Life is pretty easy to recognize. It moves, it grows, it eats, it excretes, it reproduces. Simple. In biology, researchers often use the acronym ‘MRS-GREN’ to describe it. It stands for movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. But Helen Sharman, Britain’s first astronaut and a chemist at Imperial College London, recently said that alien life forms that are impossible to spot may be living among us. How could that be possible?