Compare the Planets
Comparing the physical characteristics of the worlds in our solar system (and beyond)
The worlds of our solar system come in all shapes, sizes, and colors. Red-eyed Jupiter, ringed Saturn, and frigid Uranus and Neptune are giant gassy globes containing nearly all of the matter in the solar system. These Jovian planets, or gas giants, are huge worlds of air, clouds, and fluid that may have no solid surfaces no matter how deep you go. Everything else in the solar system is just rock, ice, and dust. The largest rockballs are known as the terrestrial planets: Mercury, Venus, Earth, and Mars, with our Moon usually considered part of the club, and now Vesta is applying for membership. Earth is the biggest of all the rocky worlds.
But the planets are not the only worlds of the solar system. All but two of the planets are orbited by moons, each of them a world unto itself. The largest moons are bigger than the smallest planets, and 16 or 17 would qualify as dwarf planets if they orbited the Sun. There are more than 100 Kuiper belt dwarf planets, but only one among the asteroids, Ceres.
Six solid worlds -- Venus, Earth, Mars, Titan, Triton, and Pluto -- have atmospheres dense enough to produce weather. Eris likely does, when it is near its perihelion. We have witnessed active geology on four worlds -- Earth, Io, Enceladus, and Triton -- and we suspect it on Venus, Europa, and Titan. Comparing the same processes across many worlds helps us to understand how each planet's unique composition and history influence its present state, and will help us predict what to expect on Earth in the future.
Pretty Pictures with Many Worlds
The Hubble Space Telescope caught Europa, Callisto, and Io transiting Jupiter and casting three shadows simultaneously on January 24, 2015. These events can be witnessed only once or twice per decade, near Jupiter's equinoxes. Because of the resonance of the orbits of Io, Europa, and Ganymede, it is not presently possible for all four moons to cast shadows on Jupiter at the same time.
Voyager 2 captured the images for this view of Saturn on July 12, 1981. Three moons are visible: Rhea below the disk, and Tethys and Enceladus on the disk. Tethys' shadow falls on the planet. The image was taken on the same day as this more garish view that was released by NASA.
This image combines a single Mastcam frame taken of Phobos behind Mt. Sharp on sol 613 (April 28, 2014) with three images from a 360-degree mosaic acquired during the afternoon of sol 610 (April 24, 2014) to extend the foreground view and balance the image composition.
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