After Worlds Collide

Since I wrote up my post “When Worlds Collide” I’ve been doing some more worldbuilding for the extremely far future of my space-opera universe, the ultimate fate of the Earth. 1.2 billion years from now, when Earth is a desert-like hothouse of a planet populated mostly by giant Venus-flytrap-like plants and pterodactyl-like flying animals, Mercury’s orbit is made elliptical enough by Jupiter to cross Venus’s orbit, throwing the inner solar system into chaos.

Mercury slams into Venus, the combined amalgam then colliding with the Moon — in my end-of-the-universe story there will be a vignette about Earth’s last creatures witnessing the lunar collision — followed by the three-in-one molten sphere of a planet colliding with Earth. At first the combined Mercury-Venus-Moon planet, totally molten due to all the energy released, makes a very close pass of Earth, a near-miss where parts of the Earth’s surface are lifted up into zero-g — most dramatic — before the other planet comes back around and slams into Earth properly, annihilating all complex life in a blaze of glory, the resulting collision forming a synestia, a doughnut-shaped rapidly-rotating cloud of vaporized rock.

Two Planets, seven Moons

After the synestia cools it condenses into a binary planet like Rocheworld, close enough for their surfaces to be deformed (so the planets become egg-shaped with lobes reaching toward each other) and their atmospheres to be shared, a planetary version of a “contact binary”; believe it or not the mathematics work out in real life, so this is a plausible sort of planet, even if it’s likely very rare in the actual universe.

The drama doesn’t end there; debris from the collision condenses into not just one, not just two, but seven moons of planetary mass. The total is nine planets, all orbiting each other closely; the binary pair at the center are obviously the closest of all, but even the other seven planets hew close.

There’s enough mass in this system for them to all be large enough to have substantial atmospheres; the four planets that went into their formation had 1.83 Earth masses, no doubt reduced a bit by ejecta from the collisions and the like. Given nine planets, the binary pair could have 0.5 Earth masses each and each of the seven moons could have 0.1 Earth masses.

These nine worlds hearken back to the nine worlds of Norse mythology; I’m thinking I’ll use Asgard and Midgard for the binary planets, with Jötunheim, Vanaheim, Alfheim, Muspelheim, Svartalfheim, Niflheim, and Nidavellir being the moons.

The idea is that these moons will all be Mars-like, but I’m planning for them to orbit close and fast to the binary planets and for them to have orbital resonances like Jupiter’s moons. In this system’s case, instead of the 1:2:4 resonance, the resonance system is 1:2:4:8:12:18:27. The innermost four moons have 2:1 resonances, with the outermost three having 3:2 resonances; I calculated their Hill spheres (i.e. their gravitational sphere of influence), and this is about the tightest system that would likely remain stable long-term.

I also figured out how close the first moon could get and still be comfortably outside the Roche limit, where moons start to break up, while taking into account the extra radius from the central planet of the system being a binary (i.e. it’s much longer than it is wide). I’ve figured their distances and orbital periods like so:

  1. 25,000 kilometers; 10.5 hours
  2. 40,000 kilometers; 21 hours
  3. 63,000 kilometers; 42 hours
  4. 100,000 kilometers; 84 hours
  5. 132,000 kilometers; 128 hours
  6. 173,000 kilometers, 192 hours
  7. 227,000 kilometers, 288 hours

The binary planets, meanwhile, each have a mean radius of 5500 kilometers, 8250 kilometers on the major axis (they’re stretched like eggs to the tune of 2:1 length to width), so each planet is 16,500 kilometers in diameter, amounting to 33,000 kilometers from end to end. Accounting for a bit of distance between the two’s surfaces, each planet would extend out 17,000 kilometers or so from the barycenter.

See the Sky rotate with the Naked Eye!

The coolest fact about this system is that both planets are tidally locked to each other, meaning their days are as long as their orbital period about the barycenter, which is very peppy: 2 hours 4 minutes. This rate of rotation causes objects (such as stars) to move in the sky at a rate of 3 degrees per minute, or 3 arcminutes per second; for comparison, our Moon is 30 arcminutes across today, so objects would travel across the sky at a rate of a tenth a full moon per second. This means that the rotation of the planet would likely be just within the visual detection threshold of the naked eye; if you were a human standing on that planet, you could see the sky rotate.

Big Moons, big Tides

The seven moons will all be easily tidally locked from the get-go, but the rub is, like the binary planets, they will be tidally locked to the barycenter, not to each other, meaning that tidal forces, all that heating and flexing, will go on unabated, similar to the likes of Europa and Enceladus. Despite being of Martian mass this should keep them toasty for a long time. As far as I understand their resonances should keep their positions stable instead of letting them recess like our Moon has.

Due to their much closer distance and larger size these moons will be much bigger in the sky; the first moon spans 13 degrees in the sky, the second moon 9 degrees, the third moon 6 degrees, the fourth moon 3.7 degrees, the fifth 3 degrees, the sixth 2 degrees, the seventh 1.7 degrees. For comparison our moon today is 0.5 degrees wide in the sky.

The distances of each of these moons is 14.84, 9.27, 5.89, 3.71, 2.81, 2.145, 1.635 times closer than our Moon. Tides scale up with the cube of the distance, so this yields tides 3268, 796, 204, 51.06, 22.1, 9.87, and 4.33 times stronger than our Moon. Multiplied by their mass, 5.86 lunar masses, this causes tides 19150, 4664, 1195, 299.21, 129.51, 57.84, 25.37 times stronger than our Moon, respectively. If they ever combine into one huge tide (which the orbital resonances probably prevent) it adds up to 25520 times as strong as our tides.

Tidal range in open ocean is usually 2 feet, so the tidal range of Asgard-Midgard should be up to 51,040 feet (almost 10 miles!); the highest tidal range caused by the innermost moon alone is 38,300 feet. This tidal bulge will be the strongest at the equator; the poles will be unaffected, as all the moons rotate in co-planar orbits with the planets’ rotation. So there will likely be equatorial oceans sloshing around, with all the land near the poles. At high tide, once every hour (!), there would be transfers of ocean between Asgard and Midgard across the L1 point, an interplanetary waterfall.

I believe the winds should flow toward the sea when the bulges are approaching, and away from the sea when the bulges are receding, as the evacuation of the water creates a low pressure zone that has to equalize. Considering these winds have to sweep across so many miles every hour, they’ll likely be very strong.

Earth in a comet-like Orbit?

I’m thinking there will be even more of a cool factor; shortly after the system’s formation a passing stellar encounter perturbs the planet into a highly eccentric and highly inclined orbit that spends much time in the outer solar system, its path resembling a comet more than Earth today, the climate being characterized by long cold winters and short hot summers. Considering it’s well away from every other major planet in the solar system, this setup should remain stable enough for a few billion years. Is it the likeliest thing? No, but it’s possible, and crafts a world very different from Earth. More importantly, it ensures habitability: with the Sun’s luminosity increasing, greater distance is needed to keep cool.

Aside from these tides which are of Miller’s Planet proportions, imagine these oceans and the torrent of liquid globules and torrential flows between the worlds’ Roche lobes freezing over, leading to an icy snowy near-zero-g thicket between the planets subject to frequent eclipses from those seven moons.

The rest of the ocean, meanwhile, freezes over, manifesting as the tidal bulge being encrusted with ice…which still moves with the underlying water. Imagine crackling waves of ice creeping up on you on an hourly basis. Like Miller’s Planet is speculated by fans to be in “Interstellar”, I imagine the terrain is very flat in the tropics, as the tides scour it clean, but the poles might be very high in altitude and with very rugged relief; think cliffs 10 miles high with broken islands of the same height, like natural arches on Earth’s beaches but much more extreme, with tides creeping up and evacuating out of these cliffs every hour.

A new Intelligence

By 3.6 billion years from now, a Cambrian explosion of life has occurred on these worlds, re-seeded by bacteria that rained back down from the ejecta of the collision after the planets cooled. Complex life thrives again, perhaps to the point of developing intelligence. In this case the sapient creatures will be similar to seabirds I think, having a global distribution, living like pueblo peoples in these cliffs, diving down and taking flight to hunt their food. They’ll most likely be able to transition directly from nomadism to industrialism.

Rocheworlds: easy Spaceflight?

Spaceflight is trivially easy, as they can fly to the L1 point (the midpoint where the planets’ gravities cancel each other out) under their own power even and then go from there to any destination they want, assuming they have a proper vessel. This L1 point would be the logical place to site gigantic floating shipyards (think zeppelins with propulsion for stationkeeping amid strong winds, on the outside of the innermost part where waterfalls flow every hour) where space habitats are constructed and sent out.

That ability will serve them well when another passing stellar encounter ejects the nine worlds from the solar system altogether, making a close passage of Neptune on the way out, which is now host to Triton’s disintegration into a new spectacular icy ring system like Saturn’s now.

The avian civilization builds and launches interplanetary arks to escape an eternity in the inky blackness of interstellar space, many sticking around to see the close pass of Neptune before they go, leaving the new Earth, and a diehard remnant population, to its fate in the void.


Don’t worry: there’s another chapter, another flourishing of life and of intelligence, before Earth goes out in a blaze of glory, consumed by the last light in the universe, but that’s another story. In the meantime, I think my concept is a rather unique idea that will prove a compelling bridge between Earth as we know it and the distant finale in my chronicle of my space opera’s extremely far future.

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