So I tried Google's Ngram viewer on Exobiology, Astrobiology and Exoplanet, and came up with the chart shown above. As an added perk, I overlayed the yearly count of exoplanet discoveries on the graph.
As you can see, the term Exobiology was more popular than Astrobiology back then. And the word "Exoplanet" never occurred in any books prior to the 80's. But it all changed when the first confirmed exoplanet was announced in 1992. It was then that the trend of the exoplanet began, and propelled a sudden surge of interest in Astrobiology.
As new exoplanet discoveries continued to pile up, so did the field of Astrobiology continue to grow, evident in the increasing usage of the term in published books. By 1997, a few years after the discovery of 51 Pegasi b, the word astrobiology overtook exobiology.
Suddenly, Astrobiology was hip. It became increasingly more popular overnight, as if riding on the wave of exoplanets.
But what happened with the word Exobiology? Why didn't it pick up as much as Astrobiology did? It's a curious historical note, and the answer could be anyone's guess. And I'll give it a try.
The term Exobiology was coined by a Nobel Prize-winning scientist named Joshua Lederberg. I do not know exactly when the word Astrobiology was adopted to replace Exobiology but all we know is that it specifically means the "study of life beyond the Earth". But since there's no known life beyond the Earth (yet!), people said it's a field with no subject matter. That may well be a plausible reason why the term didn't catch on, no matter how cool the prefix "exo" is.
And since the term Astrobiology already picked up the momentum, it also locked on as the bastion to the science of "Life elsewhere in the universe"--a phrase that makes it seem a wider field than by just saying "Life outside earth".
Now here are questions to entice the next generation. Will exobiology make a comeback? Do you think the word exoplanet will skyrocket in the next decade? And what about...ahem...Exoplanetology?
Only time will tell.
