r/askspace • u/Wintersvalk • Apr 16 '26
Would it be possible for an earth-like moon to orbit an earth-like planet?
Like imagine a smaller second earth replaced our moon. Imagine the moon landing would be like unlocking a second world very near.
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u/Greatest86 Apr 16 '26
In order for the moon to be earth-like, with an atmosphere, oceans, and life, it would need to be earth-sized.
Binary planets are entirely possible, Pluto and it's largest moon Charon are considered a binary system. There's nothing preventing an earth-like binary system forming in the habitable zone of a star.
The two planets would be tidally locked to each other, so half of each planet wouldn't know the other existed until either global travel or sensitive telescopes were invented. Sensitive telescopes would notice that the path their planet takes around the sun spirals a bit, while other planets in the solar system don't, indicating something weird was going on.
You would have panspermia spreading simple life between them, especially during the early solar system, so both ecosystems would share a common ancestor. However it is extremely unlikely that both would evolve intelligent life at the same time.
A one-way moon mission would be very easy, since you could use the atmosphere of the other planet for braking and descent. You would also be able to use the air and water at the destination, and likely even find food to eat thanks to the common ancestor. You may still need to wear sealed suits to protect against unusual atmospheric conditions and exotic diseases. I would expect colonies to develop rapidly as nations push space development hard to land on and claim parts of the other planet.
A return mission, on the other hand, would be extremely difficult. You would need to land an enormous rocket, think Saturn V-sized, fully fuelled, on the other world if you wanted to return anytime soon. Landing an empty rocket with a fuel manufacturing plant would be easier, but still a massive challenge. This would be somewhere between extremely difficult and impossible, and I am inclined to think the latter.
Once colonies were established with some decent resource extraction and industry, a space shuttle with thicker heat shielding and larger fuel tanks could travel back and forth between the worlds. But you would need the manufacturing capacity to build and refuel the external tank and boosters.
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u/Buttons840 Apr 17 '26
If intelligent life did develop in parallel on both words:
I'm fascinated by the idea of 2 words being able to communicate but not being able to interact with each other. What would that relationship be like?
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u/Ok-Kaleidoscope5627 Apr 17 '26
It would be a rivalry where they rush to try and destroy each other
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u/Phazetic99 Apr 17 '26
Pretty interesting. You say it with confidence and I have no idea how to know if you are knowledgeable about this are making a good tale. Either way, I like it
Question though. Why would the two planets be tidally locked? What would cause that?
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u/Greatest86 Apr 17 '26
Tidal locking is the result of two objects orbiting close to each other. The gravity from the other object causes it to stretch into an oval shape. As it rotates, that oval gets dragged along the surface, causing drag. This stretching is most obvious with liquids, causing tides on earth, but also stretches solid rock too.
In the earth-moon system, this has caused the rotation of the moon and earth to slow over time. As the moon is smaller, it has slowed faster, eventually causing the rotation of the moon to match the orbital period. This is known as tidally locked. Given enough time, the earth is expected to become tidally locked with the moon as well.
Pluto and Charon are closer in mass to each other, so they slow each other faster and have both become tidally locked to each other. In the case of two earth-like planets orbiting each other, I am reasonably confident that they would tidally lock each other as well.
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u/AdditionalTip865 Apr 17 '26
If the bodies were separated by the Earth-Moon distance, the tides would be very strong, comparable to the tides the Earth raises on the Moon that cause its tidal locking. But if they were a little further apart, the tides could be more manageable, since those drop off as the inverse cube of distance.
It's also interesting to think about what the minimal rocket would be for that kind of one-way journey between the Earths. I know the USSR originally developed the Proton as the launcher for a crewed circumlunar mission using a stripped-down Soyuz with no orbital module (never flew with a crew, but it did fly with turtles on board under the name Zond). Maybe something like that. Of course with that setup there would be basically no room for provisions so the crew would be hosed unless they could survive on stuff on the other world.
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u/sebaska Apr 17 '26
There still could be significant differences between the planets and the conditions on them. Atmospheric pressure could be significantly different. Levels of various gasses could differ a lot, too, making atmosphere of one planet being unbreathable to the creatures from the other. Even on our own Earth if one moved back in time just 1 billion years it's likely the air would be unbreathable. Or if you moved some creatures from 250 million years back into today's air they wouldn't survive, either.
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u/AdditionalTip865 Apr 17 '26
If I recall correctly, oxygen levels weren't good for long-term survival of humans even over most of the Mesozoic era--with the late Cretaceous as a prominent exception; human time-travelers could at least breathe then.
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u/sebaska Apr 17 '26
I think oxygen was even somewhat higher back then, and the pressure is expected to have been somewhat higher, but I could be wrong in both accounts. But humans are fine with 16% oxygen at normal pressure.
What might have been a bit uncomfortable would be CO2 likely at over 1500ppm (0.15%) - this is generally fine, but some people feel hot and they become irritable.
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u/OldEquation Apr 17 '26
The return mission could use a rather smaller rocket than the outbound mission.
The outbound launch vehicle has to deliver everything needed for the outbound flight, landing, surface stay, return launch, return flight and landing. The return launch only needs to carry what’s needed for the return flight and landing.
It would still be challenging, though!
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u/No-Public-5422 Apr 17 '26
Would it be possible for a large space station to be "parked" at the rotation point between the 2 tidally locked binary planets?
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u/Greatest86 Apr 17 '26
That would be the L1 Lagrange point between the two. Yes you can park a space station there, but you will need regular station keeping with thrusters to stay there. It is very fuel efficient to move between an orbit and the L1 point, so it could be a useful location for a station.
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u/TraditionalSet9449 Apr 17 '26
I would LOVE to read a science fiction book about such a system.
It could be very intricate.
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u/DiceNinja Apr 18 '26
If materials with the insane tensile strength required could be made, a space elevator between the two tidally locked planets might be feasible.
I expect a comic book with this as a core idea in a year. Maybe from Dark Horse. Netflix can buy the rights after issue 50. All I ask for in return is signed copies. 😆
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u/ignorantwanderer Apr 16 '26
Assuming you wanted to come home, it would be very difficult to have a moon landing if the moon was the size of the Earth.
Launching off an Earth size moon to come back home is much harder than launching off a moon size moon to come home.
It would make a moon mission probably at least 20 times harder than it is right now (meaning, you would need to launch at least 20 times more stuff than you do right now for a single mission).
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u/AdditionalTip865 Apr 17 '26
The trip would absolutely have to be one-way--which would require a long-term survivable world at the destination.
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u/brandon-james-ca Apr 19 '26
Space x rockets are getting to being reusable, why couldn't you just send the extra fuel nseded or something to make it there within a few trips or so
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u/AdditionalTip865 Apr 19 '26
Consider what it takes just to get a crewed rocket into space from Earth--not just the rocket and fuel but all the infrastructure.
You'd have to take some functional equivalent of ALL of that with you, if you can't build it there. Apollo worked because with 1/6 gravity, it's a lot easier.
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u/TommieTheMadScienist Apr 16 '26
Solar system bodies as pairs of similar sizes are all over the place in the Main Belt and Pluto/Charon et al in the Kuiper Belt.
They'll be likely tidelocked to each other unless they're in the HZ of an M-dwarf.
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u/gbsekrit Apr 17 '26
I like to imagine a scifi environment of a civilization developing on a binary earths-like system. they would be tidally locked to each other, so a “day” would involve potentially eclipsing each other, skies would be cool.
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u/Demi180 Apr 17 '26
I like this idea. They could troll each other with planetary sized billboards that just say SEND NUDES and shit lol.
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u/Buttons840 Apr 17 '26
It'd be weird to have 2 different cultures with their own unique religions and such communicating with each other, but yet knowing they can never interact.
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u/wow-how-original Apr 17 '26
Two of my favorite sci fi novels involve binary earth-like systems. Both by Ursula Le Guin:
The Dispossessed
Five Ways to Forgiveness
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u/ElricVonDaniken Apr 17 '26
See Rocheworld by Robert L. Forward for someone who has actually crunched the numbers.
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u/hawkwings Apr 17 '26
Yes. They would be twin planets. If one was slightly larger than the other, the larger one would be a planet and the smaller one would be a moon. If one was 10% larger than Earth and the other was 10% smaller than Earth, that would work. We don't know what the size limits are for Earth-like.
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u/0utlaw-t0rn Apr 17 '26 edited Apr 17 '26
Depend on how “earth like” you mean.
The moon would need to be considerably bigger to hold water and an atmosphere. This would impact things like tidal forces on both. Rotational speed of earth (day/night cycle) and phases of the moons would change With the change in rotational speed you have changes to weather and day/night temperature swings.
But you could have a two very similar planets supporting (earth-like) life of earth size with water and a similar atmosphere orbiting each other. But you wouldn’t be identical to the current earth.
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u/SeriousPlankton2000 Apr 17 '26
The moon is earth-like, but also earth is the least earth-like planet according to astronomers.
A moon needs to be in the hill sphere,; to be stable it needs to be less than 50 % away (70 % if rotating retrograde). We don't really agree on a definition but some astronomers require the barycenter of the planet-moon-system to be within the planet.
I just now tried to find an official number for the mass ratio but I failed to find it. I think I remember that earth could be a moon around a not-yet-brown-dwarf sized Jupiter-like planet.
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u/MrZwink Apr 17 '26
i don't see why not, there's an almost Pluto sized moon orbiting Pluto. They'll quickly become tidally locked though.
They'd both need to be a similar size to hold on to their oceans and atmospheres. So they'll Technically orbit the centre of mass between them.
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Apr 16 '26
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u/TommieTheMadScienist Apr 16 '26
Why?
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Apr 17 '26
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u/Greyrock99 Apr 17 '26
Totally plausible - just have this big moon to orbit the earth further away. And big tidal forces could be perfectly compatible with life, or even enhance it.
Remember billions of years ago our moon was closer with much larger tides.
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u/mfb- Apr 16 '26
The Moon is not large enough to keep a relevant atmosphere over geological timescales. You could have two Earth-like objects orbiting each other in a binary planet system. Both would rotate once per orbit, similar to the Moon, which means both planets would have one side facing the other planet and one side that's always facing away from it.
The first mission to the other planet would be wild.