The Solar System quest is one of the Voice of the Night Sky sub-quests. This quest concerns finding all of the
coins engraved with the planets in our solar system. These coins are Mercury, Venus, Terra, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and Eris. Including the Solar and Lunar coins needed to complete this collection, you've got a total of twelve coins that you need to find in order to complete this quest.
In your notebook, the quest details, before completion, read as follows:
"It looks like you can collect coins for the planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the dwarf planets, Pluto and Eris. Add the sun and the moon to complete the set."
After finding all of the coins and looking at them in your collection book, the strange voice will come over the radio and say:
"...Exodus speaks...when the guardian of the old castle...dances...by the window, you shall...be granted permission...by the heavens..."
In your notebook, the quest has been updated to note this:
"The mysterious voice mentioned "the window" of an "old castle". Try diving with GG."
When you dive with GG at Valka Castle, head to the Underwater Gallery, the area in the Castle with the most windows. When you do, a cutscene will play, and GG will notice something massive and buglike darting at high speeds by the windows. GG said that it looked like an Anomalocaris, a prehistoric and extinct creature, and attempts to rationalize what he's seeing before concluding that he's just imagining things.
You will also find the Dragon Bones BCD, a unique device. Back on Nineball Island, the notebook will update with nothing more than a cheery "You found a rare item!", mentioning nothing about the Anomalocaris.