No one actually expects you to repeat as World's Most Desirable Woman (a title won by Eva Mendes in '09, as we reported yesterday). But falling 80 spots?

That's the fate of Grey's Anatomy star Katherine Heigl, who came in at a modest #81 after taking home the title in 2008. What a difference a year makes.

Why the precipitous decline? A few theories:

• Lack of celebrity gossip.
• Her chain smoking habit.
• She's been off the market (marrying at Christmas 2007).
• Her desirability mirrors that of her much-maligned TV show.
• She rarely, if ever, shuts her damn yap and offers about as many quotes dissing Knocked Up as she did Knocked Up quotes.

We may never know the reason, but it certainly isn't because Katherine Heigl got any less hot. Case in point: click to enlarge some classic pics of her below ...