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
This artist's illustration shows the scale and comparative brightness of Pluto's small satellites, as discovered by the Hubble Space Telescope over the past several years. Pluto's binary companion, Charon (discovered in 1978), is placed at the bottom for scale. Two of the moons are highly oblate. The reflectivity among the moons varies from dark charcoal to the brightness of white sand. Hubble cannot resolve surface features on the moons and so the cratered textures seen here are purely for illustration purposes.
As Cassini approached for its close flyby of Enceladus on December 20, 2010, it caught Mimas passing through the field of view. The Sun was nearly in front of Cassini so both moons are lit only as very skinny crescents; the lighting highlights Enceladus' plumes, and shines off of tiny particles in the E ring, which provides a lighter backdrop to the dark night sides of both moons. Mimas and Enceladus are actually very similar in size, but Enceladus was much closer to Cassini when the photo was taken.
On September 28, 1977, Viking Orbiter 1 observed a dust storm over the site where it had dropped its lander, a little more than a year previously. As Viking 1 orbiter watched, the shadow of Mars' inner moon Phobos passed over the cloud tops. The sequence of images has been artificially colorized and is displayed at a speed ten times that of real time. (Viking images f467a31–f467a69)
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