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So I have a planet with tectonic plates that are separated by massive canyons that reach all the way down to the mantle. Obviously this means that the planet is very unstable and violent, experiencing earthquakes and volcanic eruptions regularly. Also, it rotates so fast that it has a ten (10) hour day/night cycle, and orbits its star at high speed. Its other characteristics aren’t really important for the question, but suffice it to say that this planet is not exactly a tourist destination.

So how can this planet hold itself together? Will it need to be abnormally large or made of some exotic materials?

I would like to have it relatively close to Earth’s gravity, but heavier is OK as long as it is survivable and possible to leave the planet. The planet also doesn’t have to have any sort of atmosphere or surface water, so no worries there.

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    $\begingroup$ "as long as it is survivable and possible to leave the planet. The planet also doesn’t have to have any sort of atmosphere". I think you should make your mind clear... $\endgroup$ – L.Dutch Jan 26 '18 at 20:02
  • $\begingroup$ Survivable in the sense that the gravity won’t immediately crush you like an empty soda can. So if a human could stand on the surface in a protective space suit with its own air supply and not immediately die, that’s what I want. $\endgroup$ – Nick Jan 26 '18 at 20:18
  • $\begingroup$ The "massive canyons that reach all the way down to the mantle" are merely the ultimate rift valleys with added vulcanism. As answered below by @Renan it only needs Earth normal gravity to hold the planet's mass together. Even lower or higher gravity will work as well. An atmosphere is probable & possibly surface water too. Just keep the tourists away. $\endgroup$ – a4android Jan 27 '18 at 7:50
  • $\begingroup$ Only "issue" the faster rotation causes is that launching things from surface to orbit from the equator will be easier. Well, it will also affect tidal currents and diurnal winds but those are generally strictly local issues. $\endgroup$ – Ville Niemi Jan 27 '18 at 9:29
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(...) orbits its star at high speed.

High speed - relative to what? If you mean faster than Earth's orbit around the sun, then either one or both of these affirmatives is true for your planet:

  • Its orbit is closer to its star than the Earth's orbit is close to our sun;
  • Its star is more massive than the sun.

So how can this planet hold itself together?

The same way that all other planets hold themselves together - through gravity:

According to the definition of planet adopted by the International Astronomical Union in 2006, one defining characteristic of planets and dwarf planets is that they are objects that have sufficient gravity to overcome their own rigidity and assume hydrostatic equilibrium.


Will it need to be abnormally large or made of some exotic materials.

No. It kinda matches what Earth should have looked like in its first half-billion years, give or take a hundred million years.

Seriously. The link above is for the Hadean era of Earth's geological history. Right after that we had the Eoarchean, which is "the first era of the Archean Eon of the geologic record for which the Earth has a solid crust". For some time during the Hadean and its transition to the Eoarchean, Earth had lots of mantle exposed.

So you see, your planet could be made of the same materials as the rock planets from our solar system.

Another interesting thing: Earth's rotation period is actually becoming longer. The slowing of Earth's rotation is mostly loss of energy due to tidal effects from the Moon. Since the Moon was actually formed around the Hadean period, Earth's spin should be much faster back then - young Earth could probably have the same sol as your planet.

I would like to have it relatively close to earth’s gravity, but heavier is ok as long as it is survivable and possible to leave the planet. The planet also doesn’t have to have any sort of atmosphere or surface water, so no worries there.

No problem there. So far what your planet seems to me is like a geologically active, tectonic plated, faster rotating version of Venus.

For something smaller, you may think of Io. It is very active geologically. You could scale it up and give it tectonic plates and it would match what you want as well.

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  • $\begingroup$ I think the massively deep "canyons" are the only thing missing from your explanation. $\endgroup$ – Michael Richardson Jan 26 '18 at 19:50
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    $\begingroup$ @MichaelRichardson when I say that the OP's planet matches our Earth on its first few hundreds of millions of age, it is because it does. I have included a link to the wiki for the Hadean era of Earth's history. Right after the Hadean, we had the Eoarchean, which is "the first era of the Archean Eon of the geologic record for which the Earth has a solid crust" In the transition from one period to the other you would expect to have lots of exposed mantle. $\endgroup$ – Renan Jan 26 '18 at 20:19
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    $\begingroup$ @Len, it won't be able to sustain life as we know it. Also notice that the question finishes with "The planet also doesn’t have to have any sort of atmosphere or surface water, so no worries there" - I think any visitors are expected to bring their own life support. $\endgroup$ – Renan Jan 26 '18 at 21:01
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    $\begingroup$ @Renan, exactly! I just need a planet that people can exist on with suitable and protection and life support. As long as they don’t instantly die as soon as they land it’s all good. $\endgroup$ – Nick Jan 27 '18 at 1:40
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    $\begingroup$ doesn’t have to have any sort of atmosphere or surface water, I imagine any body with "earth like mass" will have at least some atmosphere. And the more volcanic the more atmosphere it probably will have. $\endgroup$ – ArtisticPhoenix Jan 27 '18 at 3:03

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