I want to explore the possibility of a planet that is extremely misshapen. If we were to extrapolate what we have observed from our solar system and the extra-solar planets discovered so far, we would have to be conservative about such a possibility because nothing of the sort has been discovered. Moreover, our understanding of physics tells us, all else equal, the more massive something is the more spherical it becomes (not a perfect sphere typically). This phenomenon is easily seen in asteroids since they are less massive than planets, they can take on a Siamese twin shape:

Itokawa Asteroid, discovered 1998 by LINEAR

Now if we were considering very small dwarf planets, this would be a piece of cake, because like asteroids, the mass can be small for misshapen bodies. However, I'm interested in Siamese twins that are . So with all of the aforementioned odds stacked against this idea, I still wonder if somewhere in the vast, vast universe there might be a freak accident of astronomical proportions that could allow for a planet to have a Siamese twin. A few ways I thought of to approach are below. The bold face denotes the part of the idea that I feel needs the most of a :

  • the Siamese twin planets appears big, but have hollow cores, allowing for its mass to be small enough for the planets to be misshapen.
  • the Siamese twin planet shape is maintained via equilibrium of forces. Something is pulling the planets apart at just the right amount against gravity for the Siamese twin shape to exist. Maybe a rogue, black-hole at just the right distance? enter image description here

Question: Can any of my ideas for my Earth mass Siamese twin planets to take form be plausible? If you so desire, feel free to hypothesize your own set of "perfect storm" circumstances.

Success Metric: Create Earth mass Siamese twin bodies (can't really call them planets any more, because they are not round) that remain so for at least 100 million years (in other words, it doesn't have to be permanent). If the solution is statistically remote, that's ok. The key is just not violating any known laws of physics. Further Clarifications:

  • Mass: both twins have near Earth mass

  • Fusion Onset: can be at formation or later on

  • Area of Fusion: equatorial region of planets
  • Fusion Degree: configurable. Could be a slight join or a deep join. Quick reference in the diagram below:

enter image description here

  • Other: Other things like Planet composition, nearby objects are entirely configurable
  • 6
    $\begingroup$ The definition of a planet involves a body that has been rounded by its own gravity. If it is not round, it is not a planet. $\endgroup$ Jun 29, 2018 at 19:55
  • $\begingroup$ You might want to consider downsizing the images by adding an "m" before the file extension ("blahblah m .jpg"), minus the quotation marks and spaces. $\endgroup$ Jun 29, 2018 at 20:52
  • 6
    $\begingroup$ Think of an earth size planet as being a big bag of molten iron with a skin the thickness of an orange peel. Slap two of them together, and they will immediately blob together into a bigger blob or molten iron and the skin will reform as it cools. The "blobbing" process will happen in hours, although it may take years for the shock waves to settle out. $\endgroup$
    – pojo-guy
    Jun 29, 2018 at 22:13
  • 1
    $\begingroup$ related: worldbuilding.stackexchange.com/questions/4460/… $\endgroup$
    – David Cary
    Jun 30, 2018 at 14:43
  • 1
    $\begingroup$ Possible duplicate of What would happen to the Moon if it was lowered onto the Earth? $\endgroup$
    – Jasper
    Jun 19, 2019 at 23:55

4 Answers 4


TL;DR: Not likely. What you are looking for is called a contact binary. But you could have non-contact binaries.

Once they enter the slight join phase, any speed or other force keeping them apart will be outperformed by their mutual attraction. The effect of your rogue black hole can't be stable, and the relative speeds of the planets will slow down as they fuse.

A planet with enough mass will necessarily become rounded by its gravity in the short term (astronomically speaking). This is called hydrostatic equilibrium.

The gravitational influence of other bodies, or its own spin can make it more eccentric (a spheroid rather than a perfect sphere), but it will still be pretty round.

How much mass gets an astronomical body to hydrostatic equilibrium (HE)? It will depend on the planet's composition, but having a liquid interior helps a lot. Anyway, the answer is far less than Earth's.

As a reference, Saturn's moon Iapetus is the biggest object in the solar system known not to be in HE, but still round-ish:

enter image description here
Iapetus as seen by Cassini. Taken from Wikipedia

Its mass is 1/5000th that of Earth. About anything above that would become rounded.

Although a mass limit above which everything becomes in HE is difficult to obtain, it helps having a few ideas in consideration:

  • Ceres, with half of Iapetus's mass, is in HE
  • Mimas, with a 25th of Ceres's mass, has a HE shape
  • The result of the collision of two planets will necessarily generate enough heat to partly liquefy them, making the subsequent HE easier.

As for non-contact binary planets, they are possible. Bear in mind the closer they get, the more likely they are to be tidally locked.

If they get close enough, tides will slow them down and make them less able to keep distance. Once they get even closer, they'll become a contact binary, but it will be short lived in astronomical scale. their atmospheres will cause friction and they'll quickly decay into a (catastrophic) collision.

There is a number of contact binary stars, almost touching, but separated by their own pressure. After several million years they start sharing a common envelope (atmosphere-analog), but they are theorized to either expel part of their envelope, losing contact, or merge within a time frame of years (not millions, sorry).

Contact binary stars seem to be less frequent as the mass of the involved stars decrease, because they become less stable.

  • 1
    $\begingroup$ That's HUGE! You might want to consider downsizing the images by adding an "m" or even an "s" before the file extension ("blahblah m .jpg"), minus the quotation marks and spaces. $\endgroup$ Jun 29, 2018 at 20:53
  • $\begingroup$ @FoxElemental, thank you. Didn't know that $\endgroup$
    – Rafael
    Jun 29, 2018 at 22:26

Can any of my ideas for my Earth mass Siamese twin planets to take form be plausible?

No, not for an Earth mass body

the Siamese twin planets appears big, but have hollow cores

This configuration is highly unstable: the more dense material will sink toward the center of mass. So the hollow center would not last, especially after an impact which will necessarily melt part if not all of the planets

Something is pulling the planets apart at just the right amount against gravity

you start from a sphere. you can either get an ellipsoid because of rotation, or a bulged shape by constant pull from one direction.

Can any of my ideas for my Earth mass Siamese twin planets to take form be plausible?


  • $\begingroup$ I think the external pull hypothesis could be explored further. Suppose we had two Siamese twin, Earth mass ellipsoids? Maybe they are not planets in the conventional sense of the word, but if they have Earth mass and are co-joined, that's all we need for success on my question. (I stopped using "planet" to avoid confusion) $\endgroup$ Jun 29, 2018 at 20:08
  • $\begingroup$ @ArashHowaida, above a certain mass you cannot escape the spherical form... $\endgroup$
    – L.Dutch
    Jun 29, 2018 at 20:10
  • $\begingroup$ Yes, I conceded that in my question. The point is rather, if Twin A is about to collide into Twin B but a rogue black hole (or something else) pulls on Twin A at just the right position and intensity that it facilitates the Siamese twin shape. I don't think this contradicts the link between mass and spherical form. It's just a lucky circumstance that results in the Siamese twin shape, no? $\endgroup$ Jun 29, 2018 at 20:18
  • $\begingroup$ @ArashHowaida, then it will two partially molten masses, which will reach equilibrium again (spherical shape) $\endgroup$
    – L.Dutch
    Jun 29, 2018 at 20:24
  • 1
    $\begingroup$ @ArashHowaida There are very few freak accidents that "chill" planets (it's way easier to add heat than remove it). Moreover, the tidal stresses caused by the proximity of the black hole and the other mass would be more than sufficient to keep both masses molten until equilibrium was reached. $\endgroup$
    – jdunlop
    Jun 29, 2018 at 21:25

The idea has been explored in fiction. I wonder if you may be interested in examples?

Hal Clement wrote a classic of physics-based science fiction about a massive planet, fast-spinning hence very oblate and with many times greater weight felt at poles than equator: https://en.wikipedia.org/wiki/Mesklin

In 'TV Century 21' children's weekly, one story of comic strip 'Zero-X' featured a 'first known Siamese Twin planet'- two linked doughnut-shapes! (I recall seeing it then but haven't traced it further. The first issue was published on 23 January 1965; with issue #105 (21 January 1967) Zero-X was added, by Angus Allan and Mike Noble, continuing into 1968.)

Various works have featured a hollow Earth (notably Edgar Rice Burroughs' inner realm Pellucidar, with a central sun of its own and an internal moon) or other hollow worlds - for example, among Neil R. Jones' many Zoromes stories is 'The Sunless World' (Amazing Stories, December 1934).

Happy reading!


Yes. It is called a Rocheworld, or Contact Binary

The earliest fictional example I know of is from physicist and hard-SF novelist Dr Robert L. Forward, who uses the term Rocheworld. The term Contact Binary also exists, though it is more commonly used for stars.

Here is another example made by another physicist, Dr Luke Campbell. Side-note: Vergeworlds is a quite fascinating hard-SF setting, and I recommend looking at it for both its tech, Fermi-compatible aliens, and Relativity-compatible FTL method (two words: "causality attack").

The reason it works despite being in each-other's Roche limits is because Roche Lobes and, at those scales, the fluid nature of the bodies mess things up.

There are two interesting consequences to keep in mind, on such a world. Despite the variable gravity, it is always normal to the surface: the ground is always horizontal, even in the weird region where the two lobes are touching. And the atmosphere has the same pressure everywhere: if the minimal surface gravity zone is too weak to keep atmosphere, there won't be any atmosphere on the entire surface.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .