A Warning
The novel could be seen as an allegorical warning to show what could happen if the humanity doesn't work on improving life rather than destroying each other. It portrays a possibility of the planet becoming inhabitable due to human carelessness and greed, how easily it can be brought to destruction. The novel shows the inability of humanity to learn from mistakes to improve which only brings forth repetition of unfortunate past. It raises a question of how many opportunities for betterment the humanity can waste before it's too late. This repeating cycle is shown in a story from the voyage to Planet Blue, the story of the boy who kept being given opportunities by an angel to re-direct his life, and he kept wasting every single one. "And his angel came, and sat by him, and took pity on him once again, and interceded for him, and..."
The Bird
From the part Easter Island, the bird represents a symbol of authority, of a greater power, because the bird is able to leave the island, rise above the Natives and see everything that they can't. It shows how people make unique symbolic connections with things that are related to their own experience. The natives are unable to leave the island because they don't have the resources left for that, so the object of their symbolic admiration is a thing that can. To Billy, who is an outsider, this is not a connection valid to his own experience, but he can see its meaning.
The white planet
"The white planet was a world like ours" is what Handsome on the way to the Blue Planet says when talking about a mirage of a planet that is left after its destruction. He talks about the possibility that there were worlds before and that their world is a product of a second opportunity given to the world of the white planet, just like they are on their way to be given a second opportunity again on the Planet Blue, or Earth, just like the allegory of a boy and his angel.
