Earth-like planet with 2x surface area, but same surface gravity?

[AndromedaRXJ]

How feasible is such a planet? Can the planet still be dense enough to be rocky and not gaseous?

 

[mfb]

Surface gravity is proportional to mass/radius², surface area is proportional to radius². Double the surface are and you need mass to increase by a factor 4, but the volume increases by a factor 8. That means half the density. Difficult to do with rock. An ocean world would work. A rocky core, then a thick layer of (high pressure) ice and a global ocean. Not very Earth-like, however.

 

[jim mcnamara]

Or did you mean a same mass planet with much bigger continents? Other than increasing very arid land area and decreasing open water area, it would "work". Assuming everything else is Earthlike - whatever that is.

 

[Torbert]

Couldn't a solar system form in a low iron space? Silicon and carbon are the main ingredients of many an asteroid. Form the planets without heavy metals and the mass of a larger planet could equal half the density of the smaller planets with iron cores.
Again, not very Earth like and I have to consider the consequences from a total lack of magnetic field.

 

[AndromedaRXJ]

Or did you mean a same mass planet with much bigger continents? Other than increasing very arid land area and decreasing open water area, it would "work". Assuming everything else is Earthlike - whatever that is.

I mean 2x the geometric surface area (whether it's land or ocean). i.e. The radius is some multiple of Earth's such that the surface area is double of Earth's. But the surface gravity is roughly 1g.

 

[AndromedaRXJ]

Surface gravity is proportional to mass/radius², surface area is proportional to radius². Double the surface are and you need mass to increase by a factor 4, but the volume increases by a factor 8. That means half the density. Difficult to do with rock. An ocean world would work. A rocky core, then a thick layer of (high pressure) ice and a global ocean. Not very Earth-like, however.

How deep would the ocean be compared to real-life Earth? And is it downright impossible to have continents, or just very unlikely? What about a few islands or one small continent?

Also, this is kind of a separate question from what I said in the OP, but what about an artificial planet created by an advanced civilization, and the planet has some hollowness? But it also has an ecosystem?

 

[member 656954]

For planet building questions I always come back to the main response that it's sci-fi and most readers won't know about the mechanics of mass vs. surface area vs. gravity, they'll just take it as written.

Those who know and care will have to make a decision as to whether to continue reading or not, and likely that'll hinge on how compelling the story is and how engaging the characters are. If they're invested in that, an impossibly massive planet can be put aside.

So 'yes' is the answer to your question, however you want to configure your planet, it will work if you make it work.

Good luck with the novel

 

[mfb]

How deep would the ocean be compared to real-life Earth? And is it downright impossible to have continents, or just very unlikely? What about a few islands or one small continent?

~200 km before you reach the ice, even more before you reach rock, islands or continents don't work. If some intelligence is involved then you can make floating islands of arbitrary size.

Hollow planets don't work with any known or plausible material, you would need applied magic, but if you do that you can just ignore the details and say this planet exists with these properties, done.

Couldn't a solar system form in a low iron space? Silicon and carbon are the main ingredients of many an asteroid. Form the planets without heavy metals and the mass of a larger planet could equal half the density of the smaller planets with iron cores.

I'm not sure if that would work naturally, but artificially: Sure.

 

[Nik_2213]

IIRC, having a much smaller core / mantle ratio gets you up to ~ 1½ surface area, but there may be long-term problems with magnetic field, plate tectonics etc etc. Put bluntly, internal processes soon stall out, stellar wind strips the exposed atmosphere and you end up with a 'Big Mars' at best...
YMMV.

 

[Vanadium 50]

I always come back to the main response that it's sci-fi and most readers won't know about the mechanics of mass vs. surface area vs. gravity, they'll just take it as written.

As an example, take Silverberg's Majipoor. It's much larger than Earth, and its density is lower, and it's crust is metal-poor. And that's all the geology we get.

Good enough for a Hugo nomination and a Locus. And sold about a zillion copies.

 

[Dr Wu]

"It's exogeology, Jim, but not as we know it."

 

[Dr Wu]

One or two follow-on-thoughts: if the planet concerned is thematically central to a given novel (say, Arrakis is to Dune), then the writer will probably have to roll his or her sleeves up and get stuck into the planet's physics, and do so in a fairly convincing way. Otherwise, if the planet merely functions as a useful backdrop, then perhaps just one or two (telling) details may suffice to keep the reader onboard. It's possible, of course, to have the story's characters flagging up the gaps in understanding they may have in their own comprehension about the planet's structure. Competing pet theories could be forwarded, argued over, etc. . . a case where mystery and doubt could actually add to the novel's believability, in fact. Not having all the answers has always been part of the human condition, after all. That being so, there's no reason why the future - even an advanced spacefaring one - should be exempt from this shortfall. Whatever else the future holds, you can be darn sure puzzlement will have a place in it.

 

[gleem]

Lets put in some actual numbers.

gravity =g =kM/r2 = kρr M is the total mass and r the radius of the planet and k is a constant.

Gravity increases as the radius assuming the average density ρ remains the same.

The gravity for a new radius and new density is g' =kρ' r'

If the surface area is increase by 2 then r' = r √2

Since we require g = g' then r'ρ' = rρ thus

ρ' = ρ/√2

The average density of the Earth is 5130 kg/m³ so the new density would have to be 3627 km/m³ which is obtainable with the right mixture of layers and substances.

 

[mfb]

Oh, I used 4 times the surface area in post 2. 3600 kg/m² is still very low for terrestrial planets.

 

[gleem]

The density of Mars is 3933 Kg/m³

 

[mfb]

We are looking at something larger than Earth, not smaller. If you just keep adding Mars-like material to a planet you get a higher density from a higher pressure. Small and low density is easy, large and low density (but rocky) is difficult.

 

[Vanadium 50]

I think mfb is right. This is going to be hard. You might think you can solve the problem by making the Earth's core small, but it's already pretty small. The mantle density goes from 3-1/2 to 4-1/2 as you go down just from the weight above it, so the Earth would be a little lighter but not hugely lighter if you replaced the core with mantle, and once you made the Earth bigger, the problems show up soon. R=1.5 looks doable. R=2 looks hard.

Ironically, if you go to very large, I think it gets easier. You make your mantle/core out of ice. At these pressures, it would have a density around 1.7. You'd need to keep it relatively cold, say below 1000 K.

 

[AndromedaRXJ]

I'm surprised I got more replies to this thread. I really appreciate it! I guess I'll take the opportunity to ask more questions.

I saw magnetic fields mentioned and how it pertains to having a smaller core, which would cause issues. What about an artificial magnetic field, and the planet it self has little to no natural magnetic field so we can get the density of the planet as low as possible?

I'm open to using magic as well, but it's more of a last resort sort of thing. I want to use as much sci-fi elements as I can. Of course, extremely advanced science and technology in sci-fi can be indistinguishable from magic.

As an example, take Silverberg's Majipoor. It's much larger than Earth

From what I'm looking up, that thing has 10x the radius of Earth. My planet would be like, what, 1.4 Earth radius or something?

Competing pet theories could be forwarded, argued over, etc. . . a case where mystery and doubt could actually add to the novel's believability, in fact. Not having all the answers has always been part of the human condition, after all. That being so, there's no reason why the future - even an advanced spacefaring one - should be exempt from this shortfall. Whatever else the future holds, you can be darn sure puzzlement will have a place in it.

This has been something I'm thinking about. The planet in my story is a planet colonized by human civilization, but it was a planet that was inhabited by another far more advanced alien civilization that has since abandoned the planet. I've been thinking that the planet it self is either highly modified, or nearly entirely artificial, and it's properties would indeed be bewildering for the human civilization that has took it over.

 

[mfb]

Artificial planet-scale magnetic fields are no problem in science fiction as long as something (can be robots) takes care of maintenance.

You can reduce apparent surface gravity over most of the planet if you spin it quickly. It would make days very short. Surface gravity at the poles would get larger.

 

[AndromedaRXJ]

Artificial planet-scale magnetic fields are no problem in science fiction as long as something (can be robots) takes care of maintenance.

How would the apparatus generating the magnetic field be situated? Would it be a structure inside the core, throughout the surface, or something in orbit of the planet? Or all the above?

 

[mfb]

The easiest approach with technology we can imagine: Build superconducting rings around the planet.

If the magnetic field is just there to protect the atmosphere it can also be in the planet/star Lagrange point L1.