Anything powerful enough to destroy a planet would have to involve a hell of a lot of energy.
But that would definitely be a "blink and you miss it" thing. Not because the event would only take an eyeblink, but because passing through its image in a starship would.
Before and after the event, the star system might look pretty much the same from a distance. Planets at first, rubble later, but the difference would not be all that noticeable, especially as the Reliant
would be maintaining the "blink-and-miss" speeds well past the 15 ly distance where the explosion itself would be visible, and only slow down at a distance corresponding to a date allowing for a full decade of cooling and settling.
Supposedly, studying of targets by their "onion shells" of historical vistas is not something Starfleet would be bothered to engage in anyway, as they can get realtime information with FTL sensors at certain distances (in TNG, typically less than a dozen lightyears) and can reach those distances easily enough if they are interested in the target in the first place.
The ability to say "there's a Class F planet in this system, with signs of early industrial era" at a distance is consistent with "realistic" scanning techniques: information like this can easily be decades old and divined from low-resolution spectrography. Yet it is very rarely that our heroes are able to tell that such a planet has been recently bombarded, deserted or made to disappear, unless they utilize FTL techniques reserved for targets of established interest. And "going there" is a good FTL technique, trumping "looking" because eventually you are going to go there anyway!