Characteristics of planets
I. Relative size of planets
The Sun and the gas giant planets like Jupiter are by far the largest objects in the Solar System. The other planets are small specks on this scale, as the following figures illustrates. (Images taken from http://www.kiroastro.com/writings/perspective)
Relative size of Terrestrial planets (and Pluto)
Relative size of all planets
Relative size of Sun and planets
Our Sun is a fairly typical star. But it is quite a minnow compared to other stars. Take a look at the next two images.
Relative size of Sun and other (big) stars
II. Mass and densities
The total mass of all planets are also concentrated in the Gas Giant planets Jupiter, Saturn, Uranus, and Neptune, as the following graph indicates. In fact, Jupiter, by itself, has more mass than all the other planets combined! (Pictures below are from: http://csep10.phys.utk.edu/astr161/lect/index.html).
III. Planetary orbits
The average separation of the planets from the Sun (in astronomical units)
The eccentricities of the planetary orbits
Here is the entire Solar System to scale for the orbits. Position of planets is for Fall, 1996:
Notice the enormous amount of empty space in the outer Solar System. To show the entire Solar System to scale, the inner Solar System becomes so compressed that the planet orbits almost appear to run together. The very large eccentricity of Pluto's orbit is also obvious. Pictures from: http://csep10.phys.utk.edu/astr161/lect/index.html
The planetary orbital planes are
inclined at small angles to the ecliptic:
Note: Pluto's orbit is highly tilted (17 degrees) relative to the plane of the ecliptic. . Picture from: http://csep10.phys.utk.edu/astr161/lect/index.html
Side view of the inner Solar System, Fall, 1996
In this figure the white portion of the orbit is above the ecliptic plane and the yellow portion is below. Notice that the orbits of the inner planets are nearly, but not quite, in the same plane. The orbit of Mercury, in addition to being the most eccentric, has the largest tilt (7 degrees) with respect to the ecliptic plane. . Picture from: http://csep10.phys.utk.edu/astr161/lect/index.html
Most of the planets rotate about their rotation axis in the same sense, which is prograde. Venus rotates in the opposite, retrograde sense:
IV. An closer look at the Sun and each planet
Tables from Lowrie Textbook
Summary of Observations in the Solar System
A lot more information (and better pictures) can be found in many places on the web, for instance on the Solar System site on Wikipedia.
IV. Exoplanets - planets outside our Solar System
Prior to the Kepler mission, more than 200 exoplanets had been found. Most discovered planets are Jupiter-size (the more massive the planet, the easier it is to detect). The grph below shows the mass of exoplanets vs year of their discovery. As you can see, there is a convergence toward being able to detect Earth-like size planet. Indeed, many Earth-like size planets have now been found.
Mass of discovered planets vs year of discovery
The Kepler spacecraft mission has revolutionized our search for Earth-like planets. Announcements of the discovery of Earth-size planets now come at rapid frequency (every month or so). The first planets to have Earth-like mass were Kepler 20e and 20f, reported on December 20, 2011. Moreover, many are found in the so-called "habitable zone".
The bar-graph below, taken from the Kepler website (credit: NASA), shows the probability that a star in the Milky Way has orbiting planets that are similar to those in our Solar system. One in six stars has an Earth-sized planet in a tight orbit. About a fourth of all stars in the Milky Way have a super-Earth, and the same fraction have a mini-Neptune. Only about 3 percent of stars have a large Neptune, and only 5 percent a gas giant at the orbital distances studied. Since 17 percent of stars have an Earth-sized planet, and given that the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there!
Here are a couple of images from the Kepler website (credit: NASA).