The series is a response to Peter Douglas Ward's book "Rare Earth," which claims worlds with microbial life may be common but multicellular life is rare, and intelligent life vanishingly rare. Ward lists all Earth's quirks that influenced our unique path of evolution, then demands other worlds match every one. This approach is common in exobiology, but specious. Even if Earth were absolutely ideal for life (patently untrue) and no other path was possible (fantastically unlikely), a world may be unearthly in several ways that effectively cancel out...
His example variant Earths retain the familiar topology (the shape of Africa, the mid-Atlantic ridge etc.) but with vastly less water, more sunlight, less gravity (etc.) He suggests what the planet would look like, and what life forms might have evolved.
Lyr: 15 times more water than Earth, and much stronger gravity
Serrana: small, with 1/15 of our water
Oisin: Iced up and stressed by the gravity of its companion (like Europa)
(A more comprehensive listing of his variant planets may be found here)
Sorry, some readers will think this is a dreadful waste of time even to scan quickly, but I find it really interesting. I like to imagine what it would be like to live on a planet like that, and the way it all fits together logically is quite satisfying. And, finally, I am quite Mulder-esque in that I want to believe there is intelligent life in space. I like Chandra Wickramasinghe's theories of interstellar DNA for the same reason.