Seventy years ago - and damn, but is it going to take me a while to get used to 1940 being "seventy years ago" now - neither writers nor readers batted an eye at the prospect of canals on Mars, jungles on Venus, or intelligent species scattered across the solar system. It wasn't until the latter half of the 20th century, when more sophisticated Earth-based detection systems were developed and the first interplanetary probes were launched, that Barsoom was transformed from possibility to fantasy.
Being proven wrong is an eventuality that every sf writer will face. It's an intrinsic fact of the genre; we're none of us clairvoyants, and though some may forecast more accurately than others, what we put down in our words can't ever match the future that will eventually unfold. It's too complicated for that. What sf writers can do, however, is try to minimize this as much as possible.
Right now, the stories I've got in the working or planning stages are relatively near future pieces, set in the Earth-Luna system in the late 2070s. For a while I've had plans to go further afield once I develop the setting, to do 22nd-century stories where distant interplanetary and even interstellar journeys become possible. The stars beyond Sol have been ciphers for all of human history - for almost all of the time that science fiction has been written, Earth's intrepid explorers might find anything under other suns.
This decade might see this change a bit. NASA's Kepler telescope, launched last March on a multi-year mission to discover strange new worlds, has been hard at work, and on Monday details were released of the five new exoplanets detected by its efforts. All five of the newfound planets are hot worlds, orbiting extremely close to their parent stars, and only one is smaller than Jupiter. Going back to the discovery of Bellerophon in 1995, many of the exoplanets so far detected have been such "Hot Jupiters." Kepler is poised to change that, with its sensitive instruments said to be capable of detecting Earth-mass planets orbiting other stars. Not only that - in the near future, telescopes may well be able to detect individual moons orbiting these gas giants. Presumably, only the exotic properties of the mass relays will prevent Kepler from detecting them as well.
While this is great news to me, as a scientifictionist it poses a problem for the immediate future. Back in 1940, spaceflight itself was widely considered a fool's dream - whether or not Mars had canals and Venus jungles, if the writers of the time were concerned about accuracy, for all they knew it might have been a hundred years until they were proven wrong or right either way. I feel like I'm in a similar place now as to when Larry Niven wrote and sold The Coldest Place in 1964, written when the leading consensus was that Mercury was tidally locked, a consensus that was only broken by radar mapping - and after The Coldest Place had been bought, but before it was published.
I ran into a similar problem while writing You Source of Tears, which will be appearing in an upcoming issue of On Spec magazine. Originally it was set around Halley's Comet, though the only artifact of this is the title itself. I changed the setting to the fictional Comet Veale for two reasons - one, because a three hundred-year orbital period was more convenient for me than Halley's seventy-six, and two, because there was the possibility that some future expedition to Halley's Comet would prove me wrong. As an sf writer, I make enough assumptions about the future already - there's no need for me to dump falsifiable predictions into the mix.
Kepler, if successful, will revolutionize our understanding of the interstellar neighborhood. In ten years, maybe we'll know that a planet of Tau Ceti or a moon of a gas giant at 55 Cancri has an oxygen-nitrogen atmosphere and liquid water. We wouldn't be speculating blind anymore, and while we may end up just building new Barsooms, there's a certain magic in that. I'm willing to hold my pen until they make the first maps of those strange new worlds.