Yaris Serpentine belt snapped

[jcuk]

Hi Guys,

I've recently attempted to jump start another car and while idleing I gave the Yaris a rev to add some extra power to the jump.

However, when i revved it I heard a crack and a piece of something came flying out of the engine.

Car was still running okay but battery light came on, so timing belt is still fine.

However upon inspection, some sort of pulley wheel had completely fallen to pieces underneath the car and the drive belt wasn't attached to the alternator.

I'm not sure what it was that broke off however I'm wondering if anyone else can help? It's like a hard plastic wheel which was ribbed around the edge from what I can make out.

Could anyone suggest what this part is and is it easy to replace?

The car is a 2007 Toyota Yaris 1.8SR and currently has around 37,000 miles on the lock.

Many thanks,
Jamie