this is a long shot and may be a

ache but unless youve wiped your system since you discovered this system, every system youve visited is recorded in log files on your machine. these logs can be found in C:\Users\[your logon]\Saved Games\Frontier Developments\Elite Dangerous <assuming your using win10>
however you will likely find there are 100's of these little SO

and they tend to hold a lot of info. if you happen to recall when you discovered the planet you may be able to narrow it down a little.. these files capture a lot but im only guessing they capture first discovered, scanned etc and\or if your first to hand in the discovery.. if it captures at least discovered, scanned etc you can go check the map in game to see if your name is there
iirc there are a few log analisers arround which may help, this one is for exploration.. sorry found that, on google
edcodex.info
or you may find something more useful here
edcodex.info