Space colonization stories are a subgenre of SF. Space colonization stories in which the Earth has become a backwater world, cut off from thriving colony planets, are a thriving sub-subgenre.
At first glance, this seems odd. Earth is rich in resources and offers humans a shirt-sleeve environment . Why wouldn’t it continue to be the leader of the pack?
Sometimes it’s because we have trashed the Earth, rendering it uninhabitable. Stories like Thomas Scortia’s Earthwreck, Arthur C. Clarke’s “If I Forget Thee, O Earth,” and Joe Haldeman’s Worlds Apart are set on Earths where nuclear and biological weapons have turned the surface of the planet into a death trap . Any humans remaining have two options: flee or go extinct.
(In reality, even a radiation-soaked Earth would be still more habitable than any world in our Solar System. SF authors ignore or downplay that because they want to tell stories about extraplanetary societies.)
A few authors go that extra 1.6 kilometers and obliterate the Earth entirely. Wil McCarthy’s Queendom of Sol setting imagines a communications system with a failure mode that can and does turn the Earth into a small black hole. Nothing encourages the settlement of other worlds quite like having your old one reduced to the size of a marble.
Sometimes the issue isn’t that we’ve made Earth too hostile, but that our exuberant embrace of advanced technology has created something far too friendly for our own good.
Wil McCarthy seems to like destroying the Earth. In Bloom, nanotech beasties swarm the face of the planet, absorbing everything they encounter like an unstoppable katamari. The only recognizable humans left in the system are those lucky enough to escape the planet in time.
Similarly, the backstory of Michael Swanwick’s Vacuum Flowers postulates the abrupt appearance of a terrestrial mass mind known as the Comprise. The only sure way to avoid incorporation into the light-speed-limited Comprise is to gain enough distance from it that the Comprise cannot maintain cognitive continuity.
In Poul Anderson’s “Epilogue,” human travelers return to Earth after the passage of many years to find that it is now the domain of an ecosystem composed entirely of robots
When humanity fails to render the Earth undesirable, aliens can do the job for us. The classic example is, of course, John Varley’s Invaders, who suppress advanced technology to preserve the terrestrial species they prefer.
Some books don’t imagine compelling physical reasons to shun the Earth.
The Earth of Vonda McIntyre’s The Exile Waiting isn’t uninhabitable, but it is worn out and poor, with little to offer traders or visitors. In Melissa Scott’s Silence Leigh books, Earth has been deliberately sequestered from the greater interstellar community in order to better control it.
In John D. MacDonald’s Ballroom of the Skies, Earth is kept carefully sequestered from the galaxy because it is the sole source of an irreplaceable resource that would vanish if Earth were ever brought into the galactic mainstream. (Explaining more would be getting into spoilers.)
Sometimes it’s hard to say why the two groups, terrestrial and extra-terrestrial, avoid each other.
In Pamela Sargent’s Venus trilogy, Earth’s Nomarchies and the space-based Habbers view each other with suspicion and condescension.
In Varley’s Thunder and Lightning series, off-worlders like Podkayne Strickland-Garcia-Redmond view people from Earth with contempt, taking steps to ensure that as few people from Earth are allowed to emigrate as possible. There is no mention, however, of anyone wanting to build any sort of wall around Earth , at least.
I suspect that some isolated or ignored Earths, particularly the more backward ones, owe their inspiration at least in part to a combination of American Exceptionalism and what might be called historical orthogenesis: having founded off-world colonies which in this mythology must outshine the mother world, the Earth’s insistence on continuing to exist is at least a little rude, if not misguided. Best to underline the point by making it clear the Earth is now at best a second-rate locale….
In the words of Wikipedia editor TexasAndroid, prolific book reviewer and perennial Darwin Award nominee James Davis Nicoll is of “questionable notability.” His work has appeared in Publishers Weekly and Romantic Times as well as on his own websites, James Nicoll Reviews and Young People Read Old SFF (where he is assisted by editor Karen Lofstrom and web person Adrienne L. Travis). He is surprisingly flammable.