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I seek to have one of the setting's worlds be smaller than earth but is also conveniently earth-like in its gravity, 1g at the surface. A possible solution to this has been to have its core be made of osmium instead of iron and nickel, but I don't know how such a core would affect the rest of planet. My understanding of osmium is that it's rather toxic but whether that would bleed upwards and make the crust utterly inviable for life or have any other kinds of weirdness I'm uncertain. It can be assumed that the rest of the planet is made of more common/normal/expected materials with an earth-like atmosphere and with water content that has been adjusted to account for the smaller size of the planet.

How would a planet having an osmium core affect life living on it?

'Life living on it' can also mean colonists trying to establish a colony should the osmium core make it unlikely life will ever naturally come about on it.

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    $\begingroup$ instead of adding a ton (not literal) of a certain element, just up the amounts of Iron Nickel or radioactive dense elements already present in Earths core. Depending how much smaller you want the planet to be than earth. Someone more skilled in the math than I may help with this. The only major effect I can see is the shorting of time it may take for the core to cool down (thus the magnetic dynamo) by a billion years or so. $\endgroup$
    – Gillgamesh
    Nov 17, 2021 at 18:30
  • $\begingroup$ I am led to believe that there is already enough gold in the Earth's core to form a sphere roughly 65km in diameter. $\endgroup$
    – EvilSnack
    Nov 17, 2021 at 21:56
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    $\begingroup$ Don't set your heart on osmium just due to its surface environment density. There is argument for Iridium being denser due to crystal structure. However, we know materials change crystal structure depending on pressures. There are around19 types of water ice. With that you can speculate on some heavier ones that would work. Adding in other heavy elements or speculate on their crystal structures. Then combine them with some more common things. This article talks about exotic rocks: nature.com/articles/s41467-021-26403-8 $\endgroup$
    – David S
    Nov 17, 2021 at 22:51
  • $\begingroup$ @Lemming in my answer I try designing planets with iridium cores surrounded by normal Earth materials, trying to design planets with as low a surface gravity, and as high an escape velocity, as possible. So far the closest I came are surface gravity 0.96 g and 5.232 kilometers per second escape velocity, or surface gravity 1.28 g and 6.679 kilometers per second escape velocity. $\endgroup$
    – M. A. Golding
    Nov 18, 2021 at 3:47
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    $\begingroup$ Do humans need to survive here, or only life that's native to that planet and/or its solar system? Several answers mention toxic gases, but it might not be toxic to native life. After all, we live in a highly oxygenated atmosphere, which would have been deadly to all the life that was on Earth 2.4 billion years ago. $\endgroup$
    – N. Virgo
    Nov 18, 2021 at 9:07

4 Answers 4

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With an entire planetary core of osmium (even half of 5.9 * 10^21 tons is an absurd amount), diffusion, geological, and tectonic processes guarantee that large quantities of osmium compounds are present on the planetary crust and surface, much like compounds of the iron and nickel that make up the Earth's core are plentiful on the surface of the Earth.

Note also that osmium tetroxide forms by reaction of osmium with plain old oxygen, sublimes into a gas at room temperature, and is toxic in very small quantities. The "sublimation" part is important since any formed in the core or mantle will migrate towards the surface since a gas is less dense than the surrounding material and, along with that, anything that forms at the surface will stay in the atmosphere. Assuming an Earth-like nitrogen-oxygen mix in the atmosphere, the presence of significant quantities of this toxic gas seems probable.

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  • $\begingroup$ The Wikipedia article says that the max permissible exposure over 8 hours is an average of 2µg/m^3. With constant exposure, it would be reasonable to assume that the "very small quantities" is somewhere in the neighborhood of 1 part per billion of the atmosphere. $\endgroup$
    – Rob Watts
    Nov 17, 2021 at 22:16
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I think the biggest impact will depend on whether osmium, at the temperature and pressure of the core the planet, would allow for the onset of a planetary magnetic field.

Based on what we know, a planetary magnetic field helps protecting the planet atmosphere from depletion caused by the interaction with the stellar wind, which in turn will allow life to evolve (if the other conditions for this to happen are met). Basically it is the difference between Mars and Earth.

Due to its high density, I doubt osmium would get in big quantities in the upper layers of the planet geological structure.

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  • $\begingroup$ Tungsten is only "15% less dense" than osmium; same question: magnetosphere or not? "On the other hand, the needle is quite unmistakably attracted to a magnet, which it should not be if it were solid osmium. It might be plated, or it might be one of several osmium/iron alloys that are discussed in connection with phonograph needles." theodoregray.com/periodictable/Elements/076/index.html $\endgroup$
    – Mazura
    Nov 17, 2021 at 22:39
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    $\begingroup$ @Mazura whether a dynamo is possible is an entirely separate question to whether the material is attracted to a magnet. It's pure coincidence that the iron in Earth's core happens to be both a ferromagnetic element (it's actually not ferromagnetic at those temperatures) and an electrically conductive fluid (which is what's required for a dynamo). Dynamos are also possible with sodium, metallic hydrogen, salt etc.. $\endgroup$
    – leftaroundabout
    Nov 18, 2021 at 14:45
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Short Answer:

Because osmium reacts with oxygen to form the highly toxic osmium tetroxide, I substituted Iridim, which is almost as dense as osmium and much less toxic, in my calculations. Unfortunately, iridium is about as rare as osmium.

A planet made of solid iridium with only a thin surface layer of other maderials would probably have too high a surface gravity for humans to tolerate. So it would not be suitable for humans or any beings used to Earth gravity to settle on. On the other hand, a planet with an oxygen rich atmosphere would need a high enough escape velocity to retain that oxygen for geologic eras of time.

So I had to find online calculators for both the surface gravity and the escape velocity of planets I designed.

I designed planets with varying abomunts of iridium in the core and materials with the same average density as Earth surrounding the core. That is a sort of an over simplification, but I figured If I could design a fairly plausible planet that way someone who is more expert in planetology could do more expert calculations. And of course a planet with a large amount of iridium is not very realistic.

I designed a planet with iridium with 0.01 the mass of Earth and 0.0024467 the volume of Earth, and materials with the average density of Earth in 0.09 times the volume and mass of Earth, for a total mass of 0.1 of Earth and radius of 0.280497 Earth, or 1,787.0514 kilometers, and diameter of 2,574.1028 kilometers. That planet would have a surface gravity of 1.28 g and an escape velocity of 6.679 kilometers per second.

Accepting the surface gravity limit of 1.25 to 1.5 g and the minimum escape velocity of 6.25 kilometers per second from Habitable planets for Man Stephen H. Dole, 1964,

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

I decided that the surface gravity of 1.28 g was a little high but acceptable and the escape velocity of 6.679 kilometers per second was on the low end but acceptable.

Anywone who wants to calculate a planet with a lower surface gravity and a higher escape velocity can try it.

Long Answer:

Here is a link to a question, and its answers, including mine:

https://astronomy.stackexchange.com/questions/41590/can-a-habitable-planet-be-smaller-than-0-58-earth-radii/41599#41599

You will note that PM 2Ring's comment to my answer there suggests using another heavy element as the planet's core.

Can we switch from osmium to iridium? ;) They have almost the same density (depending on the crystallisation state), but osmium is nasty stuff. Its oxide is quite toxic and rather volatile, whereas iridium is fairly inert

Since GrumpyYoungMan's answer here says that osmium would react with the common element oxygen to form a poisonous gas, osmium tetroxide, which would seep upward from the core of the planet and poison life, iridium seems like a better alternative to Osmium.

The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest metal known.[11][12] Some ambiguity occurred regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately,[13] but, with increased accuracy in factors used for calculating density, X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.[14]

https://en.wikipedia.org/wiki/Iridium

Unfortunately Iridium is about as rare as osmium, making planets with large amounts of iridium about as impossible to form naturally as those with Osmium, and about as difficult for an advanced civilization to build.

The planet Earth has an average density of 5.513 grams per cubic centimeter (which of course is partially the result of gravitational compression of matter deep in its interior) while Iridium has a density of 22.56 grams per cubitc centimeter. That is 4.0921458 times the average density of Earth.

In my answer at https://astronomy.stackexchange.com/questions/41590/can-a-habitable-planet-be-smaller-than-0-58-earth-radii/41599#41599 I quoted from a source which gives minimum masses for habitable planets calculated various ways as 0.1 Earth mass, 0.i2 Earth mass, and 0.23 Earth mass. The authors concluded that worlds with 0.25 Earth mass are at about the lowest possible mass for habitable worlds, though various factors could drive the limit up or down in specific cases.

So in my answer I caculated the radii of planets which were almost entirely made of osmium and had masses of 0.1 and 0.25 that of Earth.

But iridium would not be nearly as toxic as osmium.

So a hypothetical iridium planet with 0.1 times the mass of Earth would have a volume which was 0.1 times that of Earth divided by 4.0921458, and a hypothetical iridium planet with a mass of 0.25 Earth mass would have a volume which was 0.25 times the volume of earth, divided by 4.0921458. Thus a 0.1 Earth mass planet would have 0.024437 times the volume of Earth, and thus 0.180019587 times the radius of Earth, and a 0.25 Earth mass planet would have 0.061926 times the volume of Earth, and thus 0.245430278 the radius of Earth.

So a hypothetical iridium planet with 0.1 times the mass of Earth would have a radius of about 1,146.9042 kilometers, and diameter of about 2,293.8084 kilometers, while a hypothetical iridium planet with 0.25 times the mass of Earth would have a radius of about 1,592.75 kilometers, and diameter of about 3,185.5 kilometers.

The hypothetical Iridium planet with mass of 0.1 Earth would have a surface gravity of 3.09 Earth gravity or g, and an escape velocity of 8.337 kilometers per second.

The hypothetical Iridium planet with mass of 0.25 Earth would have a surface gravity of 4.01 g, and an escape velocity of 11.29 kilometers per second.

https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc

https://www.omnicalculator.com/physics/escape-velocity

Those escape velocities should be sufficient to retain a oxygen nitrogen atmosphere in case a writer wants to make the planet habitable for humans. But the surface gravities would be too high for humans to endure for more than minutes or hours. Humans would never colonize such planets or visit them for long without anti gravity, nor would human like intelligent beings evolve on them.

However, native plants and land animals should have no trouble evolving adaptions to the high surface gravity, and thus native intelligent beings shaped like centaurs, elephants, catapillars, etc. should manage to evolve on a hypothetical irridum planet with a relatively thin layar of more common elements necessary for life.

Let's try a planet with 0.05 Earth mass in Iridium and 0.2 Earth mass in stuff with the same average density as Earth. The irridium core would have 0.05 times the volume of Earth divided by 4.0921458, or 0.0122185 the volume of Earth while the other 0.2 of Earth's mass would have 0.2 of Earth's volume. so the total would be 0.2122185 of Earth's volume.

Such a sphere would have 0.3700 the radius of Earth, or a radius of 2,357.27 kilometers and diameter of 4,714.54 kilometers. and it would have a surface gravity of 1.83 g and an escape velocity of 9.195 kilometers per second.

The surface gravity would still be too high.

Suppose that the planet had 0.05 of Earth's mass in iridium and 0.25 Earth's mass in lighter elements with the same density as the Earth. The irridium core would have 0.05 times the volume of Earth divided by 4.0921458, or 0.0122185 the volume of Earth while the other 0.25 of Earth's mass would have 0.25 of Earth's volume. so the total would be 0.2622185 of Earth's volume.

The planet would have a radius of 0.39706 Earth's radius, or 2,529.6692 kilometers, and a diameter of 5,059.3384 kilometers. It would have a surface gravity of 1.91 g and an escape velocity of 9.723 kilometers per second.

Suppose the planet has 0.025 of Earth's mass in Irridium and 0.275 of Earth's mass in materials with Earth's average density. The irridium core would have 0.025 times the volume of Earth divided by 4.0921458, or 0.0061092 the volume of Earth while the other 0.275 of Earth's mass would have 0.275 of Earth's volume. so the total would be 0.2811092 of Earth's volume.

The planet would have a radius of 0.4063 of Earth's radius, or 2,588.5373 kilometers, and a diameter of 5,177.0746 kilometers. It would have a surface gravity of 1.82 g and an escape velocity of 9.612 kilometers per second.

Suppose that the planet has 0.01 of Earth's mass in iridium and and 0.29 of Earth's mass in materials with Earth's average density. The irridium core would have 0.01 times the volume of Earth divided by 4.0921458, or 0.0024437 the volume of Earth while the other 0.29 of Earth's mass would have 0.29 of Earth's volume. so the total would be 0.2924437 of Earth's volume.

So the planet's radius would be 0.417766698 of Earth's radius, or 2,661.591 kilometers, and the diameter would be 5,323.182 kilometers. The surface gravity would be 1.72 g and the escape velocity would be 9.48 kilometers per second.

Suppose that the planet has 0.01 of Earth's mass in iridium and and 0.39 of Earth's mass in materials with Earth's average density. The irridium core would have 0.01 times the volume of Earth divided by 4.0921458, or 0.0024437 the volume of Earth while the other 0.39 of Earth's mass would have 0.39 of Earth's volume. so the total would be 0.3924437 of Earth's volume.

So the planet's radius would be 0.45418165 Earth's radius, or 2,893.5909 kilometers, and the diameter would be 5,787.1818 kilometers. The surface gravity would be 1.94 g and the escape velocity would be 9.01 kilometers per second.

After some more calculations, I tried a planet with 0.01 Earth mass in iridium and 0.04 Earth mass in substances with the average density of earth, for a totalof 0.05 Earth mass and 0.0424437 Earth's volume. Such a world would have a radius of 0.228539075 Earth radius, or 1,4560219 kilometers, and a diameter of 2,512.0438 kilometers.

And it would have a surface gravity of 0.96 g, and an escape velocity of 5.232 kilometers per second. The surface gravity would be great, very close to Earth's, but the escape velocity would be a bit too low, oging by the minimum of 6.25 kilometers per second in Habitable Planets for Man, Stephen H. Dole, 1964.

So the next design I tried was a planet with 0.01 Earth mass in iridium with a volume of 0.0024437 of Earth's volume, and a mass of 0.09 earth mass in material with the average density of Earth. That planet would have 0.1 times the mass of Earth and 0.0924437 the volume of Earth. It would have 0.280497872 the radius of Earth, 1,787.0514 kilometers, and a diameter of 2,574.1028 kilometers.

It would have a surface gravity of 1.28 gravity, which is more or less within acceptable limits, and an escape velocity of 6.679 kilommeters per second,w hich should b ehigh enough to be acceptable.

And no doubt trying different figures can come up with a planet with a lower surface gravity and a higher escape velocity.

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The core of a planet is so deep it's hard to imagine it having an impact on life

With any terrestrial planet, the structure will be that there is the crust, the mantle, a molten outer core, and a solid inner core.

The mantle can be divided into the upper and lower mantle. The lower mantle is about 670 kilometers before the Earth's surface. The molten outer core is about 2900 km below the Earth's surface, and the inner core is 5100 km below the Earth's surface.

So let's assume this fictional planet has a core made almost entirely of osmium.

That inner core is surrounded by a massive layer of molten rock (the outer core), which is surrounded by layers of solid rock (the mantle). Then all that is wrapped up by the Earth's crust, and somewhere at the very outermost layer of the Earth's surface, you have the soil where all the animals and plants reside.

My point is that a core is surrounded by so other material that its composition probably doesn't matter at the end of the day. Even if the core is made of osmium, it's not as if said osmium is just going to bleed towards the surface. It's too deep.

If the gravity was allowed to be different from the Earth I might have suggested that a denser core might lead to a change, but you mentioned this planet has the exact same gravity as Earth, and I'd assume a slight change in composition wouldn't lead to a significant change in mass anyways.

I think it's safe to say life would not face any issues or major changes if the composition of the core changed to osmium.

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    $\begingroup$ By this logic, all of the uranium on Earth should be in the core. Yet it isn't. $\endgroup$
    – Jon Custer
    Nov 17, 2021 at 19:52
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    $\begingroup$ @JonCuster But osmium is highly siderophilic, so you would actually expect to have more of it in the core and mantle relative to the crust, even without mass fractionation. $\endgroup$
    – Charles
    Nov 17, 2021 at 22:45
  • $\begingroup$ @Jon Custer I'm not sure I understand your argument. Could you please elaborate? $\endgroup$
    – Nyctophobia457
    Nov 17, 2021 at 22:48
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    $\begingroup$ The thought process is: what would happen to a pure Os inner core surrounded by an Earth-like Fe-Ni liquid (you probably want a magnetic field?). Osmium is quite happy to dissolve in Fe or Ni liquid, so some of it will and then diffuse through the liquid. Upon reaching the mantle, one finds that reasonable amounts of Os can be accommodated in the various solid Fe allotropes (in the 10-30 at.% range), and there are various Os-Si phases. Both these would act to carry Os into the mantle. With a huge chunk of Os at the center, even if highly siderophilic, you will have some on the surface. $\endgroup$
    – Jon Custer
    Nov 17, 2021 at 22:59
  • $\begingroup$ Some, but would it be enough to impact the health of an ecosystem as the original question was suggesting? $\endgroup$
    – Nyctophobia457
    Nov 17, 2021 at 23:26

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