A Hubble image shows the Crab Nebula. Everything needed for life on Earth is known to exist across the universe.
NASA/ESA/JPL/Arizona State University
A 2004 image of the Trifid Nebula. Many astrobiologists believe that if even the tiniest microbe is discovered on another planet, the odds that life exists elsewhere in our galaxy increases dramatically.
To find the smallest signs of life, scientists are turning to harsh environments looking for "extremophile" organisms. Extremophile researchers Gaetan Borgoni, far right, and Derek Littauer, far left, collect water samples more than 1.2 miles below the surface in a mine owned by Northam Platinum in South Africa.
Marc Kaufman-The Washington Post
Researcher James Hall tests water obtained from a South African mine for signs of life, such as nematodes.
Courtesy of Lisa M. Pratt; the Trustees of Indiana University; NASA; National Science Foundation
A false color photomicrograph of a nematode, at 55 times magnification.
Kent Wood/Photo Researchers Inc.
Other extreme environments include glaciers in Antarctica.
Kike Calvo via AP
New research has found that microbes live and reproduce in Antarctica's deep glaciers -- sometimes miles deep.
Shawn Doyle/Louisiana State University
The Atacama Desert of northern Chile is one of Earth's driest regions. Experts have generally accepted that about half of the life on Earth consists of single-cell organisms living below the ocean and below the ground.
David Nunuk/Photo Researchers Inc.
Microbes have been found around "black smoker" hydrothermal vents on the ocean floor. The pressure there is four hundred times as great as Earth's surface atmosphere, and temperatures at the vent mouths can reach 750 degrees Fahrenheit.
Science Source/Photo Researchers Inc.
Extremophiles are found everywhere on Earth, including the arsenic-laced waters of Mono Lake in California.
George D. Lepp/Photo Researchers Inc.
Gullies on Mars are believed to be formed eons ago by running water. Somehow the planet lost its ability to hold water on its surface, and scientists have speculated that primitive organisms survived by descending below the surface.