U N D E R C O N S T R U C T I O N ! ! ! ! !

Venus is the brightest object in our sky, after the sun and moon. Swirling clouds of sulfur and sulfuric acid and a deep atmosphere obscure the planet’s surface, inhibiting study of it from earth, but space vehicles, outfitted with probes, have successfully returned data. Probes determined that Venus is the hottest of the planets, with a surface temperature of about 462° C (864° F). Scientists believe a greenhouse effect causes the extreme temperature; the planet’s thick clouds and dense atmosphere allow the sun’s radiant energy to reach the planet, but inhibit the escape of the heat.

Earth

An oxygen-rich and protective atmosphere, moderate temperatures, abundant water, and a varied chemical composition allow earth to support life, the only planet to do so. The slightly pear-shaped planet is composed of rock and metal, which are present in molten form beneath its surface. This photograph, taken by the Apollo 17 spacecraft in 1972, shows Arabia, the African continent, and Antarctica (most of the white area near the bottom).

Unpiloted spacecraft from the United States, launched between 1964 and 1976, have supplied the most detailed information on Mars. From this data, scientists determined that the planet’s atmosphere predominantly consists of carbon dioxide (CO2), with small amounts of nitrogen, oxygen, and water vapor also present. Due to the thinness of the atmosphere, daily temperatures often vary as much as 100° C (180° F). In general, surface temperatures are too cold and surface pressures too low for water to exist in a liquid state on Mars, so the planet resembles a cold, high-altitude desert.

Jupiter is the largest of the planets, with a volume 1400 times greater than that of the earth. Jupiter’s colorful bands are caused by strong atmospheric currents and accentuated by a dense cloud cover. The massive planet, upper right, is shown here with the four largest of its sixteen satellites: Europa, center, nearest Jupiter, Io upper left, Callisto lower left, and Ganymede lower right.

Saturn, distinguished by its rings, ranks as the second largest planet—Jupiter is the largest—in the solar system. In 1610 the Italian physicist and astronomer Galileo discovered the ring system using one of the first telescopes ever made. Although the planet formed more than 4 billion years ago, it continues to settle and contract, generating three times as much heat as it receives from the sun. The Hubble Space Telescope obtained this image of Saturn on August 26, 1990.

Liaison International

Uranus’s blue-green color comes from the methane gas present in its cold, clear atmosphere. The dark shadings at the right edge of the sphere correspond to the day-night boundary on the planet. Beyond this boundary, Uranus’ northern hemisphere remains in perpetual darkness because of the way the planet rotates. Scientists compiled this view of Uranus from images returned from Voyager 2 in 1986, when the probe was 9.1 million km (5.7 million mi) away from the planet.

The 1989 Voyager 2 mission produced this false-color image of Neptune showing the different components of Neptune’s atmosphere. The red layer shows scattered sunlight from a haze around the planet, the blue/green indicates methane, and the white areas are high-level clouds that reflect sunlight above the atmosphere.

Pluto is farther from the sun than the other planets in the solar system, although it occasionally moves in closer than Neptune due to an irregular orbit. The small, rocky, and cold planet takes 247.7 years to revolve around the sun. This artist's rendition depicts Pluto, foreground; its moon, Charon, background; and the distant sun, upper right.