Katy Perry's 'E.T.' And More Surprising 'American Idol' Performances

By Alex Chapman

Ever since the birth of "American Idol" (remember Brian Dunkleman??), the singing competition has become the premier pit stop for already-established singers to get their "sanger" on. Whether it's Miley, Gwen or Gaga, "AI" guests always put their best vocal foot forward — after all, it's not just some random awards show; it's a show about singing, and they need to prove their pipes worthy of the illustrious "Idol" stage. Or something like that.

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That's why "Idol" performances are oftentimes especially special: They make the shiniest of stars work for their spot just as hard as the contestants, and nothing makes me happier than seeing super-rich/successful people actually having to try (evil laugh). All that being said, we've made a little list of some of the most memorable, sometimes surprising guest gigs — may there be many more!

1. Miley Cyrus

I can't say I wasn't a little skeptical when Miley came on the show not only to sing, but to mentor the contestants for the week's performances. But the super-young singer proved herself a few times over. Not only did she actually give really good advice, she gave a jaw-dropping performance of her ballad "When I Look at You" that gave her a leg up in legitimacy and the respect of "AI" viewers everywhere.

2. Gwen Stefani

Although Gwenny's long been known for her impressively imaginative artistry and distinct sound, I'd never thought of her as a particularly exceptional singer. But boy oh boy, did Ms. Stefani turn on the heat when she performed "4 in the Morning" on "Idol" via satellite. Not only did the singer/designer/superwoman's pitch-perfect vocals breathe new life into the already good track, but her showmanship proved to Stefani skeptics everywhere that the singer is far more than a pretty face; she's also a pretty voice!

3. Rihanna

Rihanna's always been a huge favorite of mine, but let's face it: She's proven to be less than perfect in her past TV gigs. But RiRi turned over a new leaf with her recent slot on "Idol" singing her beautiful ballad "California King Bed." The simple, heartfelt performance showed the singer's surprisingly wide vocal range, as well as her ability to straight-up own a song, something that sets her apart from a bulk of her contemporaries as a reigning princess of pop.

4. Justin Bieber

It may seem like the ubiquitous Biebs has been running the music game for a while now, but the young superstar has only really been around for a year or so. And closer to the beginning of Bieber fever, JB had a chance to sport his stuff on "Idol." Sure, he could've just performed "Baby" to the tune of a gazillion screaming girls, but instead, Biebs decided to prove that he could also carry a more traditional tune, performing the soulful "U Smile." The result? Justin not only won the hearts of his adoring fans, he also proved himself to be a musical force to be reckoned with, regardless of his age.

5. Katy Perry

While Katy Perry is a freakin' incredible singer/songwriter, her hit "E.T." is a difficult one to perform. The verses don't have many modulations, and they are lower in register, which can oftentimes cause songs like it to wash over audiences until bigger moments (in this case, the badass chorus). But being the true pro she is, Katy's vocals were on-point to the fullest when she performed the track on "Idol," never falling flat or losing attitude. Nothing like a gig on "Idol" to remind people why you're one of the most popular artists on the planet!

Which artist had your favorite "Idol" performance? Let us know in the comments!