If a planet had low gravity and an atmosphere three times denser than Earth's, animals as large as whales might find flying quite attractive, according to scientists. This artist's conception shows giant "skywhales" with wingspans of nearly 33 feet (10 meters).

While the shape and form of extraterrestrials may look alien, in many ways they might be similar to life on Earth, some scientists believe.

The illustration was made for the National Geographic Channel documentary Extraterrestrial, a look at what life on other worlds might really look like. Visit the Channel's Extraterrestrial Web site for a video excerpt, more images, and an interactive feature.

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