Voltaire was an amazingly prolific writer, producing over 2,000 books and pamphlets in his lifetime - along with over 20,000 letters. In 1752 (or possibly earlier) he published Le Micromégas, a novella about a couple of aliens who visit Earth.

The story begins with a description of Micromegas, a gigantic character from a planet orbiting Sirius:

"On a planet revolving around the star Sirius there lived a young man of great intelligence, whose acquaintence I had the honor of making during his recent visit to our little anthill. He was called Micromegas, an appropriate name for great people. He had a stature of eight leagues, or 24,000 geometrical paces of five feet each, or 120,000 statute feet...

His mind rivals the most cultivated among us; he knows many things, some of which are his own inventions. He had not yet reached his 250th year, and was studying, as was customary at his age, at the most famous school on the planet, when he solved 50 propositions of Euclid--18 more than Blaise Pascal, who, after having, according to his sister's account, solved 32 for his own amusement, became a pretty fair geometer, and a very poor metaphysician. When Micromegas was about 450 years old, and already passing out of childhood, he dissected, with the aid of powerful microscopes, many little insects less than 100 feet in diameter; he wrote an interesting book about them, which got him into trouble.

The mufti of that country, much given to hair-splitting and very ignorant, found in his work statements they deemed suspicious, offensive, rash, and heretical, and they prosecuted him with bitter animosity. The question in dispute was whether the substantial form of which the fleas of Sirius consisted was of the same nature as that of the snails. Micromegas defended himself spiritedly, and had all the ladies on his side; the trial lasted 220 years. At last the mufti had the book condemned by judges who had never read it, and the author was forbidden to appear at court for 800 years."

Micromegas then decided to go space travelling 'sometimes by means of a sunbeam, and sometimes with the help of a comet':

"Micromegas, after plenty of turns and twists, arrived at the planet Saturn. Accustomed though he was to the sight of novelties, when he saw the insignificant size of the globe and its inhabitants, he could not at first refrain from that smile of superiority which sometimes escapes even the wisest; for in truth Saturn is scarcely 900 times greater than Earth, and the citizens of that country are mere dwarfs, only a thousand fathoms high, or thereabout. He laughed a little at first at these people, in much the same way an Italian musician, when he comes to France, derides Lulli's performances. But, being a sensible fellow, the Sirian was soon convinced that a thinking being need not be altogether ridiculous because he is only 6,000 feet high. He was soon on familiar terms with the Saturnians after their astonishment had somewhat subsided. He formed a close friendship with the secretary of the Academy of Saturn, a man of great intelligence, who had not indeed invented anything himself, but excelled at describing the inventions of others, and who could turn a little verse neatly enough or perform an elaborate calculation."

The two friends then go on a trip together:

"Meanwhile our two inquirers commenced their travels; they first jumped onto Saturn's ring, which they found pretty flat, as an illustrious inhabitant of our little globe has cleverly conjectured; thence they easily made their way from moon to moon. A comet passed near the last one, so they sprang upon it, along with their instruments. When they had gone about 150,000,000 leagues, they came across the satellites of Jupiter. They landed on Jupiter itself, and remained there a year, during which they learned some very remarkable secrets which would now be appearing in the press, were it not for certain censors who find them too hard to swallow.

Leaving Jupiter, our explorers crossed a space of about 100,000,000 leagues, and, coasting along the planet Mars, which, as is well known, is five times smaller than our own little globe, they saw two moons. These have escaped the observation of our astronomers. I am well aware that Father Castel will write, and pleasantly enough too, against the existence of these two moons, but I believe those who reason from analogy. Those excellent philosophers know how difficult it would be for Mars, which is so distant from the Sun, to get by with less than two moons. Be that as it may, our friends found the planet so small they were afraid of finding no room there to put up for the night, so they proceeded on their way, like a pair of travelers who disdain a humble village inn, and push on to the nearest town. But the Sirian and his companion soon repented this decision, for they went a long time without finding anything at all.

At last they perceived a faint glimmer; it came from our Earth, and created compassion in the minds of those who had so lately left Jupiter. However, for fear of repenting a second time, they decided to disembark. They passed over the tail of the comet, and with the aid with an aurora borealis close at hand, alighted on Earth by the northern shore of the Baltic Sea, July 5, 1737."

At first they think Earth uninhabited, simply because everything is to tiny to their gigantic selves!

"The dwarf hastily declared there was not a single creature on this planet. His first reason was that he had not seen one. But Micromegas politely explained that that was not a good argument:

"For," said he, "you, with your little eyes, cannot see certain stars of the 50th magnitude which I distinctly discern; do you conclude that those stars have no existence?"

"But," argued the dwarf, "this globe is so ill-constructed, so irregular, and so ridiculously shaped! All here appears chaotic; look at these little brooks, not one of which goes in a straight line, and these ponds, which are neither round, square, oval, nor of any regular form; and all these little bristles which have rubbed the skin off my feet!"--he alluded to the trees--"Observe too the shape of the globe as a whole, how it is flat at the poles, how it turns around the Sun in a clumsily slanting manner, so that the polar climes are mere wastes. In truth, what chiefly makes me think there is nobody here, is that I cannot suppose any sensible people should wish to occupy such a dwelling."

"Well," said Micromegas, "perhaps the people who inhabit it are not sensible. But there are in fact signs of its not having been made for nothing. Everything here seems irregular, you say; but you judge by the standards of Saturn and Jupiter. Have I not told you that in the course of my travels I have always found variety?"

They then go on to make contact with the tiny Earthlings, with Voltaire's typical wit. You can read the whole thing at Wondersmith.