Question about the korean drama "Secret Garden"?

in episode 9, a little bit far from the beginning, when Kim Joo Won was going to his house and he finds out that his passcode has been changed. Gil Ra Im texted him that it was about a constellation/star. and then kim joo won had a flashback to when oska and raim was talking to each other, joo won thought about oska and typed in the passcode and he got in. what was the passcode??? and why was he like, "oh my godd.."