What is the purpose of the planets and their type?

Well the manual has an example in it about Gaia the Dragon Champion against Blue Eyes White Dragon and normally Gaia the Dragon Champion would lose out against Blue Eyse White Dragon (2600 vs. 3000), but due to their signs Gaia could actually defeat Blue Eyes White Dragon because of the system (similar to that of Rock, paper, scissors).
In addition, it changes what attack your monster will do in the 3D animation of your monster's attack. (Which is done by pressing square on the target)
For instance with Dark Magician, if I were to have his sign at Mercury, he would move up on the opponent monster and use a massive light attack. With Moon he would do a form of electrical discharge.

To give advantages over monsters, plus their types gives them another advantage if Umi, Mountain, Wasteland, Forest, or Sogen is played on the field.

Guardian stars power up certain monsters.

Mercury beats sun
Sun beats moon
Moon beats venus
Venus beats mercury

Mars beats jupiter
Jupiter beats saturn
Saturn beats uranus
Uranus beats pluto
Pluto beats neptune
Neptune beats mars.


Mars are usually fire monsters
Jupiter are usually forest monsters
Saturn are usually winged beast and some dragons
Uranus are usually wasteland monsters
Pluto are ALWAYS thunder monsters
Neptune are usually water monsters



Mercury are usually spellcasters,
Sun are some dragons and light monsters
Moon are usually dark monsters
And venus are everything else