Complex sentence: The second planet of the system covered with a single vast ocean and amazingly suitable for human life, was named Poseidon, and one major colony ship was sent there before civilization collapsed on Earth, and the colonists were given up for as lost.
Split into simpler sentences: The second planet of the system was covered with a single vast ocean and was suitable for human life, and was named Poseidon. One major colony ship was sent there before civilization collapsed on Earth, and the colonists were given up for as lost.