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In my story, which should be taken entirely seriously, mankind encounters a region of space occupied mostly by giant exo-householdobjects.

That is, planet sized kettles, saucers, teacups, cutlery, etc. There is even a giant Russell Hobbs branded teapot, because this is how serious a story it is. They orbit around perfectly ordinary stars of various sizes.

The protagonists are going to spend a significant amount of time stuck on a giant banana while observing a solar system with many such objects. They are mostly incredibly impractical people - philosophers, theologians and so on - with a small cohort of ships engineers and tradesmen, as well as a rapacious mine manager called Lobelia who only cares about the giant silverware. They're very well supplied with everything they could want except the means to get offworld.

Leave aside the problem of gravity trying to make the banana into a sphere - assume that either the banana is mostly made of a nearly infinitely strong material when you drill much beneath the peel, or better still, that gravity is excluded from operating within it and only begins at the surface (albeit with all the mass of the banana still contributing). With a sunlike star, what combination of orbital distance and rotational motion can make this pungent, ethyne laced world in the sweet spot of being inhabitable but periodically very uncomfortable or dangerous?

I'm specifically interested in:

  1. and main) What problems are generated by rotation of the banana and what if any rotational motions are amenable to life at least over a few square kilometres while still being a damned nuisance?

So, it needs a few square kilometres that have sort-of earth temperatures and pressures. I'll say -20 to 80 deg C temperature, and 0.3 to 3 bar pressure, with possible intermittent exceptions.

I'm guessing depending on the axis of rotation, there are going to be areas that are uninhabitably hot/cold. I'd like to place their town in the middle of the banana on the concave side, unless there are any serious objections or better suggestions.

For obvious reasons, answers need only stand up to limited scrutiny.

Orbiting fruit worlds and kettles raise such serious theological, philosophical and anthropological problems that the eggheads that are on it are only going to notice that their lives are in danger when the engineers are banging on their doors warning of the impending 30 day shadow that will cause temperatures to drop to dangerously low levels and they need to put their entire libraries in buggies or local flying craft and relocate.

I welcome anyone raising problems relating to atmospheric composition or ground instability that are likely to arise as the banana ripens, although I am not short of ideas in this department...unlike, you know, how these people can survive for 20s without freezing or burning or being thrown off the surface.

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    $\begingroup$ Define "giant" in terms of banana - cross-sectional diameter? Can we assume 1) a Sol-like star; 2) a magically produced breathable atmosphere (for the section around the middle that has any significant atmosphere? From the title I assumed you meant that 90% of the banana had to be inhabitable, but from the text it seems that a tiny section must be inhabitable 90% of the time - correct? $\endgroup$ Apr 21 at 7:54
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    $\begingroup$ Food for thought: since your banana is big enough to have Earth gravity, it's about the same diameter as Earth, so let's say it's 500 million times larger than a normal banana (Earth's diameter is, very roughly, 500 million inches, and an average banana is of course about 1 inch). People on its surface aren't standing on a smooth, rubbery banana peel - they're navigating the valleys between the banana's cells! Normal banana peel cells are about 100um ripe; when enlarged 500 million times their normal size, they're each a mountainous 50km in diameter. And those valleys are probably... gooey. $\endgroup$
    – talrnu
    Apr 21 at 16:54
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    $\begingroup$ Ok now someone needs to make a physics-based game on the surface of a giant banana-shaped planetoid... $\endgroup$
    – bob
    Apr 21 at 17:15
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    $\begingroup$ This question is loser-talk. Colonists on a planet-sized banana? Sheesh. . . what a hack. Gimme a call some time when you're putting colonists on a normal-sized banana. . . . $\endgroup$
    – Daron
    Apr 21 at 19:27
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    $\begingroup$ @talrnu That would be so much fun! Especially as I'm planning on having the banana ripen and/or rot. I spent hundreds of hours in front of the Magic School Bus books and CDROM when I was a child. They featured people being tiny compared to cells, etc; I could steal material shamelessly. $\endgroup$ Apr 21 at 20:57

7 Answers 7

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Not much fruit flies like a banana

The colonists from apple and orange worlds may have figured that the banana world offered a similar experience, with lower gravity. The center of the concave side is drawn by gravity to the middle of the "cylinder" of the banana, but the ends pull it away a bit. Rapid rotation of the banana around the axis of this cylinder brings gravity to virtually nothing by centrifugal force, providing a wonderful 'habited space' environment for those who miss the zero gravity aboard ship. Despite the low gravity, it's still the lowest point on the banana, so all the "stuff" that is to be found (such as water) tends to accumulate there.

Problem is, after a while the tidal forces from nearby pineapples and lawnmowers, combined with the sloshing of fluid that built up in the concave ocean, changes the rotation axis to match the largest moment of inertia, namely end over end like a boomerang. This causes everything that was casually suspended to start crushing to the 'ground'. I'll suppose the end-over-end rotation isn't so extreme as to pull large amounts of material from the tips of the banana, though any space-suited climbers who reach those heights before slipping on the banana peel may not be coming back.

The end-over-end phase lasts longer, so I suppose they can build a new city on banana firma, but eventually the axis might shift back by chance (chaotic rotation, I mean) and then the gravity cuts out again.

With this rapid of a rotation around any axis, there should be no Long Nights, and if the banana is the right distance from the sun, temperatures should be within some reasonable range.

Note: Veritasium has a nice video about a similar effect, which is certainly fun to watch. But I don't want to figure out the intermediate axis of rotation of a hypothetical banana, so I'm going to add additional handwaving regarding sloshing oceans and chaotic rotation caused by interactions with other orbiting bodies.

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    $\begingroup$ If the banana has enough of a bend to it, its center of gravity might actually be a point in space that is above the surface on the inside of the bend. That'd be an interesting challenge to any life on the surface... $\endgroup$ Apr 21 at 16:14
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    $\begingroup$ @DarrelHoffman Objects don't necessarily fall toward the center of gravity, though. A hollow shell, for example, has a CoG at its center, but objects inside the shell feel no gravitational force whatsoever - they don't fall toward the CoG. Gravity on a banana planet will be odd and vary in direction and strength depending on where you are, but I don't think it would point away from the surface anywhere. $\endgroup$ Apr 21 at 18:22
  • $\begingroup$ I love this answer so much. It has everything including a totally erratic and unpredictable crisis causing mechanism. I'll look into spontaneous switches between rotational axes. IIRC Veritasium did a wonderful video on this. $\endgroup$ Apr 21 at 20:47
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    $\begingroup$ Is there a name for the phenomenon where the axis of rotation usually ends up being the largest moment of inertia? $\endgroup$
    – Daron
    Apr 21 at 22:19
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    $\begingroup$ good answer taking full advantage of the lack of a hard-science tag, but +1 for the aggressively shoehorned joke in the title. $\endgroup$
    – Ottie
    Apr 21 at 22:45
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Banana will not be a stable shape

As soon as you place enough banana matter in banana shape in the banana system, gravity will start to work. Banana will contract and get denser, forming a roughly spherical planetoid that is mostly consisting of superheated banana mush. As the chemical separation of heavy and lighter elements happens, the core of the banana mush will become a core of solid graphite, surrounded by a carbon-potassium melt suspending green diamonds before we get to a somewhat softer dried banana mush layer, over which is the decomposing banana peel and the air.

What started out as a giant banana, turned into just another planet with banana properties. Possibly habitable, if in the goldilocks zone.

Stable banana wouldn't be inhabitable either

To live on the banana with 1 g, the banana will have the very same diameter as earth, but be roughly estimated to be a cylinder. a $42*10^6$ km long cylinder.

$$F=G\frac {M_1 M_2}{R^2}\to g=G\frac {M_P}{R^2}$$ $$M_P=\rho_{Banana}V=\rho_{Banana} \pi R^2*l$$ $$g=G\frac {\rho_{Banana} \pi R^2*l}{R^2}=\pi G \rho_{Banana} l$$ $$\frac g {\pi G \rho_{Banana}}=l$$ $$l=42922688156 m$$

However, there is a BAD thing: the gravity does always point to the center of the Banana. This means at the end of the banana, the gravity pulls you almost parallel to the surface and only in a very narrow band around the center of the Banana life is possible at all. Also, due to the bend of the banana, the calculation doesn't even hold true for the whole equator... gravity will be totally messy. There might be two small zones where there is 1g pointing down into the ground roughly orthogonal, but in general, unless the banana is almost straight, gravity will be pointing in wonky angles towards the banana on the outside curve and there will be no gravity at all on the inside curve of the banana. This is because the inside of the banana curve can be solved using the basis of the shell theorem - and so there will be much less banana gravity on the inside curve. And no compensation.

Any curve of the banana complicates things a lot too: no matter how you align the banana, spinning at any speed will create hyper extreme weather: the tips will be either both be much closer to the sun and scorch, or leave goldilocks on the other end and freeze. Due to the length, this deadly zone will be some 50% of the banana, 25% of either end. Even then, the equator of the banana will not inhabitable, as it undergoes something like 120 °C summers and -100 °C winters from moving to the edges of the goldilocks one, but much less than the banana tips.

In total, there will be not a single spot on the banana world, that suffices with your dictate unless your Banana takes the banana mush route and the banana shape is discarded for the sanest banana sphere (see above), a hollow banana ice cream ball or at least be peeled and stacked with copious amounts of banana ice cream and whipped cream as well as a topping cherry for a banana split world, which has an oblong ovaloid shape overall.

PSA:

There are exactly 42 Banana in this text.

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    $\begingroup$ if that so, we say the banana planet's mass distribution is actually a sphere-like at the center, the top and bottom curvy part is just a hollow rigid structure for the sake to complete banana shape, and call it a day. $\endgroup$ Apr 21 at 9:32
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    $\begingroup$ But why are the diamonds green? $\endgroup$
    – Daron
    Apr 21 at 12:23
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    $\begingroup$ Potassium makes Diamond green $\endgroup$
    – Trish
    Apr 21 at 12:24
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    $\begingroup$ Agree with @SeanOConnor that the shell theorem does not apply - that describes what happens inside a perfect hollow sphere of uniform density. It does not imply that mass "above" you has no gravitational effect regardless of the shape of the body. There will certainly be gravity on the inner curve of the banana. $\endgroup$ Apr 21 at 16:17
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    $\begingroup$ @NuclearHoagie Shell theorem basis can be applied, there will be much less gravity on the inner side, as mass on the left and right will have a net 0 effect on the body, leaving only the "below" part.. $\endgroup$
    – Trish
    Apr 21 at 17:17
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Pay attention to what other answers say about gravity.

433 Eros is a "fat banana" shaped asteroid. The number in its name means it's the 433rd asteroid we ever discovered.

From the wiki:

Surface gravity depends on the distance from a spot on the surface to the center of a body's mass. Eros's surface gravity varies greatly because Eros is not a sphere but an elongated peanut-shaped object.

That said, 433 Eros has a mass of about 1/10,000,000 that of our Moon. Supposing you made it much more massive, enough that a person could walk over it without achieving escape velocity, how would it feel?

To answer that, I will suggest that you familiarize yourself with a videogame called Kerbal Space Program. It is a space program simulator, not perfect, but VERY realistic and intense in physics. It presents you with a fictional stellar system, and it happens to have a couple natural satellites which are not perfectly spheroidal. The smaller one, Gilly, is actually a captured asteroid. From the KSP Wiki:

Gilly's odd shape and fast rotation give the surface near the peaks an apparent velocity near 5 m/s.

Also, from playing the game, I can tell you that from anywhere you stand on Gilly it seems like you are on midway through a mountain. But a banana-shaped world would feel different.

Anyone living in a banana world that rotates as fast as the Earth will notice this. The surface of the Earth has a linear speed of 460m/s due to rotation - we don't feel it because everything is moving at the same speed.

But think of a banana shaped world as an Earth like world with two gigantic mountains, one on each side. If the mountains are at the equator, and they are as tall as the altitude of a geosynchronous orbit, they will each be ~35,000 km high - about 5 times as long as the Earth is thick - and their linear speed will be around 3 km/s, which is 6.6x faster than the speed of Earth's surface on the Equator. The faster speed combined with the lower gravity means that people closer to the tips who are not tethered to the ground will fall off into space in a fraction of a second.

Since gravity is stronger in the middle, that's the only place where you will find an atmosphere. Since you have two mountains as thick as the Earth stuck into that atmosphere, you will have such turbulence that the whole world will be permanently ravaged by off-the-scale hurricanes. Tides will also be really extreme around those mountains. And each mountain will also block sunlight for a whole hemisphere until it's about noon on that mountain, so the world will be colder than Earth. The world will always be on the edge of inhabitability because of these features.

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    $\begingroup$ "The number in its name means it's the 433rd largest asteroid in the asteroid belt between Mars and Jupiter." Not at all. It means it's the 433rd asteroid discovered. astronoo.com/en/articles/asteroids-list.html shows that for the asteroid belt, it goes 1, 2, 4, 10, 87, 624, 52. Likewise, Eris, the most massive dwarf planet is 136199 Eris. $\endgroup$
    – prosfilaes
    Apr 21 at 18:41
  • $\begingroup$ @prosfilaes thanks for that, I'll fix it. $\endgroup$ Apr 21 at 19:45
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Forget gravity and habitable zones and atmospheric composition - your banana planet probably lacks a magnetic field (unless it has a rotating metal core or field-generating device), so stellar+cosmic radiation and CMEs are major concerns. Not only do they hurt people, but they also destroy/remove atmosphere.

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  • $\begingroup$ You're right about radiation. I'll have to hand wave these guys some heavy duty space suits and habitat modules. Loss of atmosphere isn't a problem; while people will mostly assume that the banana is ancient, it's a little green when people first land but it's actually going to ripen over the course of months and then rot, leaving the question of when it came into existence in the first place. $\endgroup$ Apr 21 at 20:44
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Ripened banana-planet-matter is denser

One comment by YowaneHaku mentioned fixing gravitational effects by concentrating the mass at the center and saying the extremities are hollow. That's pretty arbitrary, but there's a way to make it fit with the very serious premise.

Bananas often ripen from one end, and this one happens to be ripened from the "bottom" (part furthest from the stem). And for some reason, the ripened part of the banana is 100x denser than the green parts. This means the banana rotates about its bottom (whether end-over-end or longitudinally or oscillating between the two), but more importantly for the castananaways, it means that gravity near the bottom of the banana points almost directly into the banana. Atmosphere will accumulate there as well. Trying to reach the green parts of the banana would be like climbing a vertical cliff face.

As for why the ripe part of the banana is more massive, whether the banana is continuing to ripen or was "always" in this state, whether the "bottom" of the banana is actually the "top" because of how bananas grow on trees, and whether they should be worried about the ambient ethylene gas their biologist is ranting about - those are matters for the philosophers and theologists to ponder over a nice, filling teaspoon of compressed banana.

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  • $\begingroup$ Nailed it in one. That's basically exactly what's going to happen. Everyone* is going to assume this monstrous fruit has been around for millenia but it's going to ripen within months. The engineers will be frantically trying to flare off the ethyne while their intellectual and moral superiors argue over exactly that sort of thing, whether the Top should have been called the Bottom and so on. *Except some of the theologians, who will mostly be insufferably smug. $\endgroup$ Apr 21 at 21:07
  • $\begingroup$ +1 for mass in the center, nice find to solve some gravity issues without the need for cavities $\endgroup$
    – Goodies
    Apr 22 at 0:12
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To start with, I suggest watching the Veritassium video on the rotation of wing-nuts:

https://www.youtube.com/watch?v=1VPfZ_XzisU

Basically, you need to pick whether your banana rotates around its smallest or largest axis. If it rotates across the intermediate one, it'll flip back and forth.

For weather patterns, you might want to consider Jinx, the Easter Egg planet, as envisioned by Larry Niven: https://news.larryniven.net/concordance/content.asp?page=The%20Origin%20of%20Jinx

The reason for it is different (and just as wrong as your giant banana), but it identifies that the air and water would naturally attempt to pool around the equator. Th actually happens with Earth, too, but it's due to rotational momentum, and the difference is about 50 feet.

This suggests that, even with even gravity across the surface of the banana, rotation would pull the air and water to the tips and the convex middle. Humans could survive in a ring around the convex extremity, but the air would probably leak off of it at the points. More magic needed to keep it in place, I guess.

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    $\begingroup$ Man, you beat me to referencing that Veritasium video by 10 freaking minutes. Another commenter suggested chaotically changing the axis of rotation which made my wingnut thoughts rotate towards that video. Loss of atmosphere isn't a problem. It's a nice fresh banana, not an old one, so it hasn't had enough time to lose its atmosphere. $\endgroup$ Apr 21 at 21:16
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Donuts and banana-phones

This question reminded me of this other one about torus-shaped donut-shaped planets. Amazingly enough, a donut-shaped planet is stable under the right (albeit extraordinary) circumstances. One of the webpages linked from there, https://itzhakts.wordpress.com/2014/02/05/torus-earth/ , even offers fancy graphics about the strength of the gravity field of such a theoretical toroidal donut-shaped planet:

gravitational field of toroidal planet

The general idea is that such a planet spins fairly quickly (for the mass required for earth-like gravity, the figures vary between 2- and 4-hour-long days): the centrifugal force keeps the system in equilibrium.

But, you want to live on a banana. Not a banana-filled donut covered in sprinkles. And a donut is not shaped like a banana.

But, we can use a banana to approximate a section of the donut. The only thing needed would be some kind of "counterweight" in order to position the center of rotation of the banana so that the radius of curvature of the banana kinda sorta matches the radius of rotation.

Therefore I bring to you: the banana-phone binary planetary system:

bananaphone binary system

The banana is not floating alone in space but, rather, the banana-phone behaves as one body: its mass distribution kinda sorta approximates that of an ideal torus donut, and both the banana and the phone rotate around the center of mass of the banana-phone at the same rate to provide 2- to 4-hour long days.

Of course, the phone can be replaced by other object(s), but they should be massive enough to shift the center of gravity of the system. However, the more you deviate from a generally-donut-shaped mass, the more you'll have to hand-wave.

I'll point out that there can be a small moon (a coin?) bobbing up and down through the center, and that daily phone eclipses are going to be a thing to people on the inward side of the banana.

I welcome anyone raising problems relating to atmospheric composition or ground instability that are likely to arise as the banana ripens,

Since the gravity is stronger at the thicker parts of the banana, and weaker at the tips, the banana would be sloooowly squished. After all, the system would be pushing itself into a state of lower energy trying to achieve hydrostatic equilibrium. In other words: it would slowly turn into a donut. I can imagine pulp-quakes as the thicker parts are squished by their own weight and pulp is expelled through both rupturing tips.

Note that the atmosphere, if any, would envelop the whole system and not just the banana. The banana tips would be a hazard zone of high atmospheric pressures and unstable expelled pulp.

Other than that, I recommend reading through to the write-ups of donut-shaped planets linked before.

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  • $\begingroup$ I'm sticking with a banana but this is a great answer and if I wasn't hellbent on a banana you might well have persuaded me. $\endgroup$ Apr 24 at 11:20
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    $\begingroup$ OK, OK, but listen to this: What about TWO BANANAS? $\endgroup$ Apr 24 at 13:40

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