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
Prometheus (left) and Pandora (right) are small, elongated moons that orbit inside and outside the F ring, respectively. Cassini was slightly less than half a million kilometers from Pandora when it took this photo of the pair on October 29, 2005. Prometheus is 148 by 68 kilometers in size, and Pandora is 110 by 62 kilometers. Note the small streamer of material that Prometheus has drawn out of the inner edge of the F ring during a recent collision.
The eight innermost moons of Saturn, in color images collected by Cassini between June 7, 2005, and July 5, 2010. Pan and Daphnis (top left small moons) orbit within the Encke and Keeler gaps in the rings; Atlas (below Pan and Daphnis) orbits at the outer edge of the main rings. To their right are Prometheus and Pandora; Prometheus orbits just inside and Pandora just outside the F ring. Below them are Epimetheus (left) and Janus (right), which trade positions every four years, averaging out to the same distance from Saturn. Mimas orbits considerably farther away, but its gravitational effects influence the positions of gaps and waves within the rings. At full resolution, the montage has a scale of 500 meters per pixel.
On February 1, 2014, as Lunar Reconnaissance Orbiter approached the north pole, it rotated to catch a view of Earth rising above 180-kilometer Rozhdestvenskiy crater. Twelve such Earthrises happen each day for the spacecraft, which is in a polar orbit around the Moon.
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