The Sun and the five largest planets of the solar system (to scale). The Earth is the tiny dot between Jupiter and the Sun. (Source: NASA)
By far most of the solar system's mass is in the Sun itself: somewhere between 99.8 and 99.9 percent. The rest is split between the planets and their satellites, and the comets and asteroids and the dust and gas surrounding our star. Seen from afar (on the scale of distances between stars) the presence of the solar system would not be obvious. We would simply see a normal-looking star. Perhaps we would pick up the presence of Jupiter, which makes up two thirds or so of the solar system outside of the Sun, by mass.
Because Jupiter has most of the mass of what the Sun left over, we can think of the solar system as a double star with very unequal partners. The little guy (Jupiter) is 1/1000 the mass of the big one (Sun) and doesn't have enough mass to produce the pressure necessary for ignition at its center. It does have pretty much the same composition as the Sun (hydrogen and helium in solar proportions, along with some carbon, nitrogen and oxygen). Also, it does radiate heat — almost as much again as it receives from the Sun. This heat is released by the radioactive decay of elements within the planet and (perhaps) from gravitational effects, as the heavier elements move toward the center. (Earth also radiates its own heat, but it is negligible compared with energy of solar origin.)
Jupiter and Saturn together make up more than 90 percent of the mass of all planets. The bulk of their mass is in hydrogen and helium, as mentioned. Also, the compounds ammonia (nitrogen hydride), methane (carbon hydride) and water (oxygen hydride) are seen in their atmospheres. These are the most obvious combinations of the four most common elements (besides helium), and the fact that they are abundant reflects the relatively low temperature of the outer atmosphere of the two main planets. (Helium, as a noble gas, does not combine with other elements.)
The next two largest planets are Uranus (discovered 1781, by William Herschel) and Neptune (discovered 1846, by Johannes Galle; see also Adams and Leverrier). The two are roughly equal in size and mass, at about five percent of Jupiter and less than a fifth of Saturn. The composition of Uranus is much like that of Jupiter and Saturn. Also, like those two giants, it has (wispy) rings and lots of satellites.
Apparently, Neptune was first seen by Galileo (he sketched it when observing Jupiter in 1613) but he did not recognize its nature as a planet. Neptune has a composition much like the other "Jovian" planets, but has somewhat greater density. It too has debris and satellites encircling it. Pluto, the outermost planet, is roughly the size of the Moon and consists of dusty ice, much like a comet. It has a large companion, "Charon".
Pluto: The brown color of the planet is thought to be due to frozen methane deposits. No spacecraft from Earth has as yet visited Pluto. This image was taken with an Earth-based telescope. (Source: NASA)
