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
Zond-8 flew by the Moon on October 24, 1970 and returned to Earth with high quality photographs, some from as close as 1,350 km. Images were shot with the 400 mm AFA-BAM camera, on 13 by 18-centimeter frames of isopanchromatic film. A session of 20 full-Moon pictures was followed by a session of 78 lunar-surface pictures (including 17 shots of Earth over the lunar horizon).
As Galileo receded from its ssecond flyby of Earth on December 16 and 17, 1992, it captured this sequence of Earth rotating as the Moon zipped by on its orbit. There are 56 frames in total, each separated by 15 minutes, spanning about 14 hours.
On December 16, 1992, eight days after its encounter with Earth, the Galileo spacecraft looked back from a distance of about 6.2 million kilometers to capture this remarkable view of the Moon in orbit about Earth.