Another Earth

That’s not what I’m saying. I’m just saying that I’m wondering how life on Earth would have developed over, say, the last 100,000 years without the humans around. We’ve had a huge impact on the rest of the biosphere. Even, say 25,000 years ago, there were few or no humans in the Americas.

Aside from this (given we don’t have the capability of doing something like that), is it possible to have two habitable planets with stable orbits on one solar system?

Sure, you could have two habitable planets. Not a problem. One’s going to be hotter and closer to the sun, the other is going to be colder and farther out. But it’s easily possible to have two planets in the habitable zone, if we define habitable zone as “has liquid water”.

We don’t really know if there would be biotic exchange between planets. Maybe life on Earth got it’s quick start 4 billion years ago because it was seeded by bacteria that arose elsewhere. The only caveat to this is that bacterial life on Earth chugged along for 2 billion years before the evolution of prokaryotic life, and multicellular life only arose 650 million year ago.

So if we imagine a larger, warmer, wetter planet in place of Mars, that supported life that was phylogenetically related to life on Earth, it could easily be the case that all life on Earth II is just mats of photosynthetic bacteria. Or it could have multicellular life, but those animals would have evolved completely independently from the multicellular organisms on Earth.

Sure, it’s easy to imagine wormy things that develop paddly stuff and crawly stuff, and we’d get fishy-ish and buggy-ish stuff. There are dozens of phyla here on Earth that are various dubbed “worms” even though they have no phylogenetic relation. A wormy shape is just a thing that convergently evolves.

You have gravity, you have radial symmetry. When you start to move you get bilateral symmetry. The food-hole goes in front, the sensors go in front, the nerve cluster connected to the sensors goes near the sensors. And then you stick the poop-hole in the back, so you don’t constantly swim through your own poo. There’s your wormy-dude, now add some paddles for movement through water, or crawly things for scuttling on the substrate, and there’s your convergently evolved fishy thing or buggy thing. Note however that even here on Earth I there are echinoderms and molluscs and jellyfish that don’t work anything like this, as well as filter feeders who aren’t even playing the same game.

Klemperer rosettes don’t require any specific number of objects; any evenly spaced objects in a circular orbit with radial mass symmetry will do. However, you’ll also find that Klemperer rosettes are not practically stable over any significant time period as even the slightest variation in orbital parameters due to tidal effects from an outside body or any other influence will cause them to rapidly destabilize. Nor can two similar mass objects occupy an orbit in each others’ Trojan points (L4 or L5) or in opposition (L3) without destabilizing, notwithstanding the fact that Earth’s overly large Moon is going to wreck havoc with any attempt to stabilize such a system. As has been pointed out, the orbit of the Earth is not circular, so a planet in constant opposition is not feasible.

Setting aside the issue of stability, there is a bigger problem with orbital mechanics of reaching this “other Earth” (Aerth) orbiting the Sun in an opposing position; that is, because it is in Earth orbit on the same orbital speed but at a different epoch, it becomes very difficult to reach. The minimum time non-retrograde trajectory to reach Aerth (roughly six months) would require a velocity change of ~60 km/s—essentially, stopping in solar orbit, waiting six months, and speeding back up to match velocity with Aerth. Compare that to the challenge of sending a ~4 metric ton spacecraft directly to Mars (done for the Mars Science Laboratory just at the limit of our propulsion capability) and it is evident that we do not have the capability to do this for a crewed vessel, or even for a remotely-operated probe or lander. It would literally be easier to send a spacecraft into interstellar space (42.1 km/s delta-V at Earth orbit). You could potentially launch a much longer duration mission and perform multiple swing-by maneuvers around one or more planets to reduce delta-V budget requirements, but this would occur over a period of many years or even decades. We do not have the capability of keeping a crew alive and healthy without regular resupply and return to Earth, much less have habitable spacecraft with the reliability to operate for that duration or the practical means to protect against high energy cosmic radiation.

As for the planet itself, while there is no particular reason that there could not be a second planet in a habitability zone around a star (planet-finding efforts have found systems with multiple planets in a band around their star at which liquid water could exist) provided the planets are not so close as to create a destructive resonance, there is also no reason that a planet would evolve surface and atmospheric conditions that are in any way like Earth even starting from the same size and orbital inclination. Oxygen exists on our planet due almost solely to the presence of cyanobacteria and photosynthetic planet life. Oxygen concentrations in the atmosphere have fluctuated wildly across different eras from too high of a concentration of carbon dioxide to allow modern mammals to respire to oxygen concentrations that may have been near toxic. Despite the portrayal in science fiction of multitudes of extrasolar worlds ripe for human habitation once the acid-bleeding insects or lava monsters are vanquished, the odds of finding a world with the just-so composition of atmosphere even assuming rocky, near-Earth-mass planets are relatively common is highly unlikely to say the least.


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Thank you for that; I had no idea it would be so complicated. Honestly, I was just thinking about what it would be like if colonies on other worlds weren’t made impossible by being light years distant.

But really, even if it were a simple hop to another habitable world, colonization would still be extremely difficult, since everything would need to be carried there. Someone mentioned that you’d need something like a Saturn V rocket for a return flight from a hypothetical other Earth. But it’s not just the Saturn V rocket you’d need but also all of the accompanying infrastructure, including massive amount of liquid hydrogen and liquid oxygen for fuel.

Carl Sagan’s speech about the Pale Blue Dot is really true. This little planet is all we have and we need to do a better job of taking care of it.

Dude, do what the comics have done for decades. Put it in the exact same spot as our Earth but have it vibrate in a different dimension. Walk through the magic door and presto! And with infinite dimensions you have every possible Earth variant, including buxom bison!

Kids today. No imaginations.

I didn’t open this thread because I’m trying to write some sort of SF story. I just mentioned that he idea of colonies on other worlds is a common element in such stories.

In order to avoid the problems with another Earth in our orbit, let’s posit a different hypothetical planet. Let’s say Mars is larger, maybe about 50 to 100 percent larger than earth, and otherwise has similar properties conducive to life such as liquid water, a magnetic field, and an atmosphere similar to ours but with enough CO2 to maintain temperatures similar to Earth despite being further from the sun. Could we colonize a planet like that with our current technology?

Possibly, but not in the same orbit. Or you could have two habitable moons of a giant planet that was in its star’s habitable zone.

There might be exchange of biotic material between two habitable planets. We have found meteorites from Mars on Earth, and research on the ISS has shown that some bacteria can survive space travel. The risk of this gets higher if spacefaring life develops on one of the habitable planets (or moons).

If there aren’t spaceships flying between the planets or moons, the one closer to the star or planet is more likely to get contaminated by life from the more distant one than vice versa. The more massive planet or moon is more likely than the less massive one to be contaminated, and a thick atmosphere also makes it harder to get meteorites thrown off of a planet and onto another one. If Mars were bigger and had a thicker atmosphere, we would be less likely to get meteorites from Mars here on Earth.

Any answer to this question is necessarily fiction. You can’t approach it with scientific hypotheticals.

And why the hate against science fiction? It does “what if” just as you are.

No. We have yet to send a single human beyond the Earth’s sphere of influence, much less the group of people and all of the resources that would be needed to support a self-sufficient colony. And while I’m certain someone will come along spinning tales of (partially) reusable Falcon Heavy rockets, Aldrin cyclers, Nerva spacecraft propulsion, et cetera, none of these things are currently available and validated technologies.

Having a thicker, more Earth-like atmosphere would make the entry, descent, and landing (EDL) problem more tractable, as the current ‘barely there’ state of the atmosphere of Mars (mean surface level at ~1% of Earth mean sea level pressure) renders Mars the most difficult solid body in the solar system to soft land upon) but other aspects would be difficult unless there was a pre-existing biosphere and hydrologic cycle compatible with Earth, particularly fixed bioavailable nitrogeneous compounds in the solid and a high concentration of free nitrogen in the atmosphere. Also, humans (and most other mammals) can only tolerate a partial pressure of CO[SUB]2[/SUB] of about 0.70 psi to 0.85 psi for any duration; if CO[SUB]2[/SUB] levels are any higher, the atmosphere would be toxic, as would any of a number of potential contaminants in anything but trace quantities. Sunlight would be about 40% of that on Earth, which I don’t think any food crop other than potatoes and maybe other nightshades could tolerate while producing even minimally nutritious fruits and grains.

You could assume that people could live in self-contained sealed environments and grow food using nuclear-powered hydroponics, but that is stretching the definition of a “colony” to the extreme and is unlikely to be self sufficient without a massive amount of infrastructure and ready access to all manner of resources that would be impractical and cost-prohibitive to ship from Earth. When and if we do endeavor to permanent human habitation in space it will most likely be in solar orbiting habitats built and sustained using materials in space collected and processed with a high degree of automation.


It’s very likely chaotic. Which means, if you had 100 planets that were almost identical to Earth but with no humans, you probably wouldn’t get the same results on all of them. The planets start out very close to identical, but random stuff happens over time and they diverge from each other. That means it’s very hard to predict what’s going to happen (weather is another example of a chaotic system).

I’m not sure if I should start a new thread or just ask these question here:

If, somehow Earth and Mars switched their current locations, what effect would this have on Earth’s atmosphere and biosphere? Would humans still be able survive? Would the global warming trends help the Earth at all? Could we dump more greenhouse gases into the atmosphere to counteract the cooling? Also what would happen to Mars in such as scenario?

My understanding is that humans have been in North America for a relatively short time (say 25,000 years versus a much longer period in Eurasia and Africa). So before that, there were saber-toothed tigers and other alpha predators hunting bison. So are those giant herds of bison that were present as late as the nineteenth century a result of there not being predators? And New England, where I grew up, was covered in old-growth forests and there were a lot more redwood forests in California. It would be neat to see what that would have looked like. Manhattan Island was much smaller.

The orbit of Mars is near the limit of the so-called “Goldilocks Zone”, where solar radiance would permit fresh liquid surface water on a planet with a significant atmosphere. (That Mars has essentially no fresh surface water is a function of the very low atmospheric pressure which would cause fresh water to rapidly evaporate down to freezing temperatures; Mars does have surface water in the form of recurring slope lineae which are thick perchlorate and chloride salts which are able to retain water for long enough to flow when warmed and then leave salt trails once the water evaporates.) With a thicker atmosphere in a composition similar to that of Earth, Mars would be at least marginally habitable, and Earth in its current form at Mars orbit would be able to maintain liquid oceans, albeit likely with much larger polar caps and more frequent or constant ice ages. However, the aforementioned reduced level of sunlight would be challenging for many plants on Earth, as would those perchlorate salts which permeate the Martian regolith for anyone trying to colonize Mars.

If Mars were in Earth orbit? We can only speak in hypotheticals, but the lack of a magnetic field would likely mean that the atmosphere would be stripped away even more quickly (as is widely believed may have happened to Mars early in the planet’s history) in the more energetic early phase of solar evolution. Mars also lacks an active tectonic system as far as we can tell (there are signs of previous plate tectonics but they have been inactive for billions years) which has played a significant role in life on Earth in terms of isolating species and resulting diversity as well as periodically burying and releasing biomatter and waste products which have affected the global climate. If Earth-like life evolved on a thick atmosphere version of Mars that also had an active magnetic field to prevent it from being stripped away it would probably look and behave quite differently, and we can’t really say anything more definitive than that.

Of course, the presence of life has significantly modified the atmosphere, hydrosphere, and terrestrial surfaces of the Earth, likely stabilizing it from wilder swings of an unregulated climate. If early life had caught a firm toehold hold on Mars, especially if it were in a more energetically favorable orbit, it might too have become a habitable world with a protective thick atmosphere, active processes to maintain surface and subsurface hydrologically cycles, and geobiological activity to remake the terrestrial surface into a carbon sink. For that matter, subsurface life could exist today on Mars; we haven’t dug more than a few inches below the surface, and there have been samples of organic gases that could potentially be waste products of simple organisms, although they could also be (likely are) from non-biological processes driving solely by solar heating and freezing cycles. But there is no indication of anything like the pervasive presence of microbiota on Earth, much less anything driving by photosynthesis or any other currently plausible energy transfer processes.


North America during the Pleistocene had a much greater biodiversity. It wasn’t all bison and smilodons. There were mammoths, mastodons, camels, horses, pronghorns, lions, giant ground sloths, short-faced bears, giant condors, and on and on.

The vast herds of bison existed because so much of the megafauna went extinct, leaving them with fewer competitors. Yes, there were bison back in the ice age, but they didn’t dominate like they did in the modern era.

It’s difficult to speculate exactly what North America would be like without humans, because humans moved to North America during the Ice Age, and the climate has radically changed since then. There has never been a modern ecosystem without humans in North America. So for example, would there be old growth forest in New England if you still had mammoths and mastodons? Or would proboscideans and giant ground sloths that can push over small trees mean that you’d have a few mature large trees with open grassland between them?

Not just the climate, but the actual geology and geography of the Northeast was affected by the Ice Age. This recent article from The New York Times describes how some of the geological features of NYC were a result of the glacier.

It’s funny; I remember reading in school about how New England was shaped by the Ice Age (such as all the rocks we would find when trying to dig for a garden or certain lakes being a result of the glacier), and wondering how that could possibly be. Because I imagined the glacier being maybe a foot or two tall, while in reality it was something like two miles.)

I just want to say I really appreciate this detailed response. It’s definitely far more robust and well thought anything I could have said.

That is a lot of grass being eaten, and a lot of poop to grow more.
Do they graze like cattle, or tear up the roots like sheep?