Life on other worlds — if it exists — might be so alien as to be unrecognizable. There’s no guarantee that alien biology would use the same chemistries as on Earth, with familiar building blocks such as DNA and proteins. Scientists might even spot the signatures of such life forms without knowing they’re the work of biology.

This problem is far from hypothetical. In April, the European Space Agency’s Juice spacecraft blasted off from French Guiana on a course to Jupiter and its moons. One of those moons, Europa, has a deep, briny ocean beneath its frozen crust and is among the most promising places in the solar system to look for alien life. Next year, NASA’s Europa Clipper spacecraft will launch, also aiming for Europa. Both spacecraft have onboard instruments that will look for the fingerprints of complex organic molecules — a possible hint of life beneath the ice. And in 2027, NASA plans to launch a dronelike helicopter called Dragonfly to buzz over the surface of Saturn’s moon Titan, a hazy, carbon-rich world with liquid hydrocarbon lakes that might be just right for hosting life — but not as we know it.

These and other missions on the horizon will face the same obstacle that has plagued scientists since they first attempted to search for signs of Martian biology with the Viking landers in the 1970s: There is no definitive signature of life.