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
Hayabusa's star tracker shot this photo of the Earth-Moon system on May 12, 2010, within 13.5 million kilometers of its home world. The spacecraft is aimed for a June 13 return of its sample capsule to Earth. The star tracker is designed to photograph stars, so targets as bright as the Moon and Earth are overpoweringly bright. Neither body actually subtends more than one pixel; their apparent size results from their bright points of light spilling over into adjacent pixels. However, the star tracker successfully separates the light of the Moon and Earth, resolving them as distinct points of bright light.
Dione, Saturn's third largest moon at 1,123 kilometers in diameter, shares the frame with Telesto, a tiny "rock," about 35 kilometers across, that orbits Saturn in the leading Lagrangian point of Tethys' orbit.
On June 1 2001, Mars express watched as Phobos (the inner and larger of Mars' two moons) slipped past distant Jupiter. Mars Express is studying Phobos to help the Russian Phobos-Grunt mission prepare to land on the moon and grab a sample for return to Earth.
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