They had been disputing whether any nearby stellar systems might have Earthlike planets, and the elderly but still athletic Dane kept to his pragmatic, hard-nosed reputation. “If you have the means to experiment, do it! If not, then wait till experiments are possible. Theory by itself is only masturbation.”
The small club erupted in laughter. Still, Wyn was no spoilsport. And as everyone else seemed to want to speculate, he merely grumbled good-naturedly and went along.
“We’ll see about that Han interferometer,” said a woman geologist named Gorshkov, whom Stan had met off and on at conferences for decades. “The Chinese have been talking about it forever. Why can’t we answer the question with facilities in orbit right now?”
Stan shrugged. “The Euro-Russian and American telescopes are quite old by now, Elena. Yes, they’ve detected planets around nearby stars, but only giants like Jupiter and Saturn. Little rocky worlds like Earth are harder to find… like picking out the reflected glint of a needle next to a burning haystack, I should think.”
“But don’t most astrophysical models predict that sun-like stars will have planets?”
This time it was a younger Dane, Teresa’s husky friend Lars. The fellow might look like an overbuilt mechanic or an American football hero, but he obviously read a lot.
“Yes and no,” Stan replied. “G-type stars like our sun must shed angular momentum in their infancy, and since ours gave nearly all its spin momentum to her retinue of planets, most astronomers think other stars that rotate like the sun must have planets too.
“Furthermore, astronomers think early protostars give off fierce particle winds, which drive away volatile elements. That’s why there’s so much hydrogen in the outer solar system, while Mercury and Venus, sitting close in, have been stripped of theirs.”
“But Earth came out just right,” Wyn nodded. “In the middle of a zone where water can stay liquid, right?”
“The Goldilocks effect.” Stan nodded. “Life could never have started, or kept going for long, without lots of water.
“But as for Earth being ‘in the middle’ of the solar system’s life zone, well, astronomers have argued over that for more than a century. Some used to think that if our world was only five percent closer to the sun we would have fallen into the Venus trap… heat death by runaway greenhouse warming. And if we’d been just five percent farther, Earth’s seas would have frozen forever.”
“So? What’s the modern estimate?”
“Currently? The best models show our sun’s life zone is probably very broad indeed, stretching from just under one astronomical unit all the way out past three or more.”
Someone whistled. Elena Gorshkov closed her eyes momentarily. “Wait a minute. That extends past Mars! So why isn’t Mars a living world?”
“Good question. There’s evidence Mars once did have liquid water, carving great canyons we have yet to visit, alas.” To that there was a general murmur of agreement. Several raised their glasses to opportunities lost. “Perhaps there were even seas there for a while, where early life-forms made a brave start before all the water froze into the sands. The problem with old man Mars wasn’t that he spun too far from the sun. The real difficulty was that the Romans named their war god after a pygmy. A midget world, too small to hold onto the necessary greenhouse gases. Too small to keep those famous shield volcanoes smoking. Too small, by half, for life.”
“Hmm,” Lars commented. “Too bad for Mars. But if G stars have broad life zones, there ought to be many other worlds out there where conditions were right… with oceans where lightning could begin the first steps. Evolution would have worked in those places, too. So where — ?”
“So where the dickens is everybody!” Wyn Nielsen interjected, slapping the table.
So we return to the age-old question, Stan thought. Enrico Fermi had also asked it a hundred years ago. Where is everybody, indeed?
In a galaxy of half a trillion stars, there ought to be many, many worlds like Earth. Surely some must have developed life, even civilization, long ago.
On paper at least, star travel seems possible. So why, during all the time Earth was “prime real estate,” with no indigenous owners higher than bacteria or fish, was it never colonized by some earlier spacefaring race?
The amount of verbiage that had been spent on the subject — even excluding flying-saucer drivel — only expanded after the establishment of the World Data Net. And still there was no satisfactory answer.
“There are lots of theories why Earth was never settled by outsiders,” he replied. “Some have to do with natural calamities, like you lot are investigating here. After all, if giant meteorites wiped out the dinosaurs, similar catastrophes may have trounced other would-be space travelers. We ourselves may be wrecked by some stray encounter before we reach a level sufficient to—”
Stan’s voice caught suddenly. It was as if he’d been struck between the eyes, twice.
For a blessed time he had managed to banish all thought of the taniwha. So the sudden contextual reminder came like a blow. But the thing that really had him stopped in his tracks was a new thought, one that had swarmed into consciousness following the words — We ourselves may be wrecked by some encounter…
He coughed to cover his discomfiture, and someone slapped him on the back. While he took a drink of warm beer, waving concerned helpers away, he thought, Could our monster have come from outside? Could it not be man-made?
He didn’t need to make a mental note to look into the idea later. This was one that would stick with him. If only I’d been able to break free and go to the meeting in Waitomo! Somehow, he must find a way to transmit this thought to Alex!
But now was not the time to lose his train of thought. There were appearances to maintain. Where was I… ? Oh yes.
Clearing his throat, he resumed.
“My… own favorite explanation for the absence of extraterrestrials — or their apparent absence anyway — has to do with the very thing we were talking about before, the life zones around G stars like our sun. Astronomers now envision a very broad zone outward from our position, where a Gaia-type homeostasis could be set up by life. The farther out you go the less sunlight you have, of course. But then, according to the Wolling model, more carbon would remain in the atmosphere to keep a heat balance. Voila.
“But note, there’s very little habitable zone left inward from our orbit. Earth revolves very close to the sun for a water planet. In our case, life had to purge nearly every bit of carbon from the atmosphere to let enough heat escape as the sun’s temperature rose. And in a couple of hundred million years even that won’t suffice. As old Sol gets hotter, the inner boundary will cross our orbit and we’ll be cooked, slowly, but quite literally.
“In other words, we only have a hundred million years or so to come up with a plan.”
They laughed, a little nervously.
“So what’s your theory?” Nielsen asked.
Stan was wondering how to get the center of attention away from himself, so he could find an excuse to sneak away. But he’d have to do it smoothly, naturally. He spread his hands. “It’s really simple. You see, I think Earth must be relatively hot and dry, as water worlds go. Oh, it may not seem that way, with seventy percent of the surface covered by ocean. But that just means that normal life-zone planets must be even wetter!
“One consequence would be less continental land area to weather under rain.”
“Ah, I see,” a Turkish geochemist said. “Less weathering means less fertilizer to feed life in those seas. Which in turn means slower evolution?”