Beautiful Star by Yukio Mishima

The Osugi family have come to a realisation. Each of them hails from a different planet. Father from Mars, mother from Jupiter, son from Mercury and daughter from Venus. Already seen as oddballs in their small Japanese town in the 1960s, this extra-terrestrial knowledge brings them closer together; they climb mountains to wait for UFOs, study at home together, and regard their human neighbours with a kindly benevolence.

But Father, Juichiro, is worried about the bomb. He writes letters to Krushev, trying to warn everyone he can of the terrible threat. After all, humans may be terribly flawed, but aren't they worth saving? He sends out a coded message in the newspaper to find other aliens. But there are other extra-terrestrials out there, ones who do not look so kindly on the flaws and foibles of humans. And a charming young man, who claims to be from Venus too, tempts the daughter Akiko away from the family...