Addressing the gravity of the situation
"Home" is an effective goodbye episode that writes out Alara Kitan in a dignified and poignant way. It might’ve worked better if she — or anyone on this series, for that matter — had been on the show longer. I’m not sure why Halston Sage is leaving the series already (there seems to be no "official" line on the matter; various rumors are out there), but the writers have given her a way to exit that fits the character as we’ve come to know her in this short time.
She’s forced to return home when Dr. Finn discovers Alara’s physical strength is diminishing because prolonged exposure to lower gravity has induced an atrophy that, if she doesn’t return home, may become permanent, making it so she can never return home. How long she will need to remain home to recover is an open question. Varied case histories suggest it could be weeks, months, or forever.