Photo courtesy NOAA/U.S. Dept. of Commerce Hydrothermal-vent tubeworms. See more alien images.

Are there other forms of life in the universe? The scientific search for extraterrestrial life forms has been bolstered by two recent discoveries. First, the discovery of life forms in exotic environments on Earth indicates that life is very hearty and can adapt to the strangest and most hostile environments. Second, astronomers found planets orbiting stars besides our sun -- over 50 extrasolar planets have been discovered as of 2001. Are there alien life forms on any of these planets?

If alien life does exist, what might it be like? Would it be simple forms of life such as bacteria, viruses or algae, or more advanced, multi-cellular creatures, perhaps even intelligent beings? Would aliens be animals, plants or have characteristics of both? Would they have arms and legs and walk upright as we do? Would they depend upon vision as their primary sense or use another way to gather information about their surroundings? Would they "breathe" oxygen or some other gas?

Speculation about aliens has typically been left to science-fiction authors, science-fiction readers and Hollywood writers and directors. In this article, we will examine astrobiology, the scientific search for extraterrestrial life. We'll apply what we have learned about life on Earth to speculate about what alien life forms might be like.