Largest possible habitable planet

[Guardsman Bass]

I figured it might be too much of a diversion from the other topic on encapsulating a Jovian planet, but I'll pose the question here - how massive and large could you make a habitable earth-like planet without severely increasing the surface gravity (I'm thinking a maximum surface gravity no larger than 1.2 Earth's), or causing the planet to turn into a small gas giant?

I'm not particularly good at either math or geology, but I'm thinking of the possible arrangements - if this was, say, an artificially made planet, could you make a planet almost entirely devoid of metals, with perhaps a minuscule nickel-iron core? The planet wouldn't be entirely devoid of internal heat; the size of its volume compared to its surface area could probably allow it to retain some heat.

[Patrick Degan]

I figured it might be too much of a diversion from the other topic on encapsulating a Jovian planet, but I'll pose the question here - how massive and large could you make a habitable earth-like planet without severely increasing the surface gravity (I'm thinking a maximum surface gravity no larger than 1.2 Earth's), or causing the planet to turn into a small gas giant?

I'm not particularly good at either math or geology, but I'm thinking of the possible arrangements - if this was, say, an artificially made planet, could you make a planet almost entirely devoid of metals, with perhaps a minuscule nickel-iron core? The planet wouldn't be entirely devoid of internal heat; the size of its volume compared to its surface area could probably allow it to retain some heat.

You could, but why would you want to? The metals are the best material you've got to construct a terrestial world with and the most abundant.

As to how large such a world could be, with only marginally greater surface gravity, a simple calculation based on volume and mass would give you the sums right enough.

[Broomstick]

I'm curious as to how you arrived at a maximum of 1.2 g's? Just a wild ass guess or something more concrete?

[CaptainChewbacca]

I'm curious as to how you arrived at a maximum of 1.2 g's? Just a wild ass guess or something more concrete?

Without my notes I'm not sure, but I think after 1.2 g's hydrogen can't escape the planet.

[Guardsman Bass]

I'm curious as to how you arrived at a maximum of 1.2 g's? Just a wild ass guess or something more concrete?

I'm just trying to find a number where the gravity isn't noticeably much heavier than it would be on Earth, so I pulled 1.2 g's out of the hat.

[Lord of the Abyss]

You could probably make it quite large if you decided to rapidly spin it like Mesklin. As long as "habitable at the equator" is habitable enough for you.

[Steel]

Surface gravity is proportional to radius R, given uniform density. Surface gravity is also proportional to density. (Mass proportional to R^3 but field strength proportional to R^-2)

To arrive at your desired gravity, relative to earth, take the ratio of the radius of your planet realtive to earth and its density realtive to iron and voila, the answer.

eg planet with 1.5 earth radius and made of water (density of iron is approx 8x that of water) will have 1.5*0.125 = 3/16 = 0.187g.

[Coalition]

Let's make it easy.

Here

Assuming 1.2 G surface gravity, and 1.002 tons/cubic meter for density, you have a planet 84,000 kilometers (or about 50,400 miles) wide.

[Guardsman Bass]

Thanks for the calculator - I've been using it.

Does anyone know of any links or information on the density of common silicates in the Earth, plus the nickel-iron core? I'm trying to figure out what would be a minimum density you could have while still having the planet be a solid surface world that isn't like one of the Jovian moons (which have icy mantles with low density). I've got some that are pretty big, but they have densities like 1700 kg/m^3 and the like, and I think that's too little.

[Starglider]

Assuming 1.2 G surface gravity, and 1.002 tons/cubic meter for density, you have a planet 84,000 kilometers (or about 50,400 miles) wide.

If it was made of metallic lithium (with a molten core), the diameter could be about twice that (160,000 km). The density of quartz (solid sillicon oxide) is about 2.65 tones/m3 so my guess is the biggest plausible rocky (1.2g) planet would be about 32,000km diam.

[Zixinus]

Without my notes I'm not sure, but I think after 1.2 g's hydrogen can't escape the planet.

I'm sorry, but could you affirm that? I for one recall someone mentioning it was 4 g's.

[Guardsman Bass]

Assuming 1.2 G surface gravity, and 1.002 tons/cubic meter for density, you have a planet 84,000 kilometers (or about 50,400 miles) wide.

If it was made of metallic lithium (with a molten core), the diameter could be about twice that (160,000 km). The density of quartz (solid sillicon oxide) is about 2.65 tones/m3 so my guess is the biggest plausible rocky (1.2g) planet would be about 32,000km diam.

That's smaller than even the smallest gas giants in the solar system, but still pretty large (for the latter planet of silicon oxide in solid and molten form, with a very small metallic core). 32,000 Km would give the planet a radius nearly three times that of Earth's, with about six times the surface area.

[Omeganian]

Without my notes I'm not sure, but I think after 1.2 g's hydrogen can't escape the planet.

I'm sorry, but could you affirm that? I for one recall someone mentioning it was 4 g's.

Neither. Escape velocity and gravity are two different things.

BTW, for a larger planet, the gravity limit will be somewhat lower that way.