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

Jupiter and Io from Pleiades-1B (animation)

A week after its December 2 launch, the high-resolution Earth-observation satellite Pleiades-1B pointed at Jupiter to take this sequence of eight photos. The photos are separated by about 45 minutes and were taken during the calibration and commissioning phase of the spacecraft.

Filed under pretty pictures, animation, many worlds, astronomy by planetary missions, Jupiter, Io, Earth observing missions

Planets over Cumbria

Near dawn on December 8, 2012, Stuart Atkinson caught Mercury, Venus, Saturn, and the Moon near the horizon. All four were then being orbited by spacecraft: MESSENGER, Venus Express, Cassini, Lunar Reconnaissance Orbiter, and GRAIL.

Filed under pretty pictures, many worlds, amateur astrophotos, Mercury, Venus, Earth, the Moon, Saturn

The Galileans

A montage of the four Galilean moons, from Galileo.

Filed under pretty pictures, amateur image processing, Jupiter's moons, Io, Europa, Ganymede, Callisto, many worlds

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Planetary Facts

Mass, diameter, density, gravity, orbital characteristics, presented both in metric units and measured relative to Earth.

Eros' northern hemisphere as seen by the NEAR spacecraft on February 29, 2000. The view is a mosaic of six images.

Asteroids and Comets Visited by Spacecraft

A comparison of all the asteroids and comets ever visited by spacecraft, up to date as of November 10 (when Deep Impact flew past Hartley 2). Vesta is not included.