Of the more than 700 planets discovered outside our solar system, none yet fit the description alien hunters dream about: an Earth-like planet in an Earth-like orbit around a sunlike star. But some scientists want to broaden the parameters of their search. In November a team led by Washington State University astrobi-ologist Dirk Schulze-Makuch devised the Planetary Habitability Index, or PHI, a scoring system for distant worlds that measures their suitability for any kind of life, not merely life as we know it. "We can't go after only the Earth model of life," he says. "You really want to be open-minded."
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