The Terrestrial Planets From top: Mercury, Venus, Earth, and Mars. The planets Mercury, Venus, Earth, and Mars are called terrestrials because they have a compact, rocky surface like Earth's terra firma. The terrestrial planets are the four innermost planets in the solar system. The next group of planets is made up of gas giants such as Jupiter and Saturn. Terrestrial planets cannot be seen directly with the unaided eye, but they are visible to the naked eye from outdoors on a dark night when they pass between the Sun and Earth. Passages near these planets allow observers to see stars with a small telescope or sunspots with a large one. Earth's rocky surface is pocked with many holes, some as big as cities. Volcanoes erupt material into the atmosphere where it forms clouds that reflect sunlight back to space, causing global cooling and eroding the surface. Eventually all the water vapor within the volcanic gases condenses into liquid rain which fills the empty spaces in the rock with soil. This is how new islands are born. So Earth's rocky surface is really a collection of remnants from earlier volcanoes events that happened over time. Venus has very dense clouds covered by an air pressure so high that liquid methane bubbles up through its surface crust. If it had an atmosphere similar to ours, Venus would be too hot for life as we know it.