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
The small, dark spot near the top of this photo is Phobos. Phobos covers about 40 picture elements. From this data, Phobos was found to be an oval object approximately 22.5 kilometers long and 17.5 kilometers wide. Also, it was concluded that Phobos is a very dark object, with an albedo of 6.5%.
An animation of four images of Mars taken by Mars Orbiter mission, showing Mars' moon Phobos crossing the disk at upper right. The images were taken from a distance of 66,275 kilometers in October, 2014.
From orbit at Mercury, MESSENGER captured 31 photos of Earth and the Moon between 9:18 and 10:18 UTC. As MESSENGER watched, the Moon crossed into Earth's shadow, in a lunar eclipse that was witnessed by much of North America and the Pacific Ocean.
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