Transit of Venus, Sun-Earth Day 2012

National Aeronautics and Space Administration

Goddard Space Flight Center

Transit of Venus, Sun-Earth Day 2012

Venus, Earth, and Mars

View the planetary comparison video!

Most people, when they think of a "planet", imagine a large roundish body with a rocky surface and an atmosphere, just the kind that cool science fiction stories are written about. Forgetting Pluto for a moment (which has too many issues to deal with here), only Venus, Earth, and Mars share these characteristics as planets in our solar system.

When astronomers think about planets, they generally divide them into two distinctive groups, the inner or "terrestrial" planets (Mercury, Venus, Earth, and Mars) and the outer or "jovian" planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is a bit of an outlier, and generally considered a KBO or Kuiper Belt Object or TNO (Trans-Neptunian Object), referring to its probable origins outside the orbit of Neptune in a region where comets are thought to form.

The inner planets all reside within about 250 million km from the sun. They are relatively small in size with radii ranging from 2440 km for Mercury to 6378 km for Earth, and they generally rotate very slowly (242 days for Venus to 24 hours for Earth). They have rocky (or silicate-based) surfaces that you could land on or walk on with the proper space suit gear. They also have dense inner cores composed primarily of heavy elements like iron and nickel. Except for Mercury, they have substantial atmospheres containing clouds, water vapor, carbon dioxide, and other trace gases. Contrast this with the outer planets.

The outer planets are, well, in the outer part of the solar system with orbital distances of greater than 750 million km. They are much larger than their inner planet cousins with radii ranging from between four to 11 times that of Earth. Unlike the inner planets, they are composed primarily of very light gasses dominated by molecular hydrogen (H2) and helium (He), with rich mixtures of a variety of hydrocarbon and nitrile molecules such as methane and ammonia. These planets are huge balls of gas and condensed gasses (clouds) that rotate quite fast for their large size (9.5 hours for Jupiter to 17.24 hours for Uranus) - driving the fastest winds in the solar system. If you were to try to land on one of these gas giants, you would sink through denser and denser layers of gas until you were crushed by the atmospheric pressure, never to reach an actual solid surface at all! In addition, they all have complex ring systems, a relatively large number of moons due primarily to their larger gravitational fields and large, and extensive magnetic fields.

Focusing now just on Earth and its closest neighbors, we can compare just the three terrestrial planets, Venus, Earth, and Mars, and we find that, although they have many things in common as we just mentioned, they are also worlds apart in many equally important respects.