What is the best case scenario for a habitable planet?

[Euracil]

What circumstances would a planet have to be in to be the most habitable possible?

Earth is a good place to start, but just saying "Earth" and being done with it is no fun, is it?

I was thinking that perhaps there would be a binary red dwarf system with the 2 stars orbiting very close to each other and the planet in question would be orbiting around both in the habitable zone of the 2 stars combined, in which this habitable zone would be larger and farther away from the barycenter than a singular red dwarf. Hopefully this would distance the planet and make it not receive as much of the amount of unpleasant radiation as it would in a singular red dwarf system. The planet would not be tidally locked, and the red dwarfs would stay alive for a substantially longer period of time than other types of stars.

What do you think?

[EzinX]

Even if such a planet existed, in order to reach it, you would need either the technology to :

1. Create an absurdly large starship that will throw away most of it's mass in transit and keep alive a population of humans for decades in an artificial environment

OR

2. Create human beings from straight data files of their DNA and files describing the molecular structure of enough biomolecules to "boostrap" your way to a human embryo.

Either way, at this tech level, what need do you have for habitable planets? You could build machines that can live natively just anywhere and support human thought, you could build enclosed habitats that recycle everything like a planet, etc etc.

[Euracil]

Even if such a planet existed, in order to reach it, you would need either the technology to :

1. Create an absurdly large starship that will throw away most of it's mass in transit and keep alive a population of humans for decades in an artificial environment

OR

2. Create human beings from straight data files of their DNA and files describing the molecular structure of enough biomolecules to "boostrap" your way to a human embryo.

Either way, at this tech level, what need do you have for habitable planets? You could build machines that can live natively just anywhere and support human thought, you could build enclosed habitats that recycle everything like a planet, etc etc.

That's not really what I'm asking. I'm not saying we're going there, I'm just proposing something to think about potential habitability of planets, not the question of how we'd go there.

[magnemoe]

Well an dual planet would be fun, they would share biochemistry and travel between them would be easy. Getting back would be harder

An larger but lighter planet with more small continents and less would give more living area.

Now combine this with an smaller star and you can avoid the tidal locking and still have something long lasting.

[KerikBalm]

Well, red dwarfs have the problem of radiating primarily in the IR.

That means the atmosphere will block most of it. Sure it will heat up and produce liquid water, but it makes photosynthesis hard.

Next we have to consider the metalicity of the system - is there a correlation between the size of a star, and the density of the planets that form around it?

You want your planet to have a fair amount of heavier elements.

Earth once had enough U238 that there were natural nuclear reactors only 2 billion years ago (look it up, its pretty interesting). Radioactive decay keeps our core warm much longer than it it simply cooled since formation. A molten core is needed to maintain plate tectonics (perhaps important in keeping an atmosphere) and a magnetic field- neither of which mars has.

Though Mars has a smaller radius with a lower surface gravity, it should still be able to hold on to more atmosphere than it has... significant amounts may be locked in the regolith.

We need look not only at the star, but also the planet. Too big, and it may be too easy to send into a runaway greenhouse. Too small, and you get Mars.

I'm not sure how much you need a moon - the axial tilt of mars has varied a lot, and venus has no spin to speak of... lets just say a moon is nice, particularly for forming tide pools which may have been important for abiogenesis.

The downside is that after nearly 4 billion years of life, we have less than a billion left before Earth gets too hot. No interplanetary species yet... clearly a planet should have a longer lifespan to give its life time to spread.

I'd say just go with a star like our own, but a bit smaller so it lasts longer.

[Aethon]

Just a few thing off the top of my head.

It's certainly anthropocentric to assume the Earth is the best example of a habitable planet. As we learn more about the universe, we may begin to view our Earth as only marginally habitable.

A larger, more massive planet or one with a more uneven, wrinkly, lithosphere would obviously have more habitable surface area and be able to retain a thicker atmosphere, thus increasing protection from radiation. A stronger magnetic field would also lessen the radiation hazard. However a planet too much more massive than Earth might not loose it's primordial hydrogen atmosphere, which would be detrimental to the genesis of 'life as we know it'.

The ratio of surface water to land mass is also important. Giant continents, far from climate moderating oceans, could have huge, uninhabitable deserts in their midst. OTH, planets with many smaller continents or archipelagos may have more rich habitats for life to evolve. In addition, Earths' shallow seas have a much higher biodiversity than the deep ocean. It seems strange, but a planet with less water may actually be more habitable than Earth.

It also appears that Earths' biodiversity was greater during warmer periods in it's history. A slightly warmer version of Earth might have extended tropical zones that would allow for more biological variance, as long as it was stably warm over geological periods.

A stellar system with more than one habitable planet would certainly make those planets a better case for habitability. In our solar system, if the orbits of Mars and Venus could be swapped, we would probably have three habitable planets instead of just one. Such 'multi-habitable' systems might allow for panspermia and many diverse biomes where life could flourish.

The Earth has experienced a relatively steady increase in biodiversity over it's lifetime, which may indicate that life itself is able to modify a planet to make it more suitable for it's ancestors, so an older planet around a slightly less massive star than ours would be a good candidate for 'super habitability'.

Edited December 3, 2014 by Aethon

[Euracil]

Just a few thing off the top of my head.

It's certainly anthropocentric to assume the Earth is the best example of habitable planet. As we learn more about the universe, we may begin to view our planet as only marginally habitable.

A larger, more massive planet or one with a more uneven, wrinkly, lithosphere would obviously have more habitable surface area and be able to retain a thicker atmosphere, thus increasing protection from radiation. A stronger magnetic field would also lessen the radiation hazard. However a planet too much more massive than Earth might not loose it's primordial hydrogen atmosphere, which would be detrimental to the genesis of 'life as we know it'.

The ratio of surface water to land mass is also important. Giant continents, far from climate moderating oceans, could have huge, uninhabitable deserts in their midst. OTH planets with many smaller continents or archipelagos may have more rich habitats for life to evolve. In addition, Earths' shallow seas have a much higher biodiversity than the deep ocean. It seems strange, but a planet with less water may actually be more habitable than Earth.

It also appears that Earths' biodiversity was greater during warmer periods in it's history. A slightly warmer version of Earth might have extended tropical zones that would allow for more biological variance, as long as it was stably warm over geological periods.

A stellar system with more than one habitable planet would certainly make those planets a better case for habitability. In our solar system, if the orbits of Mars and Venus could be swapped, we would probably have three habitable planets instead of just one. Such 'multi-habitable systems might allow for panspermia and many diverse biomes where life could flourish.

The Earth has experienced a relatively steady increase in biodiversity over it's lifetime, which may indicate that life itself is able to modify a planet to make it more suitable for it's ancestors, so an older planet around a slightly less massive star than ours would be a good candidate for super habitability.

I do agree that the Earth is not ideally habitable, mostly because our star is too big and the planet too small (also, bigger planet = more sheer land and resources); however, it is good enough for life and, furthermore, intelligent life, so we must commend it for that.

I have always thought that switching Mars and Venus would have been really great for the habitability of both worlds. I always find it disappointing that they didn't. I mean, wouldn't it be so cool if there actually were Martians and we didn't have to go light years to find more life?

Mars proves a really good point. In kindergarten, they say "Venus is too hot, Mars is too cold, but Earth is juuust right." As with anything in kinder, it's not the whole story. MARS IS IN THE HABITABLE ZONE. It just had a number of other factors going against it, mainly size, that made Mars so inhospitable.

I think that the Earth has the perfect amount of water, not necessarily for life to evolve, but more so for the development of civilization. Civilization underwater would be impossible because it would not have agriculture, and tiny islands would be too mountainous for agriculture. However, not enough water would lead to, like you said, massive deserts. Here on earth we have enough land to form massive continents with flat lands good for agriculture and still enough water in oceans to maintain a healthy hydrologic cycle and to keep worldwide climates relatively mild through heat exchange. Also, simple geography substantially aided human development. The Mediterranean Sea was a massive boost to worldwide trade in early history. The continents (Americas, Australia, Afro-Eurasia) are also just barely so connected that the ecosystem of one would not totally collapse due to the introduction of completely unrelated species and also separated enough so that biodiversity would give us more variety in resources (Corn vs. Rice). Even Antarctica plays a huge role in shaping oceanic currents.

I actually think that intelligent life around a Yellow Dwarf is, in most cases, impossible. Life wouldn't have enough time. I think that Earth lucked out and had a huge moon that created tide pools that accelerated early evolution. If the moon was too large, Earth would be tidally locked to it and tides would be static. Taking that into account, humans might just be one of the first intelligent species to appear in the universe. Life might take tens of billions of years to reach this point for all we know. But simply thinking about the fact that habitable stars may exist for trillions of years and we just happen to appear withing the first few billion years that habitable stars could exist is pretty mindblowing.

[magnemoe]

Earth has unnecessarily much water, pacific especially is too large add some small continents in it.

Eurasia is too large too, split it up

higher temprature would be good.

http://www.worlddreambank.org/P/PLANETS.HTM

has a lot of cool templates with some interesting results.

Multiple planets with life would be good, not only would they exchange simple life but it would also give spaceflight an very huge boost after industrialization.

After you have the infrastructure to travel between planets regularly astroid mining and other large scale space industries will be far easier than for us.

The colonizes also create an backup if something goes very wrong.

[jwenting]

well, the best case would be a white dwarf star in a relatively empty part of a galaxy, with a few rocky planets and 2 or 3 gas giants.

The planet in case would be at about 1 astronomical unit from its primary, which would be about 4-5 billion years old.

Oh wait, that's earth... Which of course is no surprise, as earth is the prime example of a habitable planet that we have, in fact the only example.

[MBobrik]

Optimal planet for life. My personal subjective opinion... Well, first, farther from the star. Earth is already uncomfortably near the inner edge of solar habitable zone. Wild guess, outer third of the habitable zone. To avoid extensive snowball episodes, it would have to be larger and heavier than earth, so it can retain more atmosphere, and have stronger volcanism and plate tectonics, nice effect would be, that such a planet will retain its internal heat for much longer. And the star should be smaller, to add planet lifetime. Of course there is an upper limit on mass because too high gravity might place too strict limits on the evolution of complex lifeforms, and too thick atmosphere would prevent photosynthesis. land/ocean ratio should no be much different from earth. Similarly, there is a lower limit on the star mass to avoid tidal lock or stripping the planet from its moon by tidal forces. (and of course, red dwarf flares, and the issue with too little high energy light to drive photosynthesis ) The planet should have a large moon, preferably mars sized (the planet itself is bigger), to stabilize its axis, and, if a civilization is to develop there on its own, to tempt it to come over and terraform it.

[Aethon]

I think that the Earth has the perfect amount of water, not necessarily for life to evolve, but more so for the development of civilization.

Actually, drier, Dune like planets should have a wider stellar habitable zone (HZ).

At the inner, hotter hz boundary, drier worlds will be more resistant to a runaway greenhouse effect, because their tropics will be able to radiate more heat away compared to an atmosphere saturated with water vapor, but still should be somewhat opaque to infrared, keeping the poles from freezing.

At the outer, cooler hz boundary, lower humidity should mitigate the formation of clouds and snowfall thus lowering the albedo (reflectivity), so the world will absorb more solar radiation and make it less likely to have a runaway snowball effect. Daytime temps will also be higher on dry worlds at the outer HZ, due to their smaller thermal inertia.

http://online.liebertpub.com/doi/abs/10.1089/ast.2010.0545

Edited December 2, 2014 by Aethon

[MBobrik]

While we are writing our armchair guesses, other are busy actually computing the properties of such a world. http://arxiv.org/abs/1401.2392

[-Velocity-]

Well, just because the star can live for 100 billion, a trillion years does not mean the planet can. Earth is supposedly only two billion years away from the end of plate tectonics, even if were to somehow retain its oceans for the next two billion years. This is because the planet is cooling, and radioactive decay in the core- which helps to maintain the planet's heat- is dropping. Heck, the planet used to support natural fission reactors when uranium got concentrated enough by natural processes in one area. All that is done with. With plate tectonics ending, supposedly that does really bad things to the carbon cycle, because crust, and the carbon it contains, no longer gets subducted back into the mantle. It could also cause the remaining heat to build up under the crust until it busts through in devastating episodes of volcanic eruptions, like Venus appears to undergo. The brightening Sun is supposed to do in Earth's biosphere before plate tectonics shuts down, and the loss of water may make plate tectonics shut down before the 2 billion year mark is reached.

Another problem with planets that maintain life for very long periods is atmospheric- the atmosphere is slowly being lost to space. Water in particular is lost when it undergoes photolysis and the hydrogen escapes. But we're slowly losing the heavier atoms and molecules too.

Oh and as your planet cools, you'll lose your magnetic field.

Personally, I don't believe that red dwarf planets are good candidates for hosting habitable worlds. You have the extreme stellar flares problem and the problem of tidal locking. The long life of red dwarf stars is not a benefit for habitable worlds, as habitability has its own, independent timescale. However, of course, if red dwarf stars CAN host habitable worlds, even at a reduced rate from orange and yellow dwarfs, the large number of red dwarf stars may make red dwarf habitable worlds relatively common, even if they are not that ideal.

That said, potentially, a geo-engineered red dwarf planet could maintain habitability for a VERY long time, if the effects tidal locking were not too severe. Come to think of it, it's pretty much guaranteed that red dwarf planets/systems will be the last places in the universe where life continues to exist, in the very distant future where almost all star-forming hydrogen is used up in all galaxies, as long as there isn't a Big Rip (or vacuum instability event).

Edited December 2, 2014 by |Velocity|

[Bill Phil]

Best case...

not it a planet at all.

An artificial environment like a Stanford Torus. That way we could control the environment completely. Plus, it can be built in any star system.

[YNM]

Something you might consider silly - the HZ of O-type star. it's really silly - start from a colder world (near the outer limit of HZ), and then gradually heated - when the star becomes an LBV, the orbit is pushed outward , so it would be colder again ! Ends up outside the HZ, w/o parent star too unfortunately.

[kerbiloid]

Interstellar colonization would require: superlight speed or warp motion, artificial gravity, closed-loop ecosystem, super-migthy energy source, large ultra-protected habitat modules.

If you have all of this, why you need any planet at all?

If you don't need a planet, why to fly somewhere at all?

From my POV, Earth stays the only humanity planet forever, but of course, there will be forposts and hotels near any star you want.

[Torquemadus]

In science fiction, the galaxy is filled with habitable worlds with breathable atmospheres. Some might have challenging environments such as deserts (Tatooine) or glaciers (Hoth), but you don't need a space suit. Convenient transportation between these worlds is provided by faster than light drive technology, which is either invented in the near future by humans, or learned through contact with aliens. It's important not to confuse science fiction with real life!

It's been pointed out that many parts of the Earth are not "habitable" for humans unless we use technology to adapt to the conditions. For example, much of our planet is subject to freezing winters that we cannot survive without technologies such as clothing, shelter, and control of fire. We feed our populations through farming, which involves modifying the land and the use of domesticated plants and animals. We use technology to adapt to new environments, but we also modify our environment to better suit our needs. Living things have been modifying their environment to their benefit long before humans came along.

If we want to settle a new world, we don't have to wait for fantastical warp drive spaceships to be invented to fly to a distant solar system with a ready made Earth for us to live on. We can settle Mars with our existing technology. We can then modify the environment there over time to make it more suitable for our needs.

As well as being a destination for colonisation in itself, Mars is within easy reach of the main Asteroid Belt, allowing supplies sent from Mars to mining bases there. It has been suggested that a "triangle trade" could be set up, with high tech goods and colonists going to Mars, supplies going from Mars to the Asteroid Belt, and precious metals going from the asteroids to Earth.

The next question would be how best to settle the outer solar system in order to exploit the rich supplies of Helium 3 fuel present in the atmospheres of the gas giants. Titan has been suggested as an ideal base location to support this (Jupiter's high gravity and nasty Van Allen radiation belts make it less attractive than Saturn).

Helium 3 is not only an excellent fuel for fusion reactors, it is also an excellent fuel for fusion rockets, which can be used to reach speeds that are adequate for interstellar flight. If we can master the resources of our own solar system, the next step is to begin expanding into new ones.

It's worth bearing in mind that the Earth has only been "Earth-like" since the relatively recent Cambrian period. Before that, it had no multicellular life and no oxygen atmosphere. Post-Cambrian earths might be rare, but pre-Cambrian earths might be more common. Mars had such conditions in its early geological history and could well have been a home to simple life just as the Earth was. There could be many planets that never develop the right conditions to become a post-Cambrian earth on their own, but would nonetheless be easy for us to settle and terraform.

The discovery of a post-Cambrian Earth-like world with complex life would of course be of immense scientific interest.

[kerbiloid]

All Earth-like planets in Solar System have total surface area about 1.5 Earth surface.

And 1.0 of these 1.5 are on Venus, which is a little bit hard place to leave. Even remove its hellish atmosphere we would get a planet where one half is heated for months by Sun, another half is frozen for months without Sun, as its rotation period is several Earth months.

The same problem with Mercury, but more of that, it is very small body.

Another ~0.25 are on Mars. I.e. colonizing the whole Mars you would get only +25-50% to your Earth needs, with conditions worse that Antarctic.

Total Moon surface area is a bit above area of Africa. Moon total colonization gives just one more Maghrib, but a frozen one. Also no magnetosphere means you would live in bunkers and every minute outside of it be ready to a radiation alarm.

Asteroid surface is too small that nothing to say about them.

Titan is cryogenic, if you colonize it, you have a deep ammonia+hydrocarbon swamp far from Sun.

Jupiter has 3 more or less moons to use, but at least Europa is inside its radiation belt. Building an under-surface base you gain nothing, because you can easily do this even on Earth.

Ganymede and Callisto maybe less horrible, but you also have just a Moon (see above), but far from Earth.

Of course, no planet or moon in Solar System has any soil because there were no plants and no humus. But they are covered with regolith  a sticky abrasive which at best would not interfere with your efforts.

So, your plants would grow in aeroponics tanks because it a little bit hard to imagine how to create a planet-size soil layer in a reasonable time.

That means that all demographic problems will be anyways  with any method  solved while humanity is Earth-only. So, no "demographic" colonization will be happen at all.

Therefore there is just no reason to colonize some planet in Solar System except to create a vault for a part of humanity.

And any extraterrestrial settlement will be just a high-protected self-sufficient forpost isolated arcology (as probably Earth cities will).

Even if another star system has an Earth-like planet with air and grass, you won't colonize it before you "colonize" Solar planets in the way described above.

And if you have autonomous in-box paradise and your population does not grow, you have no need in extrasolar colonization at all.

You need just to establish your web around your lair to prevent any unpredictable events, dangerous for your Earth.

[Findthepin1]

planet characteristics: larger than earth, maybe 1-4 earth mass

oceans at both poles and ponds at tropics and temperate zones

atmospheric pressure 1-10 atm, co2 90%, nitrogen 9%, and traces of hydrocarbons and water vapor

temperature 35 celsius, extremes -70 celsius and 107 celsius

large moon to stabilize axis

extensive volcanism

moon characteristics: lunar mass or smaller, europa-like surface

no retained atmospheric pressure

global water ocean underground

the moon had life first, after an asteroid impact the planet was seeded

[ChrisSpace]

What circumstances would a planet have to be in to be the most habitable possible?

Earth is a good place to start, but just saying "Earth" and being done with it is no fun, is it?

http://io9.com/what-would-the-perfect-planet-look-like-1503442354