She walked on into what might once have been a room, a space bounded by six broken walls. Though there were many rooms close by here - clustered like a honeycomb, closer than would have been comfortable for people - it was hard to believe the inhabitants of this place had been so different from humans.

She wondered what it had been like here, before.

Once, Snowball had been Earth-like. There had been continents, oceans of water, and life - based on an organic chemistry of carbon, oxygen and water, like Earth life, and it had worked to create an atmosphere of oxygen and nitrogen, not so dissimilar to Earth’s. And there had been people here: people who had built cities, and breathed air, and perhaps gazed at the stars.

But the long afternoon of this world had been disturbed.

Its sun had suffered a chance close encounter with another star. It was an unlucky, unlikely event, Minda knew; away from the Galaxy’s centre the stars were thinly scattered. As the interloper fell through the orderly heart of this world’s home system, there must have been immense tides, ocean waves that ground cities to dust, and earthquakes, a flexing of the rocky crust itself.

And then, at the intruder’s closest approach, Snowball was slingshot out of the heart of its system.

The home sun had receded steadily. Ice spread from polar caps across the land and the oceans, until much of the planet was clad in a thick layer of hardening water ice. At last the very air began to rain out of the sky, liquid oxygen and nitrogen running down the frozen river valleys to pool atop the vast ice sheets, forming a softer snow metres thick.

She wondered what had become of the people. Had they retreated underground into caves? Had they fled their planet altogether - perhaps even migrated to new worlds surrounding the wrecking star?

‘This world itself is not without inner heat,’ the ghost said softly. ‘The deep heart of a planet this size would scarcely notice the loss of its sun.’

‘The volcano,’ Minda said dully.

‘Yes. That is one manifestation. And vents of hot material on the spreading seabed have even kept the lower levels of the ocean unfrozen. We believe there may still be active life forms there feeding on the planet’s geothermal heat. But they must have learned to survive without oxygen . . .’

‘Do you have that on your world? Deep heat, water under the ice?’

‘Yes. But my world is small and cold; long ago it lost much of its inner heat.’

‘The world I come from is bigger than this frozen ruin,’ she said, spreading her arms wide. ‘It has lots of heat. And it is a double world. It has a Moon. I bet even the Moon is bigger than your world.’

‘Perhaps it is,’ the ghost said. ‘It must be a wonderful place.’

‘Yes, it is. Better than your world. Better than this.’