When the lonely god landed on Thor I, he never expected what he would find there. The planet was one of many failed attempts by an ancient race to transplant their entire population from one planet to another. Thor I had once been habitable, covered in lush forests and inhabited by many exotic animals. An earthquake had struck without warning, and with enough force to shift the planet off its axis. The temperature on Thor I quickly dropped and the planet went into a permanent ice age, killing everything that lived on its surface. The entire planet was a sheet of ice, its forests preserved by a glittering glaze over the tree trunks. The Doctor found the place both beautiful and calming, as had his previous self. He opened his mouth to tell Rose this as he navigated the TARDIS safely through the planets atmosphere but stopped quickly, remembering.
She's gone. She's gone and you're never going to see her again.
The Doctor swallowed. He didn't understand; so many other companions had come and gone over the years, so why did this one trouble him so much? He felt he was dying of loneliness, stranded across space and time in his little blue box, and yet he couldn't bear the thought of finding a new companion. No one could replace Rose. No one.
The TARDIS shuddered to a halt, startling the Doctor from his musings. He sighed, and put on a pair of sunglasses to protect from snow blindness. Then, he stepped outside into the tundra.
The Doctor trod the familiar path across the empty expanse of white to his favorite spot on the planet. At the center of the sparkling ice forest that took up half the planet was a tree taller and older than all the rest. With branches that stuck out at all the right angles, the tree was a breeze to climb. The Doctor scaled the tree with an ease made from years of climbing and jumping over things, coming to rest on a nook between the trunk and a thick branch that was just the right size for sitting. From his perch, he could see across nearly the entire ice forest, his view ending where the trees began to thin. Finally feeling somewhat at ease, the Doctor leaned back and closed his eyes…
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He woke suddenly. Narrowing his eyes against the glare of the sinking sun, the Doctor tried to place the feeling of unease that had shocked him from his nap. Unsettled, he carefully made his way down to the base of the tree, thinking it was time to leave Thor I behind. As he hopped down to the ground, he caught sight of something that made him loose his footing on the slick ice and fall flat on his back. The Doctor sat up, rubbing his tailbone, and stared in shock at the thing lying buried in the snow next to him.
It was a baby, lying wrapped in a blanket and cocooned in the glittering snow. It had blue eyes that bordered on electric, and a shock of white hair that made the snow look like a dirty napkin in comparison. The Doctor stared in horrified wonder at the small, defenseless thing, which seemed to be unbothered by a cold that would have killed most life forms if they weren't properly protected from it. Not only did the child appear unharmed, it stared at the Doctor with a disturbing intelligence in its vivid blue eyes. The Doctor could see no footprints surrounding the tree but his own, and he knew the baby hadn't been there when he first arrived, so he had no idea how or when it had gotten there. The only explanation was that the child was alien, but he couldn't think as to what species. There was no race that he knew of that could survive unprotected in sub-zero temperatures and had white hair and blue eyes, so he reasoned it must be a shape-changer.
The Doctor sighed and cast the baby another look. It stared back at him, with eyes that seemed to bore into his soul. He shuddered. As much as he dreaded what would happen if he took the baby with him, he knew he couldn't leave it to die. As he scooped the child up and into his arms, the Doctor thought he felt it sigh against his chest. Shaking off the chills that ran down his spine, he walked out of the ice forest of Thor I and back to the TARDIS.