When was "off-world" / "offworld" coined?

The earliest instance of "off-world"—in a science fiction setting—that I could find in a Google Books search is from Jack Vance, The Houses of Iszm (1953), which uses the term several times:

He relaxed and loafed around the city for almost a week. There were few other off-world visitors; the Iszic authorities discouraged tourism to the maximum degree allowed them by the Treaty of Access.

...

"Yonder are four- and five-pod trees for the artisans. Each district has it unique requirements, the description of which I will not burden you. Our off-world exports of course are not of such critical concern, since we only sell a few standard and easily grown structures."

Farr frowned. It seemed the Zhde Patasz's patronizing manner had become more pronounced. "You could increase your off-world sales tremendously if you chose to diversify."

Zhde Patasz and Omon Bozhd both exhibited signs of amusement. "We sell as many trees off-world as we choose. Why strive further? Who appreciates the unique and exceptional qualities of our houses? You yourself tell us that Earther regards his house as hardly more than a cubicle to ward off the weather."

An early instance of "off-worlder" appears in an unidentified story in Vintage Science Fiction (1957):

The spaceman's trouble cry brought every off-worlder in the place to his feet. The bouncers started to come after him, hesitated as Bull's blaster swung toward them, and then looked to the house man for instructions.

And in Harry Harrison, Deathworld, serialized in Astounding Science Fiction (1960):

"My home planet? Just about the stuffiest, dullest, dead-en in the universe. ...I was fifteen before I learned to read—out of a book stolen from a noble school. After that there was no turning back. By the time I stowed aboard an offworld freighter at nineteen I must have broken every law on the planet. Happily. Leaving home for me was just like getting out of prison."

A similar term that also seems to have emerged in the 1950s in "off-planet." It shows up in Murray Leinster, The Forgotten Planet (1956):

They had a natural monopoly,—not of moth-fur and butterfly-wing fabric, and panels of irridescent chitin for luxurious decoration, but—of the strictly practical and detailed knowledge of insect-habits which made it possible to obtain them. Off-planet visitors who tried to hunt without local knowledge did not come back from the lowlands.

...

And also it is good sport. The planet is a sportsman's paradise. There are not too many visitors. Nobody may go hunting without an experienced host. And off-planet sportsmen tend to feel somewhat queasy after a session as guest of the folk who have made Burl their planet-president.

Andre Norton, Voodoo Planet (1959) offers examples of both "off-planet" and "off-worlder":

"If these have been in any way meddled with, I would need laboratory analysis to detect it. And I don't believe that Lumbrilo could hide traces of his work so cleverly. Or has he been off-planet? Had much to do with off-worlders?" he asked the Chief Ranger.

"By the nature of his position he is forbidden to space voyage, to have any close relationship with any off-worlder. I do not think, medic, he would choose your healing substances for his mischief. ..."

And again from Harry Harrison' Deathworld (1960):

"Thee is a search on in the city, but we're well ahead of that.I'm sure the Cassylians don't want to advertise their bad sportsmanship so there won't be anything as crude as a roadblock. But the port will be crawling with every agent they have. They know once the money gets off-planet it is gone forever. When we make a break for it they will be sure we still have the goods. So there will be no trouble with the munition ship getting clear."

Jason sounded a little shocked. "You mean you're setting us up as clay pigeons to cover the take-off of the ship."

"You could put it that way. But since we have to get off-planet anyway, there is no harm in our using our escape as a smokescreen."

It thus appears that both off-world and off-planet originated as ways to indicate "away from this world [or planet]" or "coming from another world [or planet]," rather than to specify a moon or artificial satellite orbiting a home planet and serving as an outpost or colony of that home planet.

The specific sense of "off-world" as a subsidiary satellite to a home planet seems to have emerged in the 1960s, as colonialism became a more touchy subject in the real world. For example, Charles Platt, Garbage World (1967) focuses on a small asteroid tht serves as a garbage dump for surrounding pleasure planets—and yet it is the denizens of this asteroid who call the people from those other planets "off-worlders":

"Fair enough," said Gaylord. "Never tried off-world brew."

He picked up the tumbler that Oliver had half-filled for him. His fleshy nose twitched as he sniffed the amber liquid. Then he shrugged and swallowed it all down in a single gulp. He smacked his lips and grimaced, shaking his head.

...

"All right," he said. "Kind of expected we'd have to get things out in the open, though it oughta be clear enough. You know how we feel about off-worlders. Nothing personal, mind, but we can't take their lily-white, pansy-faced pious attitudes. Don't like 'em dropping their garbage on us, then complaining because we're not clean like they are. Now, my girl Juliette, she was upset last night, with my hoard stolen and all. Fed up with the village crowd. You were the alternative, see. She got so drunk she thought it didn't matter you were off-world, even though she'd fallen for you. 'Course it couldn't work out. Full of shame now, she is. And so she oughta be."

The movie Blade Runner (1982) made the notion of "off-world" as a destination for colonists and pioneers seeking a new life away from a degrading planet famou throughout popular culture. The Blade Runner Wiki offers this description of "Off-world":

Off-world refers to the Earth's Orbital Space and also other planets, such as Mars, Kalanthia and Arcadia 234. Many locations in Off-world have been colonized and they are the homes of many humans as well as androids and replicants.

Years after the results from "World War Terminus", the United Nations encouraged people to emigrate to Off-world colonies to preserve the human race from the effects of the radioactive dust. Incentives for human leaving Earth have been promoted aggressively with advertising extolling the adventure and opportunities awaiting them if they leave, as well as promises of financial and personal incentives (for example, all colonists are promised a free android/replicant for their personal use).

Blade Runner was inspired by Philip K. Dick's novel Do Androids Dream of Electric Sheep? (1968), but I haven't read this book, and its plot evidently diverges considerably from that of the movie—to the extent that I can't tell whether the term "off-world" even appears in Dick's novel, although there clearly are colonies of Earthlings on surrounding satellites and planetary bodies near the earth. A search for "off-world" through a PDF file of what appears to be the complete novel does not yield any matches.

Update (January 2, 2019): In a comment above, Phil Sweet has pointed to entries for off-world (as a noun, an adjective, and an adverb) in Jeff Prucher, Brave New Words: The Oxford Dictionary of Science Fiction (2007). The entries are worth quoting at some length:

off-world n. 1. someplace other than the planet one is on or referring to. Compare OFF-PLANET [dating to 1972 as a noun and defined as "OFF-WORLD"]. [Earliest cited instance:] 1955 A. Norton Star Guard (1973) 49: Nature had provided him with a coat of thick curly hair, close in texture to the wool of a sheep, from which came a pungent, oily odor only apparent to those from off-world. 2. a planet other than the homeworld. [Earliest cited instance:] 1987 New York Times (Apr. 5) 24/4: Harrison Ford plays a down-and-dirty police assassin, who has been sent into this underworld to find humanoid robots who have escaped from the offworld and come to earth to find their inventor.

off-world adj. 1. in space; not on a specific planet. [Earliest cited instance:] 1950 B. I. Kahn Pinch of Culture in Astounding SF (Aug.) 85/2: A ship with a weapon this far out meant an off-world patrol. Planets not expecting trouble would have no reason for watchful expectancy. 2. from, of, or characteristic of someplace other than a specific planet. [Earliest cited instance:] 1955 A. Norton Star Guard (1973) 140: There is a space port near the Venturi holdings at Po'ult {...]. There is no regular schedule of ships, but off-world traders do come.

off-world adv. away from a specific planet; in or into space. [Earliest cited instance:] 1950 A. Coppel Rebel of Valkyr in Planet Stories (Fall) 8/1: "Where do you go now, Valkyr?" "Off-world."

Prucher thus identifies instances of off-world as an adverb and as an adjective from 1950 and as a noun from 1955. He also identifies instances of off-earth (as an adverb meaning "away from the planet Earth") from 1949 (in R. Lafayette, "Unwilling Hero," in the July issue of Startling Stories) and of off-planet (as an adjective meaning "in or from the space around a specific planet") from 1945 (in M. Jameson, "Lilies of Life," in the February issue of Astounding Science Fiction).

The 1953 Vance quotation cited earlier near the beginning of this answer includes one use of off-world as a noun, which antedates by two years the earliest corresponding cited occurrence in Prucher's dictionary; but Prucher's cited instance of off-world as an adjective in B.I. Kahn's 1950 novelette is three years earlier than the corresponding instances in Vance's novel.