Alien astronomy at our present technical level may have detected our biosphere many millions of years ago. The Great Oxidation Event occurred around 2.4 billion years ago; it was a rise in oxygen as a waste product due to organisms in the ocean carrying out photosynthesis. Long-lived robotic probes could have been sent to observe Earth long ago. I will call such a probe a “Lurker,” a hidden, unknown and unnoticed observing probe, likely robotic. They could be sent here by civilizations on planets as their stars pass nearby.
Long-lived alien societies may do this to gather science for the larger communicating societies in our Galaxy. The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
Here, in part 1, I estimate how many such probes could have come here.
To read more, click here.