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
New Horizons took these photos of Nix and Hydra as it approached for its flyby; they have been resized to the same scale of about 380 meters per pixel. The two moons appear to be very close to the same size; in this image, Nix (left) measures 30 by 48 km, and Hydra measures 36 by 52 km. We may be seeing neither moon's longest or shortest dimensions, so these dimensions are consistent with the minimum and maximum dimensions measured from Hubble (26 by 56 km for Nix and 34 by 58 km for Hydra).
This movie sequence from Cassini shows dark drapes in the inner strands of Saturn's F ring caused by the gravitational influence of the shepherd moon Prometheus. Prometheus appears first in the sequence, interior to the F ring, and Pandora follows along outside of the ring. Prometheus orbits closer to Saturn, and thus faster, than the icy particles that make up the F ring. The moon passes closest to the ring at "apoapse", when it is farthest from Saturn. It is during these apoapse passages that Prometheus has its greatest influence on the fine ring material. With time, the ring material previously affected falls behind so that on the next apoapse passage of Prometheus, a new gore in the inner ring material is made. The material closer to Prometheus orbits the planet faster than the material closer to the bright F ring core. The gores, together with the sheared-out material, create the dark, diagonal drapes. Visit the Planetary Photojournal to download a high-resolution, 170-frame version of this movie.
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