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
On November 29, 2005, Cassini caught a "mutual event" of Janus and Epimetheus from a distance of about a million kilometers (600,000 miles). Most of the specks in the image are cosmic ray hits on the camera detector, but a few that appear to move from frame to frame are background stars. The faint double band of light below the two moons is the G ring, seen nearly edge-on. The moons are observed from their night sides; they are illuminated from the lower right by the Sun into thin crescents, and their night sides are faintly lit by reflected light from Saturn.
Taken on April 8, 2009, these are the first images taken by the planet-hunting mission Kepler. The large image in the center shows a 100 degree square patch of sky containing an estimated 14 million stars. Kepler will observe this region continuously for more than 3 years, searching for signs of transiting planets in a group 100,000 pre-selected stars. The dark lines crisscrossing the image indicate the arrangement of the charged coupled devices (CCD's) in Kepler's powerful camera). The top left image is of a region 1000 times smaller than the full field, which contains the known transiting planet TrES-2. The image on the top right includes star cluster NGC 6791, at a distance of 13,000 lightyears from Earth.
Saturn as viewed by the Advanced Camera for Surveys on March 12, 2011, as a storm had encircled its northern hemisphere. Moons visible in the view include, from left to right, Dione, Enceladus, Tethys, and Rhea.
In 2016, The Planetary Society’s LightSail program will take the technology a step further.