Stephen Baxter - Space

But they could see, even from a distance, that this was no Earth. It was silent on all wavelengths. And it gleamed, almost as bright as a star itself; it must have cloud decks like Venus.

On a sleep break, Ben and Madeleine, clinging to each other, floated before the nearest thing they had to a picture window. Madeleine peered around, seeking constellations she might recognize, even so far from home, and she wondered if she could find Sol.

“Something’s wrong,” Ben whispered.

“There always is.”

“I’m serious.” He let his fingers trace out a line across the black sky. “What do you see?”

With the Sun eclipsed by the shadow of the FGB module, she gazed out at the subtle light. There was that bright planet, andthe dim red disc of rubble surrounding the Chaera black hole, from here just visible as more than a point source of light.

“There’s a glow around the star itself, covering the orbit of that single planet,” Ben said. “Can you see?” It was a diffuse shine, Madeleine saw, cloudy, ragged-edged. Ben continued. “That’s an oddity in itself. But—”

Then she got it. “Oh. No zodiacal light.”

The zodiacal light, in the Solar System, was a faint glow along the plane of the ecliptic. Sometimes it was visible from Earth. It was sunlight, scattered by dust that orbited the Sun in the plane of the planets. Most of the dust was in or near the asteroid belt, created by asteroid collisions. And in the modern Solar System, of course, the zodiacal light was enhanced by the glow of Gaijin colonies.

“So if there’s no zodiacal light—”

“There are no asteroids here,” Ben said.

“Nemoto. What happened to the asteroids?”

“You already know, I think,” virtual Nemoto hissed.

Ben nodded. “They were mined out. Probably long ago. This place is old, Madeleine.”

The electromagnetic petals of the flower-ship sparkled hungrily as it chewed through the rich gas pocket at the heart of the system, and the shadows cast by the Sun — now nearby, full and fat, brimming with light — turned like clock hands on the ship’s complex surface. But that diffuse gas cloud was now dense enough that it dimmed the farther stars.

Data slid silently into the FGB module.

“It’s like a fragment of a GMC — a giant molecular cloud,” Ben said. “Mostly hydrogen, some dust. It’s thick — comparatively. A hundred thousand molecules per cubic centimeter… The Sun was born out of such a cloud, Madeleine.”

“But the heat of the Sun dispersed the remnants of our cloud… didn’t it? So why hasn’t the same thing happened here?”

“Or,” virtual Nemoto said sourly, “maybe the question should be: How come the gas cloud got put back around this star? ”

They flew around the back of the Sun. Despite elaborate shielding, light seemed to fill every crevice of the FGB module. Madeleine was relieved when they started to pull away and head for the cool of the outer system, and that single mysterious planet.

It took a day to get there.

They came at the planet with the Sun behind them, so it showed a nearly full disc. It glared, brilliant white, just a solid mass of cloud from pole to pole, blinding and featureless. And it was surrounded by a pearly glow of interstellar hydrogen, like an immense, misshapen outer atmosphere.

The flower-ship’s petals opened wide, the lasers working vigorously, and it decelerated smoothly into orbit.

They could see nothing of the surface. Their instruments revealed a world that was indeed like Venus: an atmosphere of carbon dioxide, kilometers thick, scarcely any water.

There was, of course, no life of any kind.

The Chaera spun in its tank, volunteering nothing.

Ben was troubled. “There’s no reason for a Venus to form this far from the Sun. This world should be temperate. An Earth.”

“But,” Nemoto hissed, “think what this world has that Earth doesn’t share.”

“The gas cloud,” Madeleine said.

Ben nodded. “All that interstellar hydrogen. Madeleine, we’re so far from the Sun now, and the gas is so thick, that the hydrogen is neutral — not ionized by sunlight.”

“And so—”

“And so the planet down there has no defense against the gas; its magnetic field could only keep it out if it was charged. Hydrogen has been raining down from the sky, into the upper air.”

“Once there, it will mix with any oxygen present,” Nemoto said. “Hydrogen plus oxygen gives—”

“Water,” Madeleine said.

“Lots of it,” Ben told her. “It must have rained like hell, for a million years. The atmosphere was drained of oxygen, and filled up with water vapor. A greenhouse effect took off—”

“All that from a wisp of gas?”

“That wisp of gas was a planet killer,” Nemoto whispered.

“But why would anyone kill a planet?”

“It is the logic of growth,” Nemoto said. “This has all the characteristics of an old system, Meacher. Caught behind a wave of colonization — all its usable resources dug out and exploited…”

Madeleine frowned. “I don’t believe it. It would take a hell of a long time to eat up a star system.”

“How long do you think?”

“I don’t know. Millions of years, perhaps.”

Nemoto grunted. “Listen to me. The growth rate of the human population on Earth, historically, was two percent a year. Doesn’t sound like much, does it? But it’s compound interest, remember. At that rate your population doubles every thirty-five years, an increase by tenfold every century or so. Of course after the twentieth century our growth rates collapsed; we ran out of resources.”

“Ah,” Ben said. “What if we’d kept on growing?”

“How many people could Earth hold?” Nemoto whispered. “Ten, twenty billion? Meacher, the whole of the inner Solar System out to Mars could supply only enough water for maybe fifty billion people. It might have taken us a century to reach those numbers. Of course there is much more water in the asteroids and the outer system than in Earth’s oceans, perhaps enough to support ten thousand trillion human beings.”

“A huge number.”

“But not infinite — and only six tenfold jumps away from ten billion.”

“Just six or seven centuries,” Ben said.

“And then what?” Nemoto whispered. “Suppose we start colonizing, like the Gaijin. Earth is suddenly the center of a growing sphere of colonization whose volume must keep increasing at two percent a year, to keep up with the population growth. And that means that the leading edge, the colonizing wave, has to sweep on faster and faster, eating up worlds and stars and moving on to the next, because of the pressure from behind…”

Ben was doing sums in his head. “That leading edge would have to be moving at light speed within a few centuries, no more.”

“Imagine how it would be,” Nemoto said grimly, “to inhabit a world in the path of such a wave. The exploitation would be rapid, ruthless, merciless, burning up worlds and stars like the front of a forest fire, leaving only ruins and lifelessness. And then, as resources are exhausted throughout the light-speed cage, the crash comes, inevitably. Remember Venus. Remember Polynesia.”

“Polynesia?”

“The nearest analog in our own history to interstellar colonization,” Ben said. “The Polynesians spread out among their Pacific islands for over a thousand years, across three thousand kilometers. But by about A.D. 1000 their colonization wave front had reached as far as it could go, and they had inhabited every scrap of land. Isolated, each island surrounded by others already full of people, they had nowhere to go.

“On Easter Island they destroyed the native ecosystem in a few generations, let the soil erode away, cut down the forests. In the end they didn’t even have enough wood to build more canoes. Then they went to war over whatever was left. By the time the Europeans arrived the Polynesians had just about wiped themselves out.”