I'm interested about whether or not and how gravity would be affected on deformed planets. You could think of this as whether or not mega-asteroids would be able to sustain life? If so, would different parts of the world contribute to different ecosystems due to the shape of the planet, or would life evolve differently in different areas due to the gravity?

I'm thinking of making a planet that was conjoined by another planet, forming a peanut-shape in appearance. The idea is that these planets formed around same time, and in a similar orbital distance so that over time they would eventually, and passively collide and conjoin with one-another? Similar to this link but rather about the specifics of a peanut-shaped world.

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    $\begingroup$ A peanut-shaped asteroid would indeed have varying gravity. A peanut shaped planet (big enough to hold an atmosphere) is not realistically possible (as your link suggests). $\endgroup$
    – Alexander
    Dec 21, 2017 at 0:09
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    $\begingroup$ @Alexander what if the two planets had atmospheres before the collision? would the conditions make the atmosphere dissipate? In saying that, I now just realized that the word 'sphere' is in atmosphere lmao. $\endgroup$ Dec 21, 2017 at 0:12
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    $\begingroup$ You shouldn't accept an answer so quickly $\endgroup$
    – kingledion
    Dec 21, 2017 at 0:26
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    $\begingroup$ @kingledion okay fine I have un-accepted the answer.. For now... Pray that I don't alter the acceptance any further... $\endgroup$ Dec 21, 2017 at 0:37
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    $\begingroup$ @Alexander Not if they are rotating rapidly enough. Spheroids become unstable in favor of egg shapes or multi-lobed bodies above certain angular momentum limits. $\endgroup$ Dec 21, 2017 at 4:10

2 Answers 2


From an engineering perspective, I don't think your planet can exist. The problem is that two planets even of Mars size that came that close would ultimately collide violently, not just fuse at the outer edges. If that happened, you'd likely end up with one large planet, and a close moon of proportionately large size.

In point of fact, we believe this has already happened in our solar system, closer to home than you might realise.

In a peanut shaped planet hypothesis, the 'narrow' band between the two conjoined planets would have to be extremely dense and strong to stop the two globes from pulling more towards each other. Even assuming that this was the case, the amount of tectonic pressure on the surface of such a planet would be extreme and that would mean massive volcanic eruptions and the like on a regular basis.

Depending on which axis the rotation of such a planet occurred, there would also be problematic weather patterns and potentially even issues retaining an atmosphere in the first place. If it spun with the two globes effectively orbiting each other for instance, the angular velocity at the 'equator' points could be enough to lose atmosphere to space in this model, although I haven't run any math to prove this hypothesis because it would depend on the size and mass of both planetary 'halves' and the length of the conjoining piece.

Ultimately, I suspect such a planet would either rip itself apart and reform as a planet / moon pairing, destroying all life on the surface in the process (but potentially providing a world for new life to form on more conventionally).

But for the sake of argument
If I was to design such a planet, the only way I can see that it could have formed in the first place and maintain its form would be two celestial bodies that were approaching each other from opposite directions VERY quickly. This means that one would have to have been orbiting the sun in retrograde; perhaps an extra-solar capture? Then, instead of colliding head on, strike each other in a glancing blow.

I need to stress here that even this would likely result in a massive collision that would break apart both planets, but in this case I'm going to suspend the hard science disbelief and speculate a little.

Let's say for the sake of argument that the planets were travelling SO fast past each other that their mutual gravitational pull spins them together and converts the momentum between the two planets into angular momentum between each other. It's possible that you could end up with the planets close enough to fuse, but for the spin being generated to be sufficient to keep the two cores apart. This would cause the rotation of the planet to be on the axis describing the fusing of the two planets, meaning that they would continue to 'orbit' each other very quickly, counteracting the gravitational force pulling the two planets together.

If this happened, the planets would have to be spinning so fast that I doubt they could collect a reasonable atmosphere. That may not exclude life, but it would exclude land based complex life. That said, it's again possible that you could retain some form of ocean (water is thousands of times more dense than gas so less subject to being flung away) although this kind of rotation would lead to pooling water at both extreme ends of the 'peanut' which essentially forms a fragmented equator (this effect happens on the Earth today which is why there is not a lot of land right on the equator of the Earth). These two 'oceans' could allow complex aquatic life if you had ocean flora oxygenating the water, but you'd probably find two completely different evolutionary habitats on both peanut ends. It's unlikely (unless you can somehow put an atmosphere into the middle of the peanut) that you would have any life traversing between the two different biomes.

Additionally, the two bodies effectively orbiting each other would mean that their cores could build up a MASSIVE spin, generating an incredible magnetic field over each end of the peanut. If those fields were harmonised, it would be a very interesting field to 'see' and would further support any extremophile life on that planetary body by keeping out cosmic rays. If they were NOT harmonised (far more likely) the auroras and the like visible from the fused area of the peanut planet would be simply spectacular. You'd have a tourism industry built in right there.

Ultimately however the angular momentum would ultimately start to fade and you'd have tectonic instability and the planet would eventually collapse. Even the Earth has slowed considerably in its rotation since the event that created the Moon, showing that in time this model would prove unsustainable. That said, it could take a billion years to finally collapse. In that time, both biomes described already could have some interesting life forming in them (that would ultimately not survive the collapse)

  • $\begingroup$ Very interesting point! Thank you for your response. Just a little challenge I give you: What conditions would have to be true for this theory of making a peanut planet be at least plausible? You could think about this question as if you were making a peanut planet yourself, what and how could you make it work? $\endgroup$ Dec 21, 2017 at 0:28
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    $\begingroup$ Added my answer to the challenge in my original answer. Very speculative, but I hope you enjoy. :) BTW, while I appreciate the early acceptance of the answer, good practice is to wait for 24 hrs to let people all over the world have a crack. You'd be amazed at some of the answers you'll get in that time, and often people lose interest in questions with accepted answers. I know I'd be doing myself out of some points, but I'm here for the fun of it, not for the points. :) $\endgroup$
    – Tim B II
    Dec 21, 2017 at 0:50
  • $\begingroup$ This is awesome!! Very well detailed and thorough explanation. I've listened to your response and will be waiting to see if there's a better response in the next day or so, but right now you are the Icing on the Cake! :) $\endgroup$ Dec 21, 2017 at 1:05
  • $\begingroup$ Uh, angular momentum won't just "fade" over time without some external influence draining it away. Angular momentum is a conserved quantity. The reason the Earth's spin has been slowing is because it is transferring spin angular momentum into the orbital angular momentum of the Moon, which is slowly receding, via tidal interactions. $\endgroup$ Dec 21, 2017 at 4:39
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    $\begingroup$ And thirdly, the density of liquid has nothing to do with how easy it is to retain against loss to space. What matters is molecular mass. And water is in fact extremely easy to lose to space. The primary reason the Earth still has it is because of the tropospheric cold trap--i.e., most of it freezes out before it makes it to the exosphere where it could escape. If there's no other atmosphere, the only thing that will retain water against boiling and thermal loss is a global ice cap, like Europa has. $\endgroup$ Dec 21, 2017 at 4:44

Precisely how such a world would form is, to say the least, tricky to work out. Two planets colliding without first being captured into each other's orbit certainly wouldn't do it--at least, not in one step. Such is a collision is anything but "passive"! As Tim B suggested, it's much more likely to result in vaporizing a large portion of both planets, and creating a ring system and/or moon. If the resulting moon is sufficient large, however, such that you essentially end up with two new planets orbiting each other, that might be a good place to start. Or some other method of capturing two planets into each others' orbit. At that point, you just need some mechanism to rob angular momentum from the system, slowly drawing them closer together, until they distort under their mutual gravity and begin to merge. They have to be pretty close in size for this to work, as otherwise one body will simply break apart into a ring system when it crosses its Roche limit with respect to the other one, and draining enough angular momentum through natural processes is left as an exercise for the reader.

All in all, though, I would gloss over how the planet came to be in the first place, leaving that as a mystery (perhaps even one that you hang a lamp shade on, just so readers know you know that it's a thing, and are purposefully leaving it out rather than being oblivious), and focus on the features of the world as it is, assuming that it does exist. After all, that approach worked fine for Robert Forward in Rocheworld (aka The Flight of the Dragonfly) and sequels.

The "neck" of a peanut world does not have to exceptionally strong. In fact, it can be made of anything at all--even water, or just air (in which case, the system is conventionally known as a rocheworld, like the eponymous novel). This is because a peanut shape is in fact a gravitational equipotential surface; i.e., every point on the idealized surface is at the same gravitational potential, and doesn't want to "fall" anywhere, so no structural strength is needed to support it. That also means that water will not preferentially flow to any particular major region, and you can realistically put oceans anywhere you want. As far as the water is concerned, there is no reason to flow to the outer poles over the neck, or vice-versa. They both have the same gravitational potential.

Now you might think "that makes no sense! Surely the two lobes are gravitationally attracted to each other!" And indeed they are. The catch is that such an equipotential surface only exists if the body is rotating sufficiently rapidly, such that centrifugal force balances that mutual attraction. In effect, the two halves must orbit around each other, close enough that they are touching. And we do in fact know of astrophysical objects that act like that--they are called over-contact binary stars, and they are indeed stars shaped like peanuts (the processes that form over-contact binary stars unfortunately are not easily applicable to solid planets).

In fact, above a certain minimum spin rate, the oblate spheroid shapes that we typically associate with planets become unstable--so if you can figure out a way to start with a normal planet, and then spin it up to ridiculously high speeds, that would give you a way to form a peanut-planet.

So, the peanut planet will of necessity have very short days. Like, on the order of minutes rather than hours. As such, you are unlikely to get lifeforms adapting to the diurnal cycle like they do here on Earth.

The neck region, and generally all non-convex or inward-facing parts of the peanut (i.e., every part of the surface that is not contiguous with the entire surface's convex hull), will also tend to be colder than the outward-facing poles--and this is in addition to the usual decrease in temperatures towards the poles--because they are shaded by the mass of the lobes.

You can also expect some pretty weird wind patterns, due to the combination of high spin rate, non-trivial shape for air to flow around, and non-trivial heat distribution around that shape, but I have nowhere near the expertise to describe them precisely. At the very coarsest level, however, assuming the planet is roughly as "thick" as Earth, through the short axis of one of the lobes, I would expect more and thinner (i.e., covering fewer latitudes) major circulation cells, with stronger prevailing winds. Coriolis effect would also be intense, so you can expect lots of cyclonic storms, but not huge monolithic hurricanes (because if they got too large, they would have to cross cell boundaries).

Now, the surface is an equipotential, but that does not mean that there is equal gravitational force everywhere. Force is the gradient of potential, and while the value of the potential may be the same at every point on the surface, it's gradient is not. You would have the highest gravity at the north and south poles of each lobe, and lower gravity along the equator. Additionally, gravity will decrease from the exterior poles along the equator to the center point of the neck. The lowest gravity would be at the center of the neck on the equator, with the poles of the neck having slightly higher gravity. Depending on just how stretched out vs. squashed together this particular peanut is, the total differences could be very large (like, 2gees at the lobar poles, and a tenth of a gee on the neck), or relatively small (like, say, 1/5th of a gee difference between the highest and lowest gravity areas). The more stretched out the planet is, and thus the higher the variance, the more you would expect both animals and plants to be specially adapted to specific gravitational conditions. Note that the lowest gravity areas, however, are relatively small; the gravity drops pretty rapidly as you move from the convex inner surface of one lobe to the concave surfaces leading into the neck. While the two lobes could have significantly different average gravity between them, the total variance in gravity on one lobe, outside of the concave neck regions, will be fairly small.

Addendum: it may seem that the high rotation rate of such a planet may preclude holding a particularly thick atmosphere, since at some altitude any co-rotating air would be in orbit, and above that altitude it would be actively flung away. The trick is to ensure that the planet as a whole has a sufficiently high escape velocity, and sufficiently distant lobar Lagrange points, that you can keep a suitably deep atmosphere below the escape level, and ensure that a high fraction of air molecules that may escape one lobe nevertheless remain in orbit, to be recaptured later (this is similar to the gas torus phenomenon seen around gas giant moons, notably Io). In the case of a rocheworld, where the bridge is entirely made of air, the Lagrange points are necessarily inside the atmosphere; I am not certain how to explain in detail how such a world is capable of retaining air over the long-term, but Robert Forward thought it was possible, and I'm generally willing to defer to him on such matters. Additionally, the fact that the previously-mentioned over-contact binary stars are not universally surrounded by spiral nebulae is real-world empirical evidence that peanut-world can retain gas, even when the lobar Langrange points are inside the gas envelope; you just need a large escape velocity, which does not necessarily imply a large surface gravity, especially when you have the total mass of two lobes contributing to their mutual escape velocity, but not to each others' surface gravity.


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