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
In a six-frame animation spanning 15 minutes over January 30-31, 2010, the Mars Express Visual Monitoring Camera observed Phobos' shadow transiting Mars. It is late autumn in Mars' southern hemisphere, so the shadow of Phobos (which orbits exactly along Mars' equatorial plane) falls on Mars' high southern latitudes. Mars Express is looking down nearly onto Mars' south pole, which is in winter darkness; the bright splat at the center of the animation is the Argyre impact basin, with the crater Galle located on its rim. Phobos' shadow crosses northern Argyre and proceeds across Noachis terra. This is the first time that VMC wasknown to have imaged Phobos' shadow.
A model for the shape of asteroid (216) Kleopatra spins while its two moons, inner Cleoselene and outer Alexhelios, orbit it.
An image from the adaptive optics-equipped Keck II Telescope resolves the dog-bone shape of asteroid (216) Kleopatra and its two moons, outer Alexhelios (1) and inner Cleoselene (2). Kleopatra is about 271 kilometers long, with two lobes each 80 kilometers long joined by a neck about 50 to 65 kilometers wide. The two moons are between 5 and 10 kilometers in diameter and orbit 454 and 678 kilometers away from Kleopatra.
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