According to the Thirteenth Doctor, an orphan planet was a grading that meant that a planet had become too toxic for life, uninhabitable. In societies that let this happen, there was often a ruling elite that managed to evacuate, then signed off all responsibility for whatever they had left behind; all sentient life died and the planet was classified as orphan.
In a future after 2020, in a possible timeline, there were at least 55 orphan planets, with Orphan 55 being a future Earth. On this Earth, a spa known as Tranquility Spa was set up with a barrier surrounding it, making it look lush and lively. (TV: Orphan 55)
Behind the scenes
In the real world, an orphan planet is actually another name for a rogue planet, which is a planet that drifts through space without a sun.