Detectability of life and photosynthesis on exoplanets

‘Is there life on exoplanets?’. We refer to exoplanets as planets in other solar systems than our own. This often asked question can be further refined by asking ‘is there life on exoplanets which is so extensive that it may impact on its atmosphere, its biosphere and its optical properties?’. And if such a life exists, at astronomical distances from us, can we detect it with instruments on Earth-based or Earth-orbiting observatories? Will then, in that case, our advanced knowledge of present-day and early-day photosynthesis on Earth help us select appropriate biosignatures that may signal its presence? Here we elaborate further on these themes, based on the most recent literature, and from the point of view of photosynthesis. We also provide our considered views. Although search for chlorophyll is considered desirable, we conclude that our best bet is to look for and analyse photosynthesis-related gases, namely O2, CO2 and H2O vapour. We shall keep in mind that the evolutionary tree of life on our planet has its roots in autotrophy, and of the various forms of autotrophy, only oxygenic photosynthesis can produce massive amounts of reduced carbon and free O2, both in water and on land.