The shooting stars are neither a sun nor a particle. There is actually something in the editor that lets you create an image and have it rotate around in the sky box. I just can't remember the name of it at the moment.
Sorry about being vague. Packer is partially correct, it is a sun. Its an object called fxsunlight. Using various animation inputs and custom media, you could easily create a shooting star.
Here is an example (done in Vanilla, but it should still apply):