Some chemical formulas just don't look right. Any students who wrote H4O or Na3Cl on an exam would be well on their way toward failing because under Earthlike conditions those compounds would disintegrate instantly. But a new study suggests that some "impossible" combinations of elements might actually be common on planets in other solar systems. Astronomers hunt for distant planets, called exoplanets, by searching for subtle changes in the light from stars: the light might dim slightly as a planet passes in front of a star, for instance. It's not easy work, and the precision involved boggles the imagination; it's like standing in Maine and searching for a flea on a light bulb in San Diego. Despite these handicaps astronomers have discovered almost 3,200 exoplanets since the first one was spotted in the 1990s.
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