Canon has established that Star Fleet possesses the ability to obtain fairly detailed sensor scans of planetary systems from quite some distance. Even to the extent of determining if any life existed (or no longer existed).
Yet all our examples of our heroes discovering the loss of planets involve them going in for point blank studies, or receiving a distress call first, or both. Never do they notice that an entire star system (save for the star!) has gone missing, not until they are right atop it. It's not particularly unnatural to assume that smaller anomalies would go unnoticed even at the point blank stage.
Scanning is such a basic and engrained part of space travel in the ST universe that it's such a regulus ... er, regular function
Actually, it's extremely rarely that our heroes scan for anything without specific prompting, or that their scanners automatically inform them about a phenomenon not smack on their direct path. Failing to notice entire planets missing is consistent with TOS and with TNG; so it failing to scan "just in case".
Besides, "a planet" is exactly what they were looking for, and they would have to take a peek at every one in a system to make the determination whether is was suitable or not for their project
They are looking for a rare variant of desert planet, obviously. They can't be going blindly from system to system, or else they would all die of old age, or Starfleet would assign multiple ships to the task. The likelier scenario here is that they have a list of potentially useful worlds, extremely cursorily charted by previous ships or probes or remote observations, and they go through them and them only, one by one.
Which means that when they enter the system, they scan for the local desert world and home in on that. Such a process will
not provide them with any warning that a planet in the system has gone missing: the sensors simply scan the system, point them towards the desert world, and that is it. Other planets spotted will be ignored.
Then when they get there...no one notices that what was there doesn't match up with the map they used to go there to begin with?
It is probably bad practice to assume that maps hold true - too much can change in the Trek universe, as evidenced by this very adventure. A sensible navigator scans for that which is there, and ignores any claims of what ought to be there.
Terrell's navigator
was sensible: he or she got the ship to the local desert planet in one piece, without unnecessary detours or anything. It's too bad that Khan lurked there. But it can't be that records would have told
where Khan lurks and hence CA VI would have been fine even if CA V had to be avoided. Chekov's surprise is too total for that, Terrell's ignorance too deep. Rather, we must assume that the records tell nothing about Khan - at least not to people with Terrell's clearance level.
...the only way to mistake planet V for planet VI is if they are both the same mass and size, and planet V somehow shifted magically into planet VI's orbit.
Only if one has detailed information on the planets. Starfleet doesn't need detailed information - its ships can cope with cursory information, which is no doubt much faster and cheaper.
If Starfleet did have detailed information on the Ceti Alpha system, it would not need to send the
Reliant to study the system - it could send Terrell to the Kelvin Memorial Library to study the system!
Timo Saloniemi