When Worlds Collide

I’ve been thinking recently again about how I’m going to take my space-opera setting into the extremely far future, from millions of years from now to the end of the universe. In particular: the blaze of glory I have in mind for the end of Earth as we know it. Being cooked into another Venus and later swallowed up by the red-giant sun isn’t the only possibility: over the next 3.3 billion years there’s a 1% chance Jupiter’s gravity will perturb Mercury’s orbit into being so eccentric it crosses Venus’s orbit. If this happens it’ll send the inner solar system into chaos. Mercury could collide with Venus, Mars, Earth, or the Moon, fling any of these objects out of the solar system, be captured itself as a new moon of one of these planets, or a combination of all these possibilities!

The far Future of Life on Earth

1.2 billion years is the high estimate until all plant life dies out, assuming photosynthesis is still possible despite extremely low carbon-dioxide levels; I personally think plants will adapt just fine, though we’ll see more plants turn carnivorous over time. Rising temperatures from 1.1 billion years onward make Earth’s oceans evaporate, beginning the transformation into a Venus-like world…unless the nitrogen cycle lowers the atmospheric pressure and thus the greenhouse effect, in which case life might hang on until 2.8 billion years in the future. I’d add life might last for far longer in the cloudtops, as is speculated to be the case now on Venus.

I suspect atmospheric pressure will decrease over time and Earth will begin to grade more Mars-like, but with much hotter temperatures, the net effect by 1.2 billion years hence being a hot desert of a world dominated by carnivorous plants and pterodactyl-like flying animals. The nitrogen cycle factor makes it possible this state will continue for a billion or two years more even than that, just with the planet becoming hotter, drier, and more rarefied, but I think something like half a billion years of this trend is more than enough before it gets boring.

So I think I’ll go with 1.2 billion years in the future for the cataclysm with Mercury; it’s far enough out to be plausible scientifically, but not so far out that the geological worldbuilding gets same-y. Plus it would only take a minor contribution from the nitrogen cycle to keep Earth habitable until this stage, as opposed to pushing the biosphere’s luck for another billion years. It also increases the odds that plants and animals will still be around to witness the end of the world.

Mercury collides with Earth…and perhaps other Worlds too!

Which will come about by Mercury colliding with Earth. How exactly this will play out I’m not sure. Mercury could straightforwardly be flung into an Earth-crossing path and just smash into it, but another possibility is Mercury slamming into Venus and then the resulting amalgam being flung into the Earth, forming a new planet of super-Earth mass. The Moon’s fate is uncertain, but in my universe I think I’ll keep the Moon in Earth’s orbit. Alternatively, it’s even possible the new Mercury-Venus amalgam slams into the Moon and makes that into the new primary planet of the system, leaving the Earth as a dying moon of the new abode of life.

An even wilder possibility would be the new Mercury-Venus amalgam slamming into the Moon and then slamming into the Earth, completely destroying all four of those planets and reconfiguring their mass into a totally new form, perhaps one with a new super-Earth planet with multiple stable planetary-mass moons. Or a binary planet, even to the point of a configuration like Rocheworld, with multiple planetary-mass moons. That would be spectacular, and illustrate the difference between merely billions of years in the future, when the planet is binary, and the much further future, when they’ve all spiraled into each other and only a singleton planet is left.

Sure, it’s all kinda unlikely, but it’s a scenario that could plausibly happen. All this energy and momentum could for all I know transform the remnants of the four planets into one huge synestia, a doughnut-shaped ring of vaporized rock that over the course of about a century (a blink of an eye in geologic time) starts to condense to form new planets.

Rogue Planet?

Needless to say no form of life left on or under the surface of any of the four planets would survive such collisions, but life flung out in the ejecta of the impacts could survive inside the resulting meteoroids for an extended period, easily enough time to orbit for a while and then come back down by the time water has rained out of the atmosphere to reform the oceans, respawning the biosphere. There’s a shadow of a chance that spores, seeds, or even eggs of more complex life-forms might survive for a long time in space, but I’d imagine the resulting atmosphere would be mostly carbon dioxide like the early Earth, so aerobic life like plants and animals would die out, leaving only the anaerobic bacteria. It would be like the early Earth all over again.

Except for the Sun being much hotter than it was originally, perhaps precluding the formation of liquid water oceans. But within the next 3 billion years there is a 1 in 100,000 chance a stellar encounter, i.e. another star making a close pass of our solar system, will eject the Earth out of its orbit and into interstellar space, becoming a rogue planet. The course of my story requires it to happen at some point, and I’m thinking the best time would be shortly after the Mercury-Venus-Moon-Earth collision, enabling the oceans to rain out free of interference from hot suns. Life could continue on a rogue planet for a long time.

A new Orbit?

Alternatively, perhaps Earth is simply flung to a much further orbit, and possibly a very eccentric one; I’m thinking it could remain stable for an extended period geologically, until some perturbation or another ejects it from the solar system at a much later date, perhaps around the same time Triton spirals into Neptune and makes a new set of bright icy rings, 3.6 billion years in the future. As weird as it sounds, maybe the new planet and its multiple moons, complete with an incipient biosphere of complex life similar to our Cambrian period, makes a close passage of Neptune on its way out.

Again, this is a bit unlikely, but drama is built on the unlikely. One might think an eccentric super-Earth crossing the orbits of the gas giants would be unstable, but setups like trojan planets and horseshoe orbits between an Earth-sized and Jupiter-sized planet are stable over geological time, so a comet-like orbit where the super-Earth is highly inclined with respect to the other major planets in its passage through the outer solar system should be stable for a couple billion years. Alternatively, it might be possible for new-Earth to eventually be captured into a trojan orbit with Jupiter, always following 60 degrees ahead or behind the much-more-massive planet. That might be even cooler.

In any case, in the year 3.6 billion or so a passing stellar encounter flings the new world out of the solar system and kisses Neptune goodbye, spending eons in the inky black, colliding and reheating god-knows-who-many-time with rogue planets, black dwarfs, brown dwarfs and the like, until the time comes for the Lastborn of Mother Earth to arise under the last light in the universe: a supermassive black hole turned quasar as it swallows up the remnants of long-dead galaxies.

Cool, heady stuff as I flesh out the timeline for my most sublime story of deep time in my space-opera universe.

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