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
Taken on October 21, 2002 as part of an engineering test of the camera system, the image clearly shows the planet Saturn, its ring system, and the dark Cassini division between the brighter inner rings and darker outer rings. The image is a color composite of three taken by the narrow-angle camera through different filters, showing the colors of Saturn approximately as the human eye would see them. The spacecraft was 285 million kilometers away from the planet, nearly twice the distance between the Sun and Earth, when this image was captured.
New Horizons' long-range camera LORRI monitored Jupiter's rotation as the spacecraft approached on January 9 and 10, 2007. The 11 images in this animation were each taken an hour apart and cover one full rotation of the planet. During the animation, first Ganymede and then Io cross the field of view, casting their shadows onto the planet. There are a total of six of these movies planned for New Horizons' approach to Jupiter; the last of those should have been taken on Sunday, so stay tuned to the New Horizons site for those. There are only three other sets of images planned for the next month: a handful of "Kodak moments," images taken primarily for their prettiness.
In 2016, The Planetary Society’s LightSail program will take the technology a step further.