+0 WORD ? (unused) +2 n BYTEs 500 planet records of 85 bytes each, see PDATAx.DAT.
FHost is reported to mark destroyed planets by setting their ore content, temperature, etc. to unpleasant values (everything zero). Note that the "correct" way of destroying planets is to move them out of the way using XYPLAN.DAT, not by zeroing them out in this file: the change in XYPLAN.DAT can be communicated to the players, the change in PDATA.HST can not.
PHost considers planets non-existent when it has an Id of zero in this file (normally, planet slot X has Id X). Recent versions report that fact using a UTILx.DAT entry; in earlier versions, this is problematic, too, because it can't be communicated to the players.
