"Interplanetary Teleport" (a level 9 spell) is the obvious choice but won't be reachable in most campaigns.
It might be possible to travel at a lower level using the Shadow plane or the Ethereal plane. "Shadow Walk" (a level 5 to 6 spell) is the obvious choice for going to another place but it should also be possible to "Plane Shift" (a level 5 to 7 spell) to the Ethereal plane or Shadow plane for faster travel (although one might still need flight in the plane of Shadow.) However, for proper navigation one might need suitable Knowledge (Planes) and Knowledge (Geography) checks or the use of "Find the Path" (a level 6 spell.)
"Planetary Adaptation" (a level 4 to 5 spell, People of the Stars pg. 29 (Amazon), Distant Worlds pg. 55 (Amazon)) is the obvious choice for letting one survive the conditions on Aucturn but there may be lower level ways. "Air Bubble" (a level 1 spell) could let one breathe in outer space but there is also the matter of explosive decompression, radiation and a host of other issues. Those issues could be completely avoided if one was Undead or a Construct such as Wyrwoods though.
Presumably, once the base spells are figured out a suitable custom magic ship could be bought from a magic item market within wealth by level guidelines by even lower level characters. Magic item pricing is a black-art though.
I think the lowest level way to reach Aucturn from Golarion would to be a Wyrwood and simply "Fly" (a level 3 spell) to Aucturn from Golarion. There are no rules that state one can't simply keep on flying past the atmosphere into space when using the Fly spell. This might be a bit slow though.
