Super-Earth Scenario

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An idea for a sci-fi race was bobbing around my head, it's planet was bigger than earth and I was wondering, realistically how big could a terrestrial or 'rocky' planet get before becoming a gas giant? This isn't hardcore sci-fi though, a few stretches could be made, we can just assume they can cope with the gravity and so on.

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According to Wikipedia, the basic definition of a Super-Earth planet would be a planet with a mass between 2 and 10 Earth-Masses (some sources suggest 5 Earth masses as the proper lower-limit for a Super-Earth world). While I don't think there is a proper convention for the radius of a Super-Earth planet, I suspect even the more massive worlds would have a radius not much larger than twice that of Earth's. So, assuming 10 Earth-masses and a radius of 2, you'd get an approximate surface gravity of 2.5 gees. Heavy, but there's no technical bar to life evolving in such an environment, provided it has enough energy.

Expect a thicker atmosphere, though, and likely a higher air pressure at sea level. Beyond that, conditions could vary wildly, from mostly dry all around to a global ocean. No idea how plate tectonics would behave on such a world or even if plate tectonics would still work on so massive a planet.

Others can probably tell you more than I, but I hope this helps.

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According to Wikipedia, the basic definition of a Super-Earth planet would be a planet with a mass between 2 and 10 Earth-Masses (some sources suggest 5 Earth masses as the proper lower-limit for a Super-Earth world). While I don't think there is a proper convention for the radius of a Super-Earth planet, I suspect even the more massive worlds would have a radius not much larger than twice that of Earth's. So, assuming 10 Earth-masses and a radius of 2, you'd get an approximate surface gravity of 2.5 gees. Heavy, but there's no technical bar to life evolving in such an environment, provided it has enough energy.

Expect a thicker atmosphere, though, and likely a higher air pressure at sea level. Beyond that, conditions could vary wildly, from mostly dry all around to a global ocean. No idea how plate tectonics would behave on such a world or even if plate tectonics would still work on so massive a planet.

Others can probably tell you more than I, but I hope this helps.

As for the atmosphere would it be too much of a stretch to say when it was forming the area it was in was gas-poor so that's why the planet is huge, but didn't become a gas giant? Also could it be bigger in terms a radius or will it just compress down after a certain point?

Any ideas?

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It depends on its composition and temperature. Uranus has 14 earth masses and is pretty much all gas with a small rocky core.

But hotter, and with far less volatiles, you could have a planet with the same mass, with solid surface, water, and a thick but breathable atmosphere ... and 2.4 gees on the surface = you could move only on a reinforced wheelchair there, but the local fauna would be adapted.

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Very close to the sun, and formed from a very heavy element rich nebula, you could have something bigger, utterly uninhabitable, but nonetheless terrestrial with solid or molten rock surface. the upper limit is most probably size where its gravity well can contain volatiles all the way up to boiling point of carbon so it would either keep all volatiles and be a gas giant or be evaporated by the sun completely.

Edited by MBobrik

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As for the atmosphere would it be too much of a stretch to say when it was forming the area it was in was gas-poor so that's why the planet is huge, but didn't become a gas giant? Also could it be bigger in terms a radius or will it just compress down after a certain point?

Planets generated close to the sun or probably late in the process would not have much hydrogen and helium who are an requirement for making an gas giant.

The inner planets was generated by 50 or something bodies who was mostly mars sized. Size to weight also depend on the amount of metal over rock. Moon has an small metal core, mercury has an huge.

One other option is an water world, an earth like core and a ocean thousands of kilometer deep

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It's not the size of the object that turns it into a gas giant, it's the density. It is quite possible that an immense, rocky planet can form AND have a similar gravitational field as Earth's.

And like Magnemoe said, Mercury has a higher density compared to Earth's, and it could easily be the other way around for other planets.

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Here is a better way for me to put it: The factors are Size, mass, density, and gravity.

I don't care if it has really high gravity, in fact it would be better since the inhabitants are really strong.

I want the size to be big.

As for mass and density those are going to be high (compared to earth at least) because if it's really big and has high gravity it must have a lot of mass. And all of the mass is going to be under pressure from the high gravity, so at least going to have the same if not more than earth.

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Well, I suppose my next questions would be: how big do you want it to be? How much gravity are you looking for? If we have a number, we might be able to work forward from that.

I've been doing some reading and there does seem to be a clear relation between the mass of a planet and its radius. The details can be found in this PDF: http://www.gps.caltech.edu/~jackson/pdf/SotinJacksonSeager2010_Exoplanets.pdf but I'll skip to the essential component for our discussion. This equation determines the relationship between the mass and radius of a planet:

R/R(Earth) = a * (M/M(Earth))^b

The variables "a" and "b" are coefficients that change depending on the kind of planet. If you look at the PDF, Table 5 carries the coefficients for Terrestrial planets, ocean planets, and "super-mercury" planets.

There are other equations in the PDF - some of them quite complex - but this one should serve our purposes for now.

One other thing: one of the reasons why planets higher than 10 Earth masses are likely to become gas giants (even miniature ones like Uranus and Neptune) is because their higher gravity will trap lighter molecules like hydrogen. On worlds like Earth, hydrogen rarely sticks around; once it gets into the upper atmosphere, it can be excited to speeds exceeding escape velocity just by being exposed to sunlight, and so it's lost to the planet forever. Heavier worlds can trap hydrogen (and helium, and other light gasses) and so accumulate thick atmospheres.

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I've messed around in US, and earth 4 is probably going to be as big as it's going to get as believability will allow. But who knows? FYI one of the factors must be locked so all have the same density as the regular earth, which probably won't happen, but maybe I can punch in the right number if some one could find it for that.

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