Dataminers for Pokemon GO have been uncovering some major visual changes coming to the game, and planets were just added to the list of potential additions.
Skybox visual changes have been datamined for a few weeks now, but as each update is rolled out, more assets are uncovered by dataminers like Pokeminers, who have been compiling a list of potential updates in the future. Most of the time, the assets uncovered are eventually added to the game, but it's important not to take it as fact until Niantic makes official announcements.
Regardless, the dataminers give players a good idea of what's to come, and planets may be in Pokemon GO before we know it. They will be a purely visual addition to the game that will likely be a part of the skybox backgrounds.
The files which are found in Android installation files, or the APK, have all of the planets displayed.
Of course, Pluto is no longer considered a planet, but it was found in the files regardless. Planets aren't the only potential change for skyboxes in Pokemon GO though.
All visual codes datamined for Pokemon GO skyboxes
Based on what Pokeminers have found, it looks like skyboxes will be getting an entire visual update or overhaul, and planets are just one piece of that. Those potential changes were listed in the same way as the planets for Pokemon GO.
It's clear that the skybox update will entail far more than just planets, and rather elements of astronomy as a whole. On top of the potential raid stadium visual update in the APK files, there is a lot to look forward to for Pokemon GO players.