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
On July 19, 2013 Cassini passed into Saturn's shadow and turned toward the Sun, capturing an image of the planet's night side and the weirdly lit semi-transparent rings. Cassini also captured seven of the moons and three planets. This was the third time our home planet was imaged from the outer solar system; the second time it was imaged by Cassini from Saturn's orbit; and the first time ever that inhabitants of Earth were made aware in advance that their photo would be taken from such a great distance.
Cassini spotted Saturn and two of its moons in this approximate true color image taken on January 30, 2008. The orange ball of Titan has just emerged from behind Saturn, and is much more distant from Cassini than the bright icy dot of Tethys, in the foreground at the lower left of the image. Because Cassini was on the north side of the rings, we are seeing the rings' shadowed side, so the broad, opaque B ring looks black, while the inner, tenuous C ring looks much brighter. As Cassini's mission proceeds, the season is approaching equinox, so the shadows of Saturn's rings now fall much closer to the planet's equator than they did in the beginning of the mission. At the same time, Saturn's northern hemisphere is losing the brilliant blue color it had at the start of Cassini's mission.
In 2016, The Planetary Society’s LightSail program will take the technology a step further.