Rachel Bilson was spotted on a plane with her mom on Monday night, sporting an engagement ring on her finger.  Rachel was overheard telling a passenger on the flight, “I’m not married yet, just engaged.  I know I look like I’m 15, but I’m actually 27″.

We’re speculating that she’s engaged to her boyfriend Hayden Christensen.  If not, we’ll report the drama that ensues after he finds out the big news.