The 12th Planet lingers nearby, during its turnaround, and makes a second pass a few years later. During this period the core of the Earth is not settled. Overall, this activity has a slight warming influence, a few degrees at most depending on location. A dense cloud cover that lasts for decades, resulting from the volcanic activity and loss of atmosphere due to the stripping away that occurred during lashing by the comet's tail. The dust filled clouds are low to the ground, and create a constant gloom.
Rain occurs almost continuously.
Where sunlight cannot penetrate and seldom manages to peek through this dense cloud cover, it does warm the Earth's atmosphere and thus its warming influence is not lost on the Earth. Less warmth from sunlight, but warm wet air.