Did Anne McCaffrey plan the physics of Pern or did it develop along the way? Here's her response:
The Physics of Pern-ah, yes, well, they have been developed by Dr. Jack Cohen of Warwick University in Dragonsdawn and All the Weyrs of Pern.They are basically Newtonian physics and/or biology in which Jack holds both a Ph.D and a D.Sc. He is also a generalist, and was able to correct some of my basic errors, which were legion. I satisfied the science requirement at Radcliffe by taking cartography. Although I did not know at that point in my life that it would be a valuable asset in my life's work, being able to develop a map of new worlds has been extremely valuable. Even expert Karen Wynn Fonstad, whom I helped with details in the Atlas of Pern, remarked that most of my suppositions about Pernese terrain jibed well with what was geologically possible. It was a case of the subconscious running with an idea in the best of possible directions.
To Jack Cohen, I owe the biology of Pern, especially of the dragons and the fire-lizards who supplied the genetic material used to create-by a special and now-unknown process of the alien race-the Eridani, the dragons of Pern. (You can't just breed large examples of a species to other large examples and keep increasing the size in a few generations.) He also figured out how to decimate the malignant and possibly dangerous life-forms that inhabited the Oort Cloud with the zebedees (his name and creation, a creature that replicates itself despite its environment).
I had, you see, backed myself into a corner in The White Dragon,because there was no way the present day population of Pern could eradicate the mycorrhizoid Thread life form without the help of the original colonists. It was my notion to have a voice address artificial intelligence unit-AIVAS-as part of the settlers' equipment, and for the present day Pernese to discover this device and learn from it. I might have been ahead of the game in suggesting such a computer in 1970, but current computer techniques indicate that such a device is within the realm of possibility. What I desperately needed was a system by which the Pernese generation could access the technology left by their progenitors so that they could actively fight Thread and honor the Weyrleader's vow to end the Threat of the Red Star forever. Jack was ingenious in working out details that would forward this ambition. For instance, how the colony ships were powered. He figured out that, in order to move such a mass as a planet, there would need to be several attacks on it. He utilized the draconic ability to move between time and place, and also set up the zebedees to be sure that the mycorrhizoid spore did not again leave the Oort Cloud. According to the Hoyle-Wickramansingh theory (to which Jack does not personally subscribe but was willing to use as a mechanism in the story) about Oort clouds, many organisms might lurk within such astronomical features, as well as embryonic cometary masses, and it would be as well to disseminate a killer virus in the cloud to prevent any recurrence of Thread or similar nebular virii.
So, as far as the physics of Pern is discussed by me in the books, it is legitimate even if it hasn't happened elsewhere. Jack was adamant that I give a logical explanation of as much as I could, because then people would be more apt to trust the improbable. It's much easier to describe Pern as earth-like, and that is quite legitimate, too, especially now that we have discovered other star systems do have earth-type planets around their primaries. I try to have some basis for even the most drastic suggestions I make.
However, I went to the source, as it were, for correct astronomical dates for the latest Pern book, The Skies of Pern. Iasked Dr. Steven M. Beard of Edinburgh Observatory, and Scott Manley of the Armagh Observatory for substantiating facts in deploying a fireball above and into Pern. I went up to Armagh, also meeting Dr. Bill Napier, author of Nemesis,and other members of the observatory staff to see Scott's Cosmic Impact Program from which he directed Pern's fireball. Coincidentally, earth itself had a Near Earth Object scare about the time I finished writing the manuscript, and many concerned scientists were watching the rather busy skies near our planet to identify and forewarn of other close encounters with NEO's and PHA (Possibly Hazardous Asteroids). Such movies as Deep Impactand Armageddongave me visual evidences of such a disaster, but the dragons of Pern are unusually designed to help mitigate the worst effects of such a catastrophe. The thrust of The Skies of Pern,the need for the inhabitants to become more aware of their spatial environment, and the critical need to set up additional observatories to help prevent a recurrence of such a cosmic impact, is as modern and timely as the one currently in operation on earth, even if it requires the dragons of Pern to implement.
So, Did I plan the physics of Pern or did it develop along the way? The answer is, I didn't plan. It developed along the usual lines for any earth-type planet, and I dragged in whatever experts I needed to substantiate any unusual actions or positing. People want Pern to be believable: I've tried to make it logical as well. I also hope that the general attitude of the Pern People within the books has reflected their growing awareness of science and technology. I tried to infer this by their less restricted vocabulary and general acceptance of their historical origins.