White ravens are versions of the Raven that were hidden deep inside Atherton.
When Samuel and Isabel were in the caves beneath Atherton with Gossamer, they saw a massive white wall covered in spikes. This wall was really the White Ravens being stored, waiting to be activated. When Edgar flew back from Earth with a powder block in the Raven, he ended up activating the white Ravens, activating "Winter". When he returned, Gossamer made sure everyone was out of the Raven, and blew fire onto the wall while the raven flew back and forth, firing all its spikes into the white crystal wall. Eventually the wall broke, and thousands of White Ravens flew out. These then flew to earth (the dark Planet), causing all pollution to be destroyed.