Illegal Alien

"So mass curves space, of course, and at each near passage of C, that curvature becomes sufficient that my home world slips from orbiting around A to orbiting around B, or vice versa. We call this transfer ‘the handoff.’ Of course, there is a period of instability during the handoff. Still, when orbiting A, we ultimately settle in at a comfortably warm two AUs or so from that star. But when orbiting B, although we also settle into an orbit not much greater than two AUs from B, things are much colder, for B is a much fainter star. Our climate is temperate when we orbit A, but when we orbit B, our surface temperature drops to" — he paused, and worked controls on his pocket computer — "almost fifty degrees below zero Celsius."

"My God!" said Frank. "That’s below the freezing point for carbon dioxide. It must kill everything."

"It does not. Those lifeforms that have persisted on our world have developed a natural ability to hibernate during these times. Everything simply pauses for four hundred thousand years, until Proxima swings by again, causing a reverse handoff, bringing us back into orbit around Alpha Centauri A. Temperatures rise, the long sleep ends, and we continue on."

"That’s incredible," said Dale. "I mean, wouldn’t it require enormous luck for a world to exist in a stable configuration like that?"

"Incredible? No. Unlikely perhaps, but, then again, no more unlikely than the coincidences of sizes and orbits that makes possible the kind of perfect total solar eclipse we observed from Earth’s surface. Of all the inhabited planets in the entire universe, Earth may be the only one that enjoys such a spectacle."

"I suppose," said Dale, "but—"

"And, of course, there usually are such harmonics to orbital mechanics. Orbits of bodies around each other are often in perfect ratios: two-to-three, one-to-two, and so on. Your innermost planet, Mercury, for instance, revolves round your sun exactly two times for every three times that Mercury rotates around its own axis; its day is precisely two-thirds the length of its year." Hask’s tuft split in the center. "No, our configuration may be as unique as the perfectly eclipsing combination of your Earth, your giant moon, and your sun, but it is possible, and although it may not always have been this way, nor may it always be this way, it has indeed been stable for millions of years."

"Doesn’t the handoff cause a lot of earthquakes?" asked Dale.

"The Tosok home world has no moon," said Hask. "It probably did once, or else our world would have an even greater greenhouse effect than it does now; Clete told me that without the moon, Earth’s atmosphere would resemble Venus’s — a thick blanket covering the world. Our old moon’s gravity doubtless skimmed off some of our original atmosphere, but that moon must have been lost during one of the handoffs. Anyway, without your own moon churning up Earth’s interior through its tidal effects, you would not have any plate tectonics; Earth is, after all, unique among the planets in your solar system in having such things. Without crustal plates, you do not get earthquakes; they are unknown on our world. Except for the impact on our climate, the handoffs proceed quite smoothly."

"But if handoffs occur only every four hundred thousand years," said Frank, "your race can’t have known one yet. I mean, you guys are advanced beyond us, but by hundreds of years, not by hundreds of centuries."

"True," said Hask. He paused. "Our fossil record is scantier in many ways than is yours. But since we have no plate tectonics, no portion of our crust is ever subducted and destroyed through the passage of time; although there are fewer fossils, our record is contiguous in a way that yours is not.

We were astounded when our geologists found proof in core samples of the huge periodic temperature changes our world had undergone for at least many tens of millions of your years. But although the fossil record shows that a few species die out at each great freezing, most simply continue on immediately after the thaw. Life on our world has evolved to deal with the handoffs — or, more precisely, only those lifeforms that had an ability to survive freezing survived the first handoff, and all current lifeforms are descended from them." He paused. "Perhaps it is not coincidental that our hearts are primitive compared to yours; I understand many earthly fish and amphibians — creatures with similar hearts — can also survive periods of freezing."

"That’s why there was no hibernation equipment aboard your ship," said Dale.

"Exactly. Simple cold temperatures are enough to induce hibernation. The two centuries of sleep required for the journey from our planet to here were insignificant; we could as easily have gone many hundreds of thousands of years."

"All of that is fine," said Frank, "but then what did you mean when you said your starflight was about survival?"