Anders Celsius, inventor of the temperature scale, was also a science fiction writer

The name Anders Celsius will be familiar to readers from his invention of the universally-used temperature scale. But Celsius (1701-1744) was much more than that: he was a professor of astronomy, he went on latitude-measuring expeditions, he gave early warning about the melting of the Arctic ice cap...and he wrote science fiction.

Swedish fan writer Ahrvid Engholm tells the tale at Europa SF. In 1735 Celsius, writing an essay about astronomy, speculated the following, as translated by Engholm:

When astronomy has reached so far that we are sure of the status of the inhabitants of the planets; and physics is so forward that we find ways to travel from Earth to the planets, they none-the-less begin to make war in the air and trade and navigate on the planets, and thus they want to conquer each other. Later Earth will be considered as a united realm in opposing e.g. the Kingdom of Mars. They will then find out how to make alliances with each other, e.g. Mercury, Venus and Tellus, on one side, and on the other side Mars, Jupiter and Saturn. But as Jupiter and Saturn has conquered so many small kingdoms on their satellites; Mercury, Venus and Tellus would then likely oblige the inhabitants of the Sun to be on their side to keep the balance in the Solar system, and instead they now have Europa. By the way, if science would become as advanced that we could trade with other planetary systems, e.g. Sirus, our system would then once again be considered as one Kingdom, and then wars start between different systems, and so on ad infinitum. But since you cannot travel between systems in shorter time than a few hundred years, medicine should be improved so that people could become as old as during the time of the Patriarchs.

Who did originally think you could sail the seas, so you couldn't see see land? Who thought you could begin to do battle on water? Now we have little belief in that people in due time could sail and battle in the air. It could be possible that flesh and skin would become transparent. If one in the future could imagine such people and animals, it would become a possibility for an excellent novel – medicine would then make people long-lived.

Considering that the first work of proto-science fiction in Sweden was only published in 1734 (Olof von Dahlin's "The Story of Erik the Goth"), this would seem to make Celsius a true original — he had none of the French proto-sf stories and novels to go on, and based his speculations purely on his own imagination.

More at Europa SF.