If human intelligence was seeded on Earth by extraterrestrials a few million years ago, they must have had fun monitoring our intellectual growth over history. For example, as soon as we came up with the religious notion of God who created humans in its image, the extraterrestrials must have texted each other: “Look, they started talking about us!”
Once our spacecraft will exit the Solar system and enter interstellar territory, we might hear from them. But any message of the type: “welcome to the interstellar club” will not come sooner than tens of thousands of years from now. The interstellar probes we launched so far will take that long to traverse, merely a few light years. Voyagers 1 & 2, Pioneer 10 & 11 or New Horizons, acquired a terminal speed of only one part in ten-thousandth of the speed of light.
An interplanetary trip may have already occurred by tiny astronauts onboard natural space vehicles in the form of rocks. Primitive lifeforms, like microbes, can potentially be transferred through the exchange of rocks between neighboring planets. This hypothesis, labeled panspermia, was first proposed in the fifth century BCE by the Greek philosopher Anaxagoras and is reviewed in chapter 10 of my recent book with my former postdoc Manasvi Lingam, titled Life in the Cosmos (Harvard University Press, 2021).
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