. On the other hand, Kepler's law states that most of the time, these objects are far from the Sun. Evidence indicates that there is a spherical clouds of comets, called the Oort Cloud that extends from 20,000 to 100,000 A.U. from the Sun.
There are some interesting trends apparent in the above data.

The planets in the solar system tend to break down into two groups, dense, rocky low-mass terrestrial planets in the inner part of the Solar System, and hydrogen-rich, massive Jovian planets in the outer Solar System.

The (uncompressed) density of the planets tend to decrease with distance from the Sun. Planets in the outer parts of the solar system have a density near that of water, while the inner terrestial planets are rocky.

Planets with little or no atmosphere have large daytime/nighttime temperature fluctuations. Gases such as carbon-dioxide absorb infrared radiation and prevent the energy coming from the Sun from easily radiating away into space.

If you count the asteroid belt as a planet, then each planet is roughly twice as far from the Sun as the previous planet. An even better approximation is to start with the sequence 0, 3, 6, 12, 24, etc., where each number (except the first) is obtained by doubling the previous one. Add 4 to this number, divide by 10, and that gives a very good approximation for this distance to the planet. (In other words, Mercury would be (0 + 4) / 10 = 0.4 A.U., Venus would be (3 + 4) / 10 = 0.7 A.U., etc.) The sequence breaks down at Neptune (but works for Pluto), and the asteroid belt must be counted as a planet, but other than that, Bode's Law is a good way to predict the distance of each planet from the Sun. (In fact, Bode's Law was used to predict the existence of the asteroid belt.)

Other interesting facts about various bodies of the Solar System.

Mercury is heavily cratered; it looks almost like the Moon. But its density is much greater, almost like iron. Mercury's diurnal period is in a 3:2 resonance with its orbit; that is, Mercury rotates on its axis exactly 1.5 times every Mercurian year.

Although Mercury is much closer to the Sun, Venus is the hottest planet in the Solar System. It has a thick carbon-dioxide atmosphere with sulfuric acid clouds.

The Earth-Moon system is really a binary planet. But, as can be seen from the planets' densities, the two bodies are made of different materials. The Earth is also unique in that it has large amounts of oxygen in its atmosphere. Oxygen is quite reactive, and normally doesn't exist long in gaseous form.

Mars has icy polar caps, made of frozen carbon-dioxide and water, and a thin atmosphere. The remains of ancient rivers can also be seen. Mars' two moons, Phobos and Deimos, are asteroid size bodies.

Jupiter has a composition like the Sun, and spins rapidly. Its fast rotation produces a substantial centripetal force which flattens the planet. Jupiter has many moons and a faint system of rings. The Galilean moons in particular are remarkable for their variety.

Like Jupiter, Saturn is noticeably flattened, due to its rapid rotation, and like Jupiter it has many moons. (Its largest, Titan, has a methane-ammonia atmosphere.) Saturn is also less dense than water, which means it would float in a bathtub. But the most obvious feature of Saturn, of course, is its very complex network or rings.

Uranus north pole is tipped almost 90 degrees from its eliptic plane. As a result, during summer, the pole is tipped almost directly towards the Sun, while in winter, it's directly away from the Sun. This causes interesting atmospheric effects. Uranus has many moons, and a network of rings.

The location and properties of Neptune were predicted before the planet was discovered. From antiquity, there were five planets: Mercury, Venus, Mars, Jupiter, and Saturn. In 1781 William Hershel, while compiling star charts, discovered the planet Uranus. (It moved with respect to the other stars.) The planet's position in the sky vs. time was then predicted from its previous positions and Kepler's and Newton's laws. However, by the the 1830's it became obvious that Uranus was not following its predicted path. The assumption was that another body was gravitionally affecting Uranus and perturbing its orbit. In 1843, John Couch Adams in England and Urbain LeVerrier in France independently worked out the probable position of the perturbing planet. Adams gave his prediction to Sir George Airy, Astronomer Royal of England. Since Adams was only 2 years out of school, Airy basically ignored him, and gave the search low priority. LeVerrier, however, gave his prediction to Johann Galle of the Berlin Observatory. Galle took a picture that night, and found it. Note that if Nepture were at the distance predicted by Bode's law, Uranus' orbit would not have been noticeably affected.

Now we come to Pluto. It's round like a planet, and has a moon like a planet (called Charon), but is it a planet? Clyde Tombaugh discovered it in 1930 and called it a planet ever since. But we now know that there are probably tens (or even hundreds) of Pluto-size bodies outside the orbit of Neptune. One has already been found, and nicknamed Xena. It, too, has a moon, which has been nicknamed Gabrielle. (I'm not making this up!) Are they planets? What about the next dozen or so which are predicted to be out there?

In 2004, a bureaucratic organization called the International Astronomical Union appointed a committee to define the word planet. It failed. They then appointed another committee. This second committee came up with a definition that was 4 long paragraphs long and that admitted not only Pluto, but Charon, Xena (but not Gabrielle), and the asteroid Ceres. Everyone hated it. So another definition was tried -- planets must be round bodies ( i.e., made round by gravity), and have gravitationally cleared its orbit of other bodies. This definition excludes Pluto, since it's so small that it has had virtually no affect on any other body. (In fact, it crosses the orbit of Neptune.) Pluto and Xena are now "dwarf planets" (the IAU could not agree on a better name), and our Solar System is left with only 8 real planets.