I have always taken (and used) for granted the 1031 number of phages in the planet. Normally, this is calculated from the estimation that there are 10 phages per prokaryotic cells, and the latter are estimated to be 1030. Usually the references to these numbers are: Jiang & Paul 1998, PMID 9687430 and Whitman 1998, PMID 9618454
Today I found what might be an older reference: Bergh et al. 1989, PMID 2755508, High abundance of viruses found in aquatic environments
Once I get access to the full-text paper (“thanks to” Nature’s unwillingness to open even older articles), I can confirm the exact phage number as claimed in 1989.
If you know of a better (aka older) reference, feel free to share it.
This number (1031), by the way, can be read as: ten nonillions (by the US numbering system)