There are aliens living in the hot springs of Yellowstone National Park. Technically, the otherworldly creatures aren't all alive; many are just bacterial microfossils. They're not exactly extraterrestrials either, having spent their brief lives entirely on Earth. But if the tiny microbes prove anything, it's that living organisms can make their homes in the most unlikely places. For scientists like Sherry Cady, 43, a geologist at Portland State University in Oregon, this is big news indeed.
Cady is one of the leading lights in a brightening field known as astro- or exobiology--the study of how life could form elsewhere in the...