Comet Medal

Comet Medals are a type of item that appears in Super Mario Galaxy 2 & Super Mario. They are white octagonal coins with a golden lining and a picture of a golden comet imprinted on them. Collecting enough Comet Medals will make Prankster Comets appear, like the Prankster Comets in the first game. They are found in the first mission of every galaxy, usually in hard to reach places or requiring the use of a power-up. Once collected, if the player re-enters the level, the Medal will appear to be a clear blue silhouette of the original. It can still be collected, however. It will temporarily be added to the total, but as soon as the level is completed, the total will revert back to its previous value.

There are 49 Comet Medals total, but not every galaxy will encounter a Prankster Comet.

Super Mario (Nintendo 3DS)
Comet Medals will appear in the upcoming game, Super Mario for the Nintendo 3DS. Their logo now just has a star on it instead of a star with a tail. It is unknown what purpose they will be used for.

Trivia

 * When collected, the Comet Medal makes a similar sound effect to that of a Red Coin from Super Mario Sunshine.
 * They were originally thought to be known as Comet Metals due to a mistranslation.
 * Comet Medals have a shape that is almost identical to the shape of the Bowser Space from Mario Party 6.