Something deadly might be wafting through the clouds shrouding Venus—a smelly, flammable gas called phosphine that annihilates life-forms reliant on oxygen for survival. Ironically, though, the scientists who today announced sightings of this noxious gas in the Venusian atmosphere say it could be tantalizing—if controversial—evidence of life on the planet next door.

As far as we know, on rocky planets such as Venus and Earth, phosphine can only be made by life—whether human or microbe. Used as a chemical weapon during World War I, phosphine is still manufactured as an agricultural fumigant, is used in the semiconductor industry, and is a nasty byproduct of meth labs. But phosphine is also made naturally by some species of