People.com Celebrity 21 & Under: Hottest Breakout Stars Zac! Rihanna! Shia! Lauren! These kids aren't only the most popular, they're also the most likely to get bigger in '08. By Aaron Parsley By People Staff Published on November 14, 2007 03:00 PM Share Tweet Pin Email 01 of 07 THE HEARTTHROB Steve Granitz/WireImage With the worldwide sensation High School Musical 2 breaking records and his big screen debut in Hairspray, Zac Efron, 20, proved that he's going to be making girls scream for a long, long time. 02 of 07 THE BOMBSHELL Lester Cohen/WireImage No one was caught without an "'ella, 'ella, 'ella" this summer thanks to the season's catchiest song, Rihanna's "Umbrella." And while artists as diverse as Mandy Moore and My Chemical Romance covered the sexy hit, the ubiquitous Barbadian beauty and VMA winner, 19, became a Cover Girl herself, modeling for the makeup line. 03 of 07 THE GIRL NEXT DOOR Steve Granitz/WireImage Heroes star Hayden Panettiere has conquered – and saved! – the world on her hit show Heroes, but in real life she proved to be more than just a spokesmodel and red carpet fixture. The sweet-faced blonde, who's only 18, risked her own life as she took a stand in dangerous Japanese waters to attempt to save a school of dolphins from slaughter. 04 of 07 THE NEXT BIG THING Tammie Arroyo/AP With the bankable hit Disturbia and the summer blockbuster Transformers under his belt, Shia LaBeouf, 21, was already being compared to another superstar: Tom Hanks. But all that talk isn't going to his head as he films the next Indiana Jones with Harrison Ford, admitting recently, "I'm not that smooth." 05 of 07 THE COMIC RELIEF Jeffrey Mayer/WireImage After a brilliant run as the awkward George Michael in the fan favorite Arrested Development, Michael Cera, 19, graduated to the big screen with the super hit Superbad, a raunchy teen movie antithesis to Disney's High School Musical that even had the New York Times acknowledging his "deceptively innocent eyes." Now this winter the nerdy cute boy will get serious in the teen pregnancy indie film Juno, costarring Jennifer Garner and Jason Bateman. 06 of 07 THE INGéNUE Jon Kopaloff/FilmMagic As High School Musical 2's popular girl – not to mention Zac Efron's girlfriend! – Vanessa Anne Hudgens, 18, was the envy of every teenage girl in America. Then an Internet scandal threatened to derail her status as a Disney darling – until her heartfelt apology tempered her youthful mistake, proving she's still got the good girl charm that made her a star. 07 of 07 THE CALIFORNIA GIRL Steve Mack/FilmMagic When this Laguna Beach alum packed up and moved to Hollywood with a reality camera crew in tow, Lauren Conrad's 15 minutes could've run out fast. But when the 21-year-old It girl with the enviable wardrobe and relatable social life turned The Hills into a screaming match with nemesis Heidi Montag ("You're a bad person!"), you couldn't help but pick a side and watch her every text message.