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 major moons of Uranus to scale. These images were taken on January 24, 1986. The geologic diversity these moons show begs for another mission to explore them thoroughly. Sadly, we have never been back, and we have no plans to go back. Top row: Titania, Oberon. Bottom Row: Umbriel, Ariel. Top Middle: Miranda. Bottom Middle: Puck.
Oberon, moon of Uranus (left), and Charon, moon of Pluto (right). These worlds are of similar size and both exhibit intriguing geology. Oberon was barely glimpsed by Voyager 2 as it flew by the Uranian system on January 24, 1986, while Charon received a close encounter by the New Horizons spacecraft on July 14, 2015. Charon's complex geology makes the barely-studied moons of Uranus even more tantalizing.
Cassini acquired this sequence of images with its narrow angle camera on September 8, 2005. The view is centered on Epimetheus, nearly occulting Janus, with the F ring visible.
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