From the op-ed (by astrobiologist Paul Davis):
Genetic sequencing is used to position unknown microbes on the tree of life, but this technique employs known biochemistry. It wouldn't work for organisms on a different tree using different biochemical machinery. If such organisms exist, they would be eliminated from the analysis and ignored. Our planet could be seething with alien bugs without anyone suspecting it.
How could we go about identifying "life as we don't know it"? One idea is to look in exotic environments. The range of conditions in which life can thrive has been enormously extended in recent years, with the discovery of microbes dwelling near scalding volcanic vents, in radioactive pools and in pitch darkness far underground. Yet there will be limits beyond which our form of life cannot survive; for example, temperatures above about 270 degrees Fahrenheit. If anything is found living in even harsher environments, we could scrutinize its innards to see whether what makes it tick is so novel that it cannot have evolved from known life.