Changelings by Anne McCaffrey

Those familiar with McCaffrey and Scarborough's first SF trilogy about life on the sentient planet Petaybee will best appreciate this solid start of a new series, which picks up where Power Play (1995) left off. Murel and Ronan, the precocious twins born to Maj. Yanaba Maddock-Shongili, administrator of Petaybee, and geneticist/selkie Dr. Sean Shongili, lead an idyllic, if frigid, life on the icy planet for their first eight years.