How big could an Earth-like* planet be?

[Santino]

For reasons of fantasy/science fiction, but staying within known science, how large could a planet be that had more-or-less Earthlike living conditions such as gravity, day and night, atmosphere etc.

Could you have a Jupiter-sized planet that had a low enough mass to provide livable gravity?

 

[Silas Loom]

This is for colonising, rather than life evolving on, right? So the question is how one could build the equivalent of a space station “on” (or, perhaps, “within”) a gas giant?

I haven’t read any SF with mining colonies on gas giants, it’s a nice idea with lots of plausible perils.

 

[Santino]

No, I want people walking about on the ground, not living in Cloud City. A big planet with seas and land on it, but that's outrageously big.

 

[Shippou-Sensei]

Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radii, as 2 Earth-radii would be the upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar).

Super-Earth - Wikipedia

en.m.wikipedia.org

 

[MickiQ]

For reasons of fantasy/science fiction, but staying within known science, how large could a planet be that had more-or-less Earthlike living conditions such as gravity, day and night, atmosphere etc.

Could you have a Jupiter-sized planet that had a low enough mass to provide livable gravity?

Not naturally no, Jupiter is a gas giant with a mass 318 times that of the Earth but a surface gravity of 2.2G's however it has a much lower density than Earth. A rocky Earthlike planet the same size would have a mass of 1400 times that of the Earth and thus a surface gravity of 11.2G's so not really very homely.
There is a concept bandied about by speculative scientists, sci-fi writers and other people with too much time on their hands of a suprashell world which is an artificial shell built around a micro black hole so that the world could be as big as you wanted but with still livable gravity. Currently found only in the realms of fiction but they don't break any known laws of physics so the galaxy might be heaving with them.

 

[DotCommunist]

Bigger the planet, higher the gravity. Mass. Say humans live long enough on a Jupiter sized earth and you'd have the 'heavy g worlder' beloved of sci fi. A squat but immensely strong figure

 

[Silas Loom]

No, I want people walking about on the ground, not living in Cloud City. A big planet with seas and land on it, but that's outrageously big.

A hollow planet might work, to keep the mass down. Obviously it would be unlikely for such a planet to form naturally, but it could be built with an infinite number of engineering bots set to work on an Earth-type iron-cored planet.

 

[Chz]

No, I want people walking about on the ground, not living in Cloud City. A big planet with seas and land on it, but that's outrageously big.

As Shippou-Sensei says, you're looking at a reasonable upper limit of 2 Earth radii. There's a reason why things like Ringworlds and Dyson Spheres are popular in SF when you want something truly gigantic. A huge rocky world is either hollow or its mass is drawing in an atmosphere that makes life impossible. (even ignoring gravity, which we could hand-wave away by using "rock" that varies wildly from Earth)

ETA: Beaten by Silas Loom on the hollow world.

 

[Silas Loom]

As Shippou-Sensei says, you're looking at a reasonable upper limit of 2 Earth radii. There's a reason why things like Ringworlds and Dyson Spheres are popular in SF when you want something truly gigantic. A huge rocky world is either hollow or its mass is drawing in an atmosphere that makes life impossible. (even ignoring gravity, which we could hand-wave away by using "rock" that varies wildly from Earth)

ETA: Beaten by Silas Loom on the hollow world.

Looks like MickiQ actually got there first, although he specifies a micro black hole centre.

 

[DotCommunist]

these lads could handle the extra G's

 

[urbanspaceman]

Hycean planet - Wikipedia

en.wikipedia.org

There is also the new idea, postulated only a couple of years ago, of the Hyacean planet, a water world with up to 2.6 times the Earth's radius and 10x its mass.

 

[Santino]

these lads could handle the extra G's

That helmet on the right has a suspicious shape.

 

[DotCommunist]

That helmet on the right has a suspicious shape.

herman gelmet

 

[Spandex]

Moab (from The Ballad of Halo Jones) - larger than Jupiter but solid rock and ore rather than gas, the gargantuan gravitational pull bends both light and time, meaning that time passes at different speeds there and everything looks like it’s melting.

 

[NoXion]

For reasons of fantasy/science fiction, but staying within known science, how large could a planet be that had more-or-less Earthlike living conditions such as gravity, day and night, atmosphere etc.

Could you have a Jupiter-sized planet that had a low enough mass to provide livable gravity?

If you're OK with your setting being a largely artificial world, there is the option of it being a supramundane/suprastellar shell with a terraformed surface:

Supramundane and Suprastellar Worlds

Shells suspended above planets or stars using mass-stream technology.

www.orionsarm.com

TL;DR - It is not impossible within known physics to build an electromagnetically supported shell around a large body like a gas giant or a brown dwarf star, the surface of which can support an Earth-like environment. This gets around the "density" issue and allows the habitable surface to have Earth-like gravity.

Of course if you want something even bigger than that, you can expand the concept into what's called a Birch Planet, a multi-level artificial structure constructed around a supermassive black hole (like the one at the centre of our galaxy):

 

[Santino]

Of course if you want something even bigger than that, you can expand the concept into what's called a Birch Planet, a multi-level artificial structure constructed around a supermassive black hole (like the one at the centre of our galaxy):

I only need a surface 100 or so times the size of Earth. I'm not some kind of maniac.

 

[MickiQ]

I only need a surface 100 or so times the size of Earth. I'm not some kind of maniac.

The maths for that are easy then, To get the surface area 100 times that of the Earth you would need a suprashell world with 10 times the radius (slightly smaller than Jupiter which is 10.5x) which would need a black hole 4.7cm in diameter to give you a gravity of 1G at the surface. The actual doing might be more of a challenge though I don't think the UK in 2024 is either rich enough or advanced enough to build such a thing, hell we can't even build a fucking railway from Birmingham to Manchester.
Plus not sure where you might get the black hole from, the centre aisle at Aldi's stocks some amazing stuff but I suspect even they don't do a special on microscopic black holes.

 

[NoXion]

I only need a surface 100 or so times the size of Earth. I'm not some kind of maniac.

Assuming that the habitable surface is to be at 1 Earth gravity, then the mass of the body underneath the shell will equal how many more Earths worth of space it will have e.g. 100 Earth masses = 100 times Earth's surface area. In that case, you'd want such a shell around a planet that masses slightly more than the planet Saturn (which is 95.16 Earth masses).

 

[NoXion]

The maths for that are easy then, To get the surface area 100 times that of the Earth you would need a suprashell world with 10 times the radius (slightly smaller than Jupiter which is 10.5x) which would need a black hole 4.7cm in diameter to give you a gravity of 1G at the surface. The actual doing might be more of a challenge though I don't think the UK in 2024 is either rich enough or advanced enough to build such a thing, hell we can't even build a fucking railway from Birmingham to Manchester.
Plus not sure where you might get the black hole from, the centre aisle at Aldi's stocks some amazing stuff but I suspect even they don't do a special on microscopic black holes.

The interior body doesn't need to be a black hole, it just has to be dense enough to fit inside the shell. In the case of Jupiter that might be achieved by active cooling of the interior body, thus causing it to shrink.

 

[MickiQ]

The interior body doesn't need to be a black hole, it just has to be dense enough to fit inside the shell. In the case of Jupiter that might be achieved by active cooling of the interior body, thus causing it to shrink.

Yes but black holes are cool, I can well imagine 20,000 years from now the descendants of the bores who always talk about house prices will be talking about the size of their black holes.
I'm a follower of Isaac Arthur myself he does some excellent videos.

 

[NoXion]

Yes but black holes are cool, I can well imagine 20,000 years from now the descendants of the bores who always talk about house prices will be talking about the size of their black holes.
I'm a follower of Isaac Arthur myself he does some excellent videos.

If Earth-descended intelligent life is still a thing 20,000 years hence, then I am sure there will be bores of some kind or another, but I really hope that our descendants will have knocked capitalism on the head by then.

I'd like to hear more about Santino 's plans for an Earth-like world 100 times the area of our own, if they're comfortable sharing.

 

[Santino]

I just think it would be cool if there was a film trilogy and/or tv series set on one.

 

[youbeauty]

I only need a surface 100 or so times the size of Earth. I'm not some kind of maniac.

What about a planet that is the same size as Earth but everything on it is 100 times smaller?

 

[Santino]

What about a planet that is the same size as Earth but everything on it is 100 times smaller?

How would you tell the difference?

 

[andysays]

For reasons of fantasy/science fiction, but staying within known science, how large could a planet be that had more-or-less Earthlike living conditions such as gravity, day and night, atmosphere etc.

Could you have a Jupiter-sized planet that had a low enough mass to provide livable gravity?

If we're talking about earth-like in the sense that humans and a wide other earth-originating life (especially plants) could thrive there, it would need to have not just similar gravity, day and night length and atmospheric* contents as Earth, but also things like light levels, temperature range, year length and seasonality, rotational velocity, water availability and all the other minerals necessary for plant growth.

If you increase the size significantly, you also change many of the other factors or the relationship between them, so I'm going to suggest that the range of sizes for a genuinely Earth-like planet is actually quite small.

A low density Jupiter-sized planet might have a low enough mass to have surface gravity similar to that of Earth, but some combination of the other factors would be wrong.

* the atmosphere isn't just the stuff at the surface which we breath, it also includes the upper layers which are important in terms of, for example, heat and light.

 

[youbeauty]

How would you tell the difference?

exactly

 

[RiderOnTheStorm]

my first thought was hollow planet if you want it really big.

Or less dense material if you want a bit bigger

Or for a S/F story, what about a planet the size of a dyson sphere , but the sun is dead, and the sphere evolved to be a planet, ie covered over , snd so what they thought was a supermassive planet is actually hollow

Or a massive planet where folk have to wear harness (not big ones) so that an orbiting device (magnet? mini black hole? hoover?) attracts the device and reduces the pull? Options there for drama when people have to go places outside the shadow of the device, or murder by sabotage of the harness , etc

 

[JimW]

What about if it was flat and resting on turtles?

 

[NoXion]

How would you tell the difference?

If we were one-hundredth the size that we're used to being, then we'd definitely notice. It means as a person who is normally 6 foot 2 inches, I would be under an inch in height. Air resistance would feel way different, falls from what would seem like much greater heights could be survived, and if the effects of mammalian-style metabolisms are taken into account, humans that size would starve a lot more quickly. There's probably a whole bunch of other effects that would result as well.

 

[WouldBe]

The maths for that are easy then, To get the surface area 100 times that of the Earth you would need a suprashell world with 10 times the radius (slightly smaller than Jupiter which is 10.5x) which would need a black hole 4.7cm in diameter to give you a gravity of 1G at the surface. The actual doing might be more of a challenge though

You'd put something in place, turn around and it's been sucked into the black hole.