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The planet that Voyager uncovered is a stormy, windswept world with a vivid blue hue. Its highly active atmosphere is surprising, since it receives so little sunlight to power its weather systems. Like the other giant outer planets, Neptune has no solid surface. It also has a system of rings and more than a dozen moons.

Neptune is the smallest of the four giant outer planets. The diameter at its equator is about 30,775 miles (49,528 kilometers), as measured at a level of the atmosphere where the pressure is 1 bar (the pressure at sea level on Earth). This makes it slightly smaller than Uranus but nearly four times as big as Earth. Neptune’s mass is about 1.2 times greater than Uranus’, however, and more than 17 times greater than Earth’s. It is the third most massive planet in the solar system, after Jupiter and Saturn. Like the other outer planets, Neptune has a low density—only about 1.6 times the density of water. However, it is the densest of the four, being roughly 25 percent denser than Uranus.

Saturn was named after the ancient Roman god of agriculture. His counterpart in ancient Greek mythology was Cronus, the father of Zeus (the counterpart of the Roman god Jupiter). The planet Jupiter is Saturn’s nearest neighbor and the closest to it in size and composition. Like Jupiter, Saturn is a giant world formed mainly of hydrogen with no solid surface. It has a massive atmosphere, or surrounding layer of gases, with complex weather patterns. ESA/NASA/JPL/University of ArizonaESA/NASA/JPL/University of Arizona The planet’s extensive system of icy moons includes nine major moons and dozens of small ones. Some of the moons help create the rings and maintain their shape. Titan, the largest of Saturn’s moons, is bigger than the planet Mercury. It is the only moon in the solar system known to have a dense atmosphere.

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time

Uranus is the third largest planet in the solar system, after Jupiter and Saturn. Uranus is about four times bigger than Earth. Its diameter at the equator is about 31,763 miles (51,118 kilometers), as measured at the level of the atmosphere where the pressure is the same as at sea level on Earth. The planet is slightly larger than Neptune, but Neptune is about 1.2 times more massive. Uranus’ density is quite low—only about 1.3 times that of water, compared with 1.6 for Neptune and 5.5 for rocky Earth.

The second planet from the Sun is Venus. After the Moon, Venus is the most brilliant natural object in the nighttime sky. It is the closest planet to Earth, and it is also the most similar to Earth in size, mass, volume, and density. These similarities suggest that the two planets may have had similar histories. Scientists are thus intrigued by the question of why Venus and Earth are now so different.

Nevertheless, the search for life on Mars continues, in part because it shows signs of having been wetter in the past. Water is necessary for all known forms of life. It is possible that microscopic life once existed on Mars. Today the surface is too cold and the air is too thin for liquid water to exist there for long. It is also bombarded with ultraviolet radiation from the Sun, which is very harmful to living things.

During a sunrise, the sun is lower in the sky, which lengthens the angle at which light reaches the Earth. As a result, the shorter violet, blue and green light never reaches the eye, so only the yellow, orange and red light is visible. Because red has the longest wavelength, the first light of dawn often appears pink or red--depending on the particles in the atmosphere. As the sun moves further up the sky, the reds and yellows fade as the scattered blue light becomes more visible.

As the light from our Sun shines into the atmosphere, most of the colors are able to reach the Earth’s surface uninterrupted. However, because blue light has a wavelength that is the same size as the particulates in the air, this light is scattered in every direction. This blue light bounces from particulate to particulate until it eventually reaches your eyes. For this reason, no matter what direction you look in the sky, it appears to be blue. This blue light originated with the Sun, was bounced around in the sky many times, and then eventually reached your eyes.

Jupiter has a very faint ring system, but is totally invisible from the Earth. (The rings were discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic.

At the heart of the solar system is our sun. The four planets nearest it are rocky, terrestrial worlds — Mercury, Venus, Earth and Mars. After that are four gas giants — Jupiter, Saturn, Uranus and Neptune. Between the orbits of Mars and Jupiter lies the asteroid belt, which includes the dwarf planet Ceres. Beyond the orbit of Neptune one finds the disk-shaped Kuiper belt, in which dwarf planet Pluto resides, and far beyond that is the giant, spherical Oort Cloud and the teardrop-shaped heliopause. See the structure of the solar system (infographic).