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 sol 393 at 9:43 p.m. local time (September 14, 2013), Curiosity watched Phobos and Deimos pass each other in the sky. Like our moon, Deimos orbits Mars more slowly than Mars rotates, so appears to move from east to west across the sky. But Phobos orbits Mars more than three times per Mars day, and thus rises in the west and sets in the east.
Phobos, the larger of the two moons of Mars, passes directly in front of the Sun in this sequence of 89 images taken by Curiosity on August 20, 2013. The pace of the video matches the actual elapsed time of 32 seconds.
On November 29, 2005, Cassini caught a "mutual event" of Janus and Epimetheus from a distance of about a million kilometers (600,000 miles). Most of the specks in the image are cosmic ray hits on the camera detector, but a few that appear to move from frame to frame are background stars. The faint double band of light below the two moons is the G ring, seen nearly edge-on. The moons are observed from their night sides; they are illuminated from the lower right by the Sun into thin crescents, and their night sides are faintly lit by reflected light from Saturn.
In 2016, The Planetary Society’s LightSail program will take the technology a step further.