Wikipedia the idea was popularised by Percival Lowell from 1893. However, both of these fellows are trumped, beaten and knocked into a cocked chronological hat by the Vicar of Newlyn St. Peter, Reverend Wladyslaw Lach Szyrma. He was, after all, the first person to use the noun Martian in Aleriel or A Voyage to Other Worlds published in 1883.
Aleriel is a novel about an English vicar who meets a Venusian, who's on an incognito tour of the Solar System. The Venusian, Aleriel, describes Venus to him (it's rather like a long matins), and later sends him a letter describing the farther planets and a few of their moons. Aleriel lands his ether-car on Mars and:
buried it in the snows, so that it might not be disturbed by any Martian who might come across it.
And bam! The noun was in our language, as familiar to us now as those other great fictional creatures: the Australian and the American. Martians were originally like this:
Under the forest shade, as we descended, we saw a figure half-human in aspect - erect and dignified - but gigantic in figure. His face was very like a man's and like ourselves, but yet he had a sort of lion look also in his limbs.
We should be very scared of Martians. According to Rev. Lach-Szyrma, they are nine-foot tall, they're vegetarians, they're communists, and they speak Esperanto (or a Martian equivalent). I don't like them one little bit, though Lach-Szyrma thought they were super.
They also live in triangular stone houses with conical metal roofs and have a lot of canals.
Lach-Szyrma seems to have had terribly raw deal in the history of science fiction. There is almost nothing about him on the web*, and I had to go to the Rare Books department of the British Library to read Aleriel. This is odd because science fiction types tend to be a trifle fanatical about noting and archiving every detail of their genre.
Though I disagree with Aleriel's politics and vegetarianism, the book is very pleasantly written and is such a turning point in the history of literature and of our conception of extraterrestrials that it should be in print in some sort of scholarly edition. Its prequel is now completely lost.
It's strange to see a work that predates, essentially*, its own genre. Take this apologetic paragraph from the introduction:
I have endeavoured to avoid as much as possible any conflict with established scientific discoveries; and, indeed, have based my speculations on the known facts of astronomy, only allowing the fancy to have free play where science is, and must be, unable, in its present state, to answer the questions here considered.
Which is pretty much what science fiction is. Most of all, though, I like Lach-Szyrma. There's something cuddly about him, something so wildly hopeful yet politely apologetic. His imagination sailed the solar system in an ether-car, yet his body remained in the vicarage of Newlyn St Peter. When the Venusian describes the capital of Venus he says that it is beyond earthly comparison, but a bit like Edinburgh. The volcanoes of the moon are huge, terrifying and lifeless; but remind him of the Malvern Hills. The Venusian, who is of superhuman intelligence, believes that the strangest thing on earth is that so few people come to weekday services at the parish church.
Poor Wladyslaw. His book is a voice from another world.
I can't find a picture of Wladyslaw, so here is his father Krystyn, who was, of course, a Polish emigré. He was also a soldier, a philosopher and occasional author.
*Somebody has started typing out a brief biography of him here, and then given up. I'd watch out, though, as the page looks hacked and may be virus-ridden.
**I shall not be entering in to huge discussions of what the first science fiction novel is. The point about genre is that it is a form that the public understands and which therefore requires no explanation or apology. A book does not a genre make.