Advances in photography, astronomy, telescopes, analytical techniques, telecommunications, and space flight have given us more detailed glimpses of our solar system. Space probes have measured the chemical compositions of the atmospheres on planets, and rock samples have been recovered. We have mapped the surfaces of our Moon and nearest planets in detail. Geologic features similar to those on Earth have been identified. The theory of uniformitarianism can be applied to outer space as well as to Earth. For example, the braided patterns on the surface of Mars were probably formed by running water as they are here.
Scientists believe that our solar system—consisting of the Sun, nine planets, and numerous moons‐‐formed about the same time, 4.5 billion years ago. The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are terrestrial planets. They are called terrestrial because their densities (of 3 g/cm3 or more) are similar to Earth's. The rest are called jovian planets and have densities below 2 g/cm3. All nine planets have been studied by uncrewed spacecraft. Measurements have indicated that all the planets appear to have solid cores.
The kinds of tectonic, magmatic, and surficial processes that have shaped Earth's surface have also affected the surfaces of the other planets. Likewise, the impact cratering from meteorites so visible on other planets has also occurred on Earth and has been suggested as a reason for the mass extinction of the dinosaurs.