Well, Boom

Energy curdled back into mass as the ship translated out of light speed. After a pause for the crew to get used to experiencing time again, the ship’s instruments extended.
From the crew’s perspective, they’d finished an extensive survey of this part of space just a moment ago. For this part of space, however, 600 years had passed, so it was important to make sure nothing had changed.
Something had.
“It used to be a star like the sun,” explained the astrophysicist, whose name was Gaviria. “It had a family of planets ranging in size from a little larger than Earth to a little smaller than Neptune, all of them orbiting closer than the orbit of Jupiter.”
Marletta, the astrophysicist, spoke over Gaviria in his excitement. “So far, so similar to many other star systems. Really, it’s closer to the galaxy’s standard average than the Solar System.”
“Or it was when we translated to light,” said Gaviria.
“But?” asked Zhang.
Once a biochemist, Zhang had recently been elected to the post of “Social Coordinator,” or as he called himself, “cat herder.”
“Its planets have shrunk,” said Gaviria.
“Huh.” Zhang wondered why he was having this conversation. Aha. There it was. “You want me to authorize an away team.”
In free fall as he was, Marletta could not jump up and down with excitement. The best he could do was anchor himself to a hand-rail and vibrate in place. “It’s close. It’s super close!”
Zhang looked at Gaviria, who said, “Point nine eight light years.”
“Would a two-year-ship-time trip fit our flight plan?” Zhang’s question was directed at the ship’s computer, which cleared the mission. Soon, the three of them were packed and in their landing pod, which the ship translated into light. The team was away.
***
After either a year or no time at all, the landing pod re-materialized above a planet. Zhang, Marletta, and Gaviria watched the hazy, blue-white ball flicker in their portholes. As their pod translated itself into a safe landing trajectory, the planet vanished and reappeared, changing position and orientation. It grew closer.
Now, the planet filled all the portholes on one side of the pod. The diamond light of its home star limned its upper edge. Now, that light was tinged red by atmosphere, and the edge had become the horizon. The horizon developed mountains. Finally, the pod settled, the planet became the ground, and Gaviria, Marletta, and Zhang walked out onto it.
Zhang hopped experimentally, feeling his suit flex under the extra gee. He couldn’t smell anything except his own canned air, but his mics picked up the sound of running water, wind over rocks, and a distant bass pulse that might be surf.
They’d touched down on a hill overlooking a floodplain, where a river flowed into an ocean. The sun rose above the mountains on the other side of the plain, casting pink-yellow light onto clouds, folded rocks, and the forest growing out of the river.
The plants, if they were plants, had no leaves, branches or trunks. The green, blunt-nosed cones simply sat there, their roots – if they had roots – invisible under the water.
They might still prove to be some strange kind of geology, but Zhang allowed himself to take a leap of faith, and sighed, “Life.”
Part of the reason Zhang had accepted the post of cat herder was that there wasn’t usually much call for a biochemist. It wasn’t the first extraterrestrial biosphere that the Von Neumann Fleet had discovered, but it was a first for his individual ship.
“Samples samples aha,” Gaviria hummed to herself. She headed for a cone-plant growing in a nearby stream.
Marletta looked out over the blue ocean and green floodplain. “Life how? Six hundred years ago, this place was a sub-Neptune.”
“It’s much closer to its star than our Neptune,” said Zhang. “Right?”
Marletta flapped his hands. “And with a denser core. And not as close as Earth…really, it was intermediate between Earth and Neptune.”
“Which is unusual?”
“Well, yes. Usually you either have a terrestrial planet like Earth: a secondary atmosphere out-gassed from the rock.” Marletta held his hands apart, as if measuring a grapefruit. “Or,” He spread his hands out to the diameter of a beach ball.  “Or, you get a gas giant like Neptune, with an envelope of hydrogen and helium gathered out of the primordial matter that built its star. Those light gasses spread out farther, so the planet looks bigger from space.” He moved his palms inward, the volume between them now the size of basketball. “But then over time, heat from the planet and the star would have blown that primordial atmosphere away. That’s why we assumed that the intermediate diameters, like the one-point-five Earth diameter this planet used to have, were so rare.”
“Rarer now!” said Gaviria. She chipped away at the green cone-plant with her multi-tool. The surface of the organism was as hard as the heat shielding of their landing pod. “Since we left Earth, every planet in this system has shrunk down.”
Zhang watched Gaviria work. “And in only six hundred years,” he mused. “I’m assuming that’s much faster than any stellar or geological process could account for.”
“Don’t jump to conclusions,” said Gaviria.
But Zhang didn’t have to line up his evidence for a review board. He was a cat herder now, and he could jump to whatever he wanted. “Marletta, did the observations we made from Earth indicate oxygen in this planet’s atmosphere?”
“Well, we don’t know,” said Marletta. “We never recorded any absorption spectra through the original atmosphere. All we have are the transit and radial velocity data that told us the size and mass of this system’s planets. But…” His brain caught up with his mouth.
On Earth, Marletta would have spun around to face Gaviria and her cone-plant. In two gees, he wobbled like a penguin, but eventually got himself turned in the right direction. “You mean photosynthesis?”
“How much longer?” Zhang asked Gaviria. His logic was leading him into uncomfortable places.
Gaviria gave her cone-plant another whack with her multitool, which didn’t even scratch the surface. “Just let me get my sample. It’s not going to go off right now, just because we’re talking about it.”
“‘Go off,'” Zhang repeated.
Marletta thought out loud. “Water and carbon dioxide go in, oxygen and carbohydrates come out. O2 gas rises to mix with the H2, and now every time there’s a bolt of lightning or other spark…”
“Well,” said Gaviria, “boom.”
Marletta swore in English, and Zhang made a decision.
“We need to go.”
“Stop being paranoid, this all happened hundreds of years ago.” Gaviria scratched at the cone again.
Marletta stopped, his colleague’s assumptions overriding his sense of self-preservation. “Wait. That’s still not fast enough. The entire atmosphere couldn’t have, uh, explosively oxidized in only six hundred years.”
Zhang answered the implied question. “There must have been biological processes actively speeding things up. Sequestrating the hydrogen? Controlling the rate of reaction?” He shook his head, remembering he was supposed to be herding these cats. “Let’s go, Gaviria. We can print out better tools on the pod.” After a light-speed jump into deep space, he added silently.
“I suppose so,” Gaviria reluctantly put away her multitool and hoisted herself out of the stream.
Marletta couldn’t snap his fingers in his suit, but he tried. “And it didn’t just happen on this planet, did it? Every planet in the system lost its hydrogen atmosphere within the same six-hundred-year window!”
“Panspermia!” crowed Gaviria from the bank.
Zhang groaned because he had always hated the idea of panspermia. Also, because steam was rising from the cone-plant behind his geologist.
***
It was a good thing, they decided later, that Gaviria hadn’t been able to crack the ablative shielding on the cone-plant. If she had, it could well have exploded.
As it was, though, the plant only launched.
Back in the safety of their pod, the three humans watched as the green, ceramic-shelled organism lifted into the sky on its pillar of fire, and began its mission to spread life to other stars.




This story was inspired by William Misener’s “To Cool is to Keep: Residual H/He Atmospheres of Super-Earths and sub-Neptunes.” Thanks go out to him and everyone at the 2020 NASA Exoplanet Science Institutes Exoplanet Demographics conference.
This story was published in “Heavy Metal Jupiters,” the zine of the Exoplanet Demographics 2020 conference.