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Managing Editor, NGM.com Rob Covey <![CDATA[Producer]]> Sabi Chawla <![CDATA[Writer & Researcher]]> Jamie Bussey Flash Developer Stefan Estrada Styling Nancy Gupton and Kitry Krause Research Consultants Barbara Wyckoff /2008/10/space-special/venus-nasa-470.jpg Photograph by Magellan Project, JPL, NASA /2008/10/space-special/mercury-nasa-470.jpg Photograph by Mariner 10, NASA /2008/10/space-special/earth-470.jpg Photograph by Corbis /2008/10/space-special/marsvolcano-nasa-470.jpg Photograph by NASA /2008/10/space-special/moon-470.jpg Photograph by NASA Great Images in Nasa Collection /2008/10/space-special/jupiter-nasa-470.jpg Photograph by The Hubble Heritage Team /2008/10/space-special/saturn-rings-nasa-470.jpg Photograph by NASA/JPL /2008/10/space-special/uranus-moons-470.jpg Photograph by Kenneth Seidelmann, U.S. Naval Observatory, and NASA /2008/10/space-special/neptune-nasa-470.jpg Photograph by NASA /2008/10/space-special/planets-470.jpg Photograph by NASA Jet Propulsion Laboratory /2008/10/space-special/pluto-470.jpg Photograph by NASA, JPL, Michael W. Carroll /2008/10/space-special/makemake-470.jpg Photograph by R. Hurt (SSC-Caltech), JPL-Caltech, NASA /2008/10/space-special/eris-470.jpg Photograph by Thierry Lombry /2008/10/space-special/ceres-470.jpg Photograph by NASA, ESA, Southwest Research Institute, Cornell University, University of Maryland, College Park, and M. Mutchler and Z. Levay /2008/10/space-special/dwarf-470.jpg Photograph by NASA/JPL-Caltech/S. J. U. Higdon (Cornell University) 2 Excellent work. You are an expert on planets. With a little work, you will be a pro. You are off to a good start. You may want to try again. 0 A thick, rapidly spinning atmosphere The lack of moons The planet's distance from the sun Heavy volcanic activity 1
These two gases absorb infrared radiation, trapping heat from the sun on Venus's surface—temperatures can reach over 850 degrees Fahrenheit, hot enough to melt lead. The thick atmosphere also contributes to the intense pressure on Venus, the same pressure found at a depth of 2,953 feet in Earth's oceans.]]> 1 It virtually has no atmosphere, which allows heat to escape Its thick atmosphere, which doesn't allow sunlight to reach the surface The angle of its axis Its iron core, 75 percent of the planet's radius 1 2 Mercury Uranus Venus Neptune 3
The similarities end there, since Venus's atmosphere makes its surface unsuitable for the life that Earth's combination of air, land, and water supports.]]> 3 The tallest volcanic mountain The smallest moon The largest impact crater The most seasons 1
Located in the northern Tharsis region of Mars, Olympus Mons and other volcanoes are so huge they deform the roundness of the red planet.

Mars also boasts the solar system's largest rift valley, Valles Marineris, which on Earth would stretch from New York to Los Angeles.]]> 4 One Three Five Nine 2
Phobos, the larger of Mars's moons, is identifiable by the six-mile crater that covers nearly half the moon's width. In its present orbit, it's spiraling inward toward Mars; in 50 million years it will either crash into the planet or break up, possibly leaving a ring around the planet.]]> 5 Twice its current size Ten times larger Eighty times larger A hundred million times larger 3 6 Pieces of comets Pieces of asteroids Shattered moons All of the above 4 7 The seven dwarfs in "Snow White" Characters in William Shakespeare's plays Characters in Homer's "Iliad" Characters from Walt Disney movies 2 8 A ring around Neptune Six new moons Water on Triton, Neptune's largest moon All of the above 2
Voyager 2 also sent back images of Neptune's largest moon, Triton, and a Great Dark Spot, similar to the Great Red Spot on Jupiter.]]> 9 Mercury Venus Jupiter Uranus 4 10 It can't clear other objects from its orbit. It takes too long to orbit the sun. It doesn't have enough gravity to keep it in a ball-like shape. It orbits within the Kuiper belt. 1
Since Pluto can't clear other objects from its orbit, it now belongs to a new class of celestial bodies known as dwarf planets.]]> 11 Polynesia China Kenya New Zealand 1 12 3.7 billion miles 4.2 billion miles 6.4 billion miles 100 billion miles 3 13 1778 1801 1922 2006 2 14 Pluto Makemake Ceres Eris 3
Not all dwarf planets are plutoids, because a plutoid must orbit outside Neptune. Ceres isn't a plutoid, because it orbits inside Neptune, between Mars and Jupiter.]]>