This handy graphic by XKCD helps drive this point home:
Here, in a piece called “Space Without the Space,” XKCD’s Randall Munroe stitched together an old school, pirate-like map that shows all of the solid ground in our solar system (excluding speculative estimates solid “ground” we might find deep within the cores of gas-giants). Earth clearly wins hands down, though it’s unclear as to how Munroe incorporated the oceans of Earth in the map. Venus comes in at a close second, which isn’t surprising since it’s very similar to Earth in size. Then we have the other rocky bodies, Mercury and Mars.
What might be surprising to some is just how similar in size the planets and moons are. Three out of four of the Galilean moons (Callisto, Ganymede and Io) make up a considerable amount of the map. Ganymede, in particular, is the most noteworthy. Believe it or not, it’s actually a bit larger than the inner-most planet from the Sun, Mercury (it’s not that much smaller than Mars, for that matter). It even appears as if all of the dwarf-planets (pictured near the bottom) could fit inside any of the three largest Galilean moons.
It’s also neat that he grouped asteroids, comets and other small planetoids together. They make a small, but discernible fraction of our solar system’s rock. I’m not sure which point of view is cooler: the fact that there are so many of these objects scattered throughout our solar system that, together, they are the same size as a small moon, or that objects so numerous (there are billions, perhaps even trillions, of them) could be so small that all of them combined only add up to the size of a small moon. I’ll leave that one up to you guys.
How Planets Form:
Despite just how vastly different they are in size and composition, terrestrial planets and gaseous ones form in a strikingly similar manner (at least we think so).
It’s understood that based on our most current model, our solar system (along with all of the other planetary systems we’ve found circling distant “Suns”) formed following the collapse of a nebular cloud. From there, it’s understood that after a newly born star emerges from its cocoon, an elliptical disk of material, called a protoplanetary disk, encircles the young star.
The disk is composed of a variety of materials: including ice, water-ice, rock, grains and some heavier elements (iron, nickel, gold, etc), but gas is by far the most prevalent type of material. Within the chaotic, spinning disks, the materials collide and start to coalesce into a planet. After enough of the materials gather, gravity takes over and helps transform the oddly shaped planetesimals into the spherical planets we all know and love.
Gas-Giants v.s. Rocky Bodies:
Of course, the concentration and the quantity of the materials dictate what the planets are made of and the number of them that form, but a different mechanism — one occurring much farther out within the disk — starts to influence the proto-planets. After hundreds of millions of years of slow accretion, all at once, they start accreting gaseous envelopes (like an atmosphere). The growth can be stanched by stellar phenomena (like solar winds), but these effects are diluted over vast distances, thus allowing the more distant planets to keep on growing until they are more gas than rock.
At such distances, the temperatures also drop off, eventually becoming so cold that even gas itself freezes over. The newly acquired mass allows the large bodies to capture the frozen gas and become even more immense, until the planets become full-blown gas-giants.