This one looks too big they think, 450km across. It was one like the other. They showered the Earth over a period of about two days bashing every place until the air was unbreathable for non aquatic or burrowing animals. Most birds died & some flew high. Leaves helped to keep dust low. The same is true of fallout.
Far left.
Answer: Pluto will continue to move nearer to Neptune by it's occasional proximity & eventually find an orbit with the planet. Neptune will move nearer the sun by destabilisation & have aquatic life eventually. The dissimilar speeds & proximity are altered by interaction until a smooth pairing.