Is there any realistic explanation for Earth-like climate on a Sun-sized planet?

I know that Sun-sized planets are impossible in our universe, please, bear with me.

Eastern high fantasy (xianxia) features worlds that span millions of kilometres. For example, descriptions of distances may state that it will take 200-300 years to go from place A to place B at the speed of 5000 km per day (*). This size is 100% handwaved, cannot be explained with physics, and is absolutely necessary for world-specific reasons (combat system, travelling speeds, specifics of communication, etc.).

My question is: In a world like this, is it possible to have a set of geographic features that realistically produce Earth-like climates (can be any climate zone or type)?

I believe that it is not possible but my knowledge of climate and geography is not sufficient to be 100% sure.

(*) PcMan courteously did the maths and told me that this would make the planet 'larger than Mars's ORBIT, or a "planet" 144 million times the volume of the sun'. Therefore, for some xianxia settings planets are even bigger than I initially expected. For the purposes of this question, please, assume that the planet is the size of the Sun.

A typical planet featured in xianxia:

• is at least Saturn-sized, often bigger
• has vast deserts, seas, lakes, forests, etc.
• has myriads of mountains (so all immortals and all sects have a place to live)
• has areas with microclimates similar to Earth and areas inhospitable to life (too cold, too hot, too dry, etc.)
• many areas are vast valleys surrounded by high mountains
• gravity is 1 G
• Earth-like day/night cycles
• all normal environmental physics apply in the areas with Earth-like climate
• Sun and Moon are rarely mentioned, so it is hard to say how they work

The criteria for the best answer:

1. It will not refute the premise of a Sun-sized planet.
2. It will be based on science (whether it states the possibility or impossibility of areas with Earth-like climate).
3. It will contain a brief explanation of geographical features and their effects on climate if the answer attempts to prove that areas with Earth-like climate are possible.

This planet does not have to be a sphere. It can be any shape if it is necessary for Earth-like climate areas to appear. The only two things that matter are size and plenty of mountains.

You can assume that the planet is made of some mysterious unobtainium/handwavium that create surface properties similar to that on Earth.

If you need celestial objects (Sun, Moon, stars, etc.) to behave a specific way to make Earth-like climate possible, you are free to choose their behaviour. Sun and Moon are rarely explained in xianxia.

• Comments are not for extended discussion; this conversation has been moved to chat. Commented May 30, 2021 at 1:07
• Maybe it is a Dyson Sphere or some other kind of artificial megastructure? Commented May 30, 2021 at 19:51
• @Otkin. I added a part three to my answer on May 30, 2021, discussing a new type of hypothetical mega structure I designed on May 29 and May 30. Commented May 30, 2021 at 21:29

Let's try Ringworld type solutions. Should be mandatory reading for Worldbuilders. :-)

Better than me has "designed" Ringworlds so think of what follows as a rough outline.

You have edited your question to ask for a planet that is Sun sized. I am, for these purposes, going to assume that's an equivalent surface area to the Sun. That's a lot of area and you are free to have it shaped and molded any way you want. We're going to gloss over trivial details like where you get all the material (theoretically but out of scope here) and how you keep it's orbit stable (which is the tricky one) and work on what kind of ring you need and what you do about day-night cycles and climate.

Area and dimensions.

The Sun has an area of about $$6\times 10^{12}km^2$$ which is apparently about $$12,000$$ Earth surface areas.

To get this area we need an orbit so we're going to rather arbitrarily choose one at exactly $$1\,AU$$ (Earth's orbital distance) and that also means that's the radius of our ring. At this distance about a star conveniently the same as our Sun we magically get the right range of temperatures on our surface with no messing. That is $$1.5\times 10^8\,km$$ and doing the math that makes the perimeter of our ring about $$9.4\times 10^8\,km$$. Dividing that into the area that gives us our ring's width : about $$6360\,km$$.

• A ring at 1 AU from a star identical to the Sun
• A width of over $$6300\,km$$

Will anyone on it notice it's a ring ?

Yes, if they travel to the edges of the ring, but can make that extremely hard by having insanely tall "mountains" and the edge would, and it's probably no issue to make cloud cover and weather at altitude near the edges extremely nasty - extreme winds, no visibility, low oxygen level at altitude and extremely cold. This won't fool any advanced technological society, of course, but if you want to populate with less advanced cultures - no real issue.

Visibility is such that no one is going to be able to see the curvature. Although $$6300\,km$$ sounds a lot, by the time the ring curvature lifts up "into the sky" (as viewed from the surface), you're not going to notice something obscured by atmospheric haze anyway (and maybe cloud) that is basically hundreds of thousands of km away. At best on Earth you can see about $$300\,km$$ away (when flying in exceptionally clear conditions). So the ring nature is not really an issue in practical terms. They'll notice the curvature of their world less than we notice ours.

They might notice the distant bright areas of other parts of the ring during the "night", but will be harder than it sounds. They would be distant lines of light, about as bright as a moon (when visible clearly). With careful arrangement of more rings I think you might be able to hide these, but it's possible not an issue anyway.

Gravity

You can make this whatever you want by varying the rotational velocity of the ring. So a standard Earth surface gravity is no problem.

Day and Night

Various proposals have been made for this, including an independent ring inside the main ring that rotates at a different speed. The inner ring has partial gaps that allow it to alternatively block the Sun and let it pass to the outer ring. Again it's entirely up to the maker of this ring as to how long they want to make this.

Like Earth the surface will get warm during the "day" and cool over the course of the night as it radiates heat away.

More inner rings can control "seasonal" variation of climate (by rotating a varying light filter) and so on.

Atmosphere

Smarter people than me have worked out how to make an atmosphere that stays there exist. It's possible and you basically get an atmosphere not unlike Earth's if you want it. Maintaining this would require some advanced biochemistry and automated controls by vast machinery, but it's at least theoretically possible as you have vast amounts of solar power to play wit - nothing prevents you building solar power arrays (or something better) extending way wider than the habitable part of the rings.

Climate

Essentially you control this by designing mountains, seas, rivers, plateaus, etc. These "shape" the winds and the winds carry moisture and deposit it as they do on Earth. You lack the circulating currents we do on Earth, although you can tweak something like them by subtle control of lighting - e.g. your atmosphere "lid" can have different levels of subtle filtering to force different areas to have different solar heating levels. Seas are very important.

Ice and snow is just a matter of altitude and you should be able to arrange a similar atmosphere to Earth's with minimal control. Any technological culture capable of building the ring in the first place is not going to find this anything but a minor problem.

Climate here generally runs from side to side - along the width of the rings. The "north and south poles" would be the edges of the ring, probably capped by incredibly steep slope - vertical in places - that supports the atmosphere lid. These might be artic regions of fierce cold and wind and storm. When visible at all, they would be permanently ice capped sloped barely visible in mist and haze.

• +1 as promised :) I actually like your answer. While it does not spend much time on climate per se, it makes the premise possible and this type of world structure is very good for a story (can turn fantasy into sci-fi fantasy). Thank you for not giving up on this question. Commented May 30, 2021 at 0:11
• Climate and weater are largely controlled by the surface material (sea, lake, arid, mountain, plateau, valley, forest, grassland, etc) and the local topology - actually not so different from Earth except we have a strong circulating pattern of winds and a polar cap due to the sphere shape and it's rotation. Filtering sunlight is important to mimic the effect of latitude on sunlight levels here. Essentially any Eartlike is possible for the same reasons. Commented May 30, 2021 at 0:22
• Is moon possible in this setup? And would this world have tides? Commented May 30, 2021 at 0:28
• @Otkin Bigger surface area - just widen the ring - you can leave the radius alone. The original Ringworld novel had a huge width - I think 1 million miles wide and a radius of 1 AU. Commented May 30, 2021 at 9:36
• @Trioxidane Haven't done the math but my gut feeling is that by the time the ring rises high enough to be a feature in the night sky the tiny width I am describing (think diameter of Venus) would be not much more than a line in the sky (as I think I said in my answer). Visible but not very distracting. You don't be seeing features just a line of light (and that's on good viewing nights). That's my guess anyway. Commented May 30, 2021 at 9:40

Geocentric model

Your giant planet is the center of its universe. Its sun rotates around it. A system like this bears more than just passing scrutiny. The Ptolemaic model could explain just about every observation people made from the surface.

This sidesteps a lot of issues. Your planet does not need to move thru space or rotate and its interaction with its light source can be what you need it to be. If you want to lean into "otherness" you could have more than one sunlike light source orbit the planet which would let you smooth out heating and cooling effects that arise as a consequence of just one sun. Nights and days could be different lengths according to which sun was coming, and the light might have a different character according to which sun was in the sky. If the suns did not move at the same speed there could be days with 2 (or more!) suns and days with 1. Stuff like this is good grist for the prose mill and lets you talk about something to set the tone for your big world.

Break up the continents.

Poles aside, midcontinental weather is the most extreme weather, examples being the American midwest or west and central Asia. You can ameliorate this effect by having large water bodies break up your continents and moderate the weather.

• I like this. Since we are already handwaving such a massive object not collapsing into a black hole and somehow having 1G at the surface, it would mean a star would orbit the planet, rather than the other way around. Commented May 30, 2021 at 15:11
• From what I remember, gravity is proportional to mass, so if the planet was empty inside, it would allow it to have 1G, while still keeping greater diameter. And this by the way opens nice opportunities for extensive cave system or even world-in-world scenario. However, the structure of the planet would have to be strong enough, so that planet does not collapse on itself. Commented May 31, 2021 at 7:00
• @Spook - I was thinking exactly this. It is a hollow world. Lots of zero gravity hijinks can take place in a hollow world. Plus many Frazetta hotties live in Pellucidar! Commented May 31, 2021 at 17:20
• Minor nit: “west and central Asia” seems surprising — central, certainly, but western Asian climates are in most ways less extreme than NE Asian, I think? E.g. Ulaanbaatar or the comparatively coastal Harbin compared to Moscow, St Petersburg, or anywhere else I’ve checked west of the Caspian. Commented May 31, 2021 at 17:33
• @PeterLeFanuLumsdaine - I agree. West was meant to modify "American"; thinking Wyoming, Utah. Commented May 31, 2021 at 17:48

Not really

I'm assuming a lot, like a gravity similar to Earth, windspeeds don't change and the like. Still there are some problems. First what's ok.

Imagine a solar system with a star like our sun and the planet fixed in location with the closest part of the planet to the star as close as the Earth is. Let's put that in perspective.

The sun's diameter is about 1.4 million kilometers in diameter. The Earth is about 152 million km away from the sun. The Goldilocks zone is between 130 to 180 million km. That means your planet is fully inside the goldilocks zone with room to spare, as it is between 152 million to 154 million km away from the star. Great!

Now the problems. If your planet rotates as fast as the Earth, the crust will move at blistering speeds compared to Earth. I can't be bothered with the results in wind speeds, oceans moving, general cohesiveness of the planet and the like, but it's not looking good. So you wouldn't have normal days and nights, letting the days heat up extraordinary and the nights cool down to extremes.

Ignoring that, the whole ecosystem will be difficult. Normally the energy of the sun is hitting the Earth straight on, meaning the further you go towards the poles you'll get the light more crooked, thus the energy is more dispersed. Thus the poles are cold and the equator warm, as they each get different energy per m². Just imagine a hot directional lamp on a piece of paper held straight under, or in a crooked position. The crooked has the energy spread out over a larger area, this won't be as hot where the light hits.

Your planet will have the same, but the bands this happens are much larger. That means the transport of these energies gets bigger. You have a lot more hot air at the equator wanting to go to the cold, making hurricane winds what would be a normal tiny wind on Earth. If not, you'll have the cold and hot areas become much more hot or cold, as the heat doesn't soread easily.

Other problems include the magnetosphere. Even if it gets proportionately stronger, the amount of solar radiation on all bands does so as well. The poles likely experience near constant Northern lights, but with the amount of EM radiation they fear will kill all electronics on Earth. On that scale it'll do strange things with ozone and other particles in the atmosphere, making living on the planet potentially hazardous as even more harmful radiation will reach the ground.

We haven't spoken about the tectonic movements and many other energies, but suffice to say you'll not get what you want by following much scientific rules. Your planet must have the same reason for weather as for just having the planet exist. Because you say so, and it is important for the story.

• Talking about speed of rotation, I wonder if relativistic effects start kicking in between the equatorial and polar citizens, based on a 24h rotation of our sun. Maybe not noticeable on an everyday level, but over a long time or with very specific technology (e.g. quartz clocks)? Did a quick lookup, 4.379 million km over 24h equates to 50,682 meters per second, which is about 1/6000th of lightspeed. That's gotta count for something, no? Commented May 30, 2021 at 21:53
• @Flater 44 seconds over a century. Certainly not measurable with quartz clocks I’m afraid!
– Tim
Commented May 30, 2021 at 23:14
• Rotation speed is not an issue as long as it's constant: Air and water will just move at the same speed (if there was any speed difference, friction would have sped up air/water and slowed the planet down). Commented May 31, 2021 at 12:06
• Magnetosphere is not an issue either - the amount of energy does not matter, it's amount of energy per surface (or volume) that matters. The poles might be bombarded more, but other than that, no changes. Commented May 31, 2021 at 12:08
• I agree that the wider bands will likely cause stronger storms. It might not be as dramatic as you think, because there's more surface for surface-air friction, slowing winds down. That said, higher levels of air will still be faster - I can't do the math, but I'd find supersonic jetstreams plausible. What the author can do is to assume that the atmosphere is much higher, so the heat transport has more medium, which means the medium can move slower. Commented May 31, 2021 at 12:11

Part one of Three:

Eastern high fantasy (xianxia) features worlds that span millions of kilometres. For example, descriptions of distances may state that it will take 200-300 years to go from place A to place B at the speed of 5000 km per day (*)

The world in the example, where it takes 200-30 years to go from place A to place B at the speed of 5,000 kilometers per day, would have place A and place B separated by about 73,050 to 109,575 day's travel, at a speed of 5,000 kilometers per day. Thus the shortest route from place A to place B would be 365,250,000 to 547,875,000 kilometers long, and going all the way around the planet or other world would probably take a journey of at least 730,500,000 to 1095,750,000 kilometers.

So if such a place was spherical or cylindrical, it would have a radius of at least about 116,262,784.1 to 173,951,212.7 .kilometers, and a diameter of at least about 232,525,568.3 to 347,902,425.4 kilometers.

One Astronomical Unit or AU, the distance between Earth and the Sun, is 149,597,870.7 kilometers. So if a planet like the one in the example was spherical, it would occupy at least about 0.777168 to 1.162792 of the radius and diameter of the earth's orbit around the Sun.

So if the figures used in the question are actually taken from a specific story, the planet in the story would be almost a billion (1,000,000,000) kilometers in circumference.

It is impossible for a planet to have a solid surface such as humans need to live on and have a surface area as large as the surface area of the planet Jupiter. The apparent surface of the planet Jupiter is actually the opaque top of cloud layers in the extensive Jovian atmosphere.

It is even impossible for a planet to have a much greater diameter to the top of its atmosphere than Jupiter. With greater mass than Jupiter, planets will increase in diameter and surface area, until they reach a point where adding mass stops increasing the size of the planet, which instead will become denser and more compact. Thus no planet can be much larger than Jupiter.

There are two hypothetical exceptions:

1. if a gas giant planet orbits very close to its star and is very hot, its atmosphere will swell and the planet's diameter will increase greatly. But of course the surface of the atmosphere will not be a solid surface for humans to stand on, and the planet will have many times the temperature which humans could survive.

2. Physicists have imagined several exotic types of matter which could hypothetically exist. And it might be possible for some types of exotic matter to form planets much larger than any known planet.

But I am not an expert on such hypothetical forms of exotic matter. And of course even if such a giant planet could form out of exotic matter, it is possible that Earthly life forms like humans would find it impossible to survive on planets made of such exotic forms of matter.

So it is basically impossible for any planets or other celestial objects to naturally form with solid surfaces and having solid surface areas as large as or greater than the surface areas of the top cloud layers of giant planets.

So does this mean that it is scientifically totally impossible for there to be places suitable for humans to live which have large enough surface areas to satisfy the requirements of the question?

Part Two:

Not exactly.

Larry Niven discussed the possibilities of building gigantic artificial places for humans to live in or on, in an article titled "Bigger than Worlds", Analog Science Fiction/Science fact, March 1974, about 47 years ago.

It is briefly summarized at:

And there have probably been other discussion on that topic in the last 47 years,

So science fiction writers can imagine that highly advanced civilizations could build structures for people to live in or on that are as large as or larger than any fictional planet.

Of course there is the problem that available structural materials would be unable to handle the stresses of structures that large.

But if such technological problems could the solved, super-advanced civilizations might build super gigantic structures which might later be inhabited by people who for some reasons in the plot of a story think that their structure is a natural world.

Part Three:

A type of gigantic artificial world designed by me on May 29 and 30, 2021.

One possible design for such a giant artificial planet would be a giant framework constructed around a star. The mass of the star will determine at which distance the framework will have a surface gravity about equal to Earth.

The Earth has an average radius of about 6,371.0 kilometers. The average distance of Earth from the Sun, one AU, is 149,597,870.7 kilometers. So the if the radius of the framework is one AU, that will be about 23,481 times the radius of Earth. Since the force of gravity falls off with the square of the distance, if the star at the center had the mass of the Earth, the surface gravity at the distance of the framework would be 1 Earth gravity or g divided by the square of 23,481. Since the square of 23,481 is 551,357,361, one divided by 551,357,361 is 0.000000001, the surface gravity at a framework with a radius of one AU around an object with a mas of one Earth mass would be only 0.000000001 g.

So the object at the center of the framework would have to have a mass about 551,357,361 times that of the Earth in order for the framework at a distance of 1 AU to have a surface gravity of 1 g.

The mass of the Sun is listed as about 333,000 times the mass of Earth. So the object at the centre of the framework would have to have a mass of about 1,655.7 times the mass of the Sun for the framework to have a surface gravity of about 1 g.

One of the most massive stars known is Eta Carinae,4 with 100–200 M☉; its lifespan is very short—only several million years at most. A study of the Arches Cluster suggests that 150 M☉ is the upper limit for stars in the current era of the universe.56 The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space. However, a star named R136a1 in the RMC 136a star cluster has been measured at 315 M☉, putting this limit into question.8 A study has determined that stars larger than 150 M☉ in R136 were created through the collision and merger of massive stars in close binary systems, providing a way to sidestep the 150 M☉ limit.[9]>

The first stars to form after the Big Bang may have been larger, up to 300 M☉ or more,[10] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter (MJ), or .09 M☉, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.[11] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 MJ.[12][13] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 MJ.[13][14] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.

So about 11 stars each with a mass of 150 times the mass of the Sun would be needed to orbit inside the framework with a radius of 1 AU to provide the framework with a surface gravity of 1 g.

Unfortunately, the upper number of stars in a stable multiple star system is probably 8.

And the hierarchical structure of multiple star systems means that such a multiple star system would have to be much wider than 1 AU.

Fortunately, a supermassive black hole could be at the center of the framework.

A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun (M☉).

In fact, a supermassive black hole would be far too massive for a framework at a distance of 1 AU to have a surface gravity of only 1 g.

An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes.2 Several IMBH candidate objects have been discovered in our galaxy and others nearby, based on indirect gas cloud velocity and accretion disk spectra observations of various evidentiary strength.

A hypothetical intermediate-mass black hole with a mass of about 1,655.7 times the mass of the Sun would be right to give a framework around it at a distance of 1 AU a surface gravity of 1 g.

So how would the framework around a star or black hole be supported against the central object's gravity of 1 g?

If the central object was a star radiating light, gigantic ultralight weight solar sails could be stretched across the empty spaces between the pieces of the Framework. Those solar sails would reflect the light and stellar wind of the star back at it, providing a force to lift up the framework they were attached to.

But a black hole would not be radiating energy or a stellar wind.

If the center of the framework was filled by a star with 1 solar mass, or 333,000 times the mass of earth, the framework could have a surface gravity of 1 g at a distance of about 577.06 times the radius of Earth, or about 3,676,458.956 kilometers. That would be very close to the star, and the framework would be very hot, unless there was a way to convert the light of the star striking the inner side of the framework into energy.

But the framework and solar sails would have a combined surface area of 5,77.0615 squared times the surface area of Earth. That would be 332,998.24 times the surface area of Earth. If only 1 millionth of the surface was the solid framework, that would be 0.332998 times the surface area of Earth. If only 1 thousandth of the surface was the solid framework, that would 332.9984 times the surface area of Earth.

If the star at the center had a mass of 100 solar masses, or 33,300,000 times the mass of the Earth, a framework at a distance of 5,770.615 times the radius of Earth, or 36,764,589.56 kilometers, would have a surface gravity of 1 g.

A star with 100 times the mass of the Sun would radiate many times the Sun's luminosity. For example, BL 253 has a mass about 80 times that of the Sun and a luminosity about 750,000 times that of the Sun.

The framework would get many times hotter in the example of a star with one solar mass, despite being 10 times as far from the star.

But the framework and solar sails would have a combined surface are 5,770.615 squared times the surface area of Earth. That would be 33,299,997.48 the surface area of Earth. If only 1 millionth of the surface was the solid framework, that would still be 33.299 times the surface area of Earth. If only 1 thousandth of the surface was the solid framework, that would 33,299.99 times the surface area of Earth.

If the framework surrounded an intermediate-mass black hole with a mass of about 1,655.7 times the mass of the Sun at a distance of about 1 AU the framework would have a surface gravity of about 1 g. But what would hold up the framework if there was no radiation from the black hole to press against the gigantic solar sails?

The inhabitants would have to make the black hole produce light by moving matter from far beyond the framework through gaps in the framework and send that matter toward the intermediate-mass black hole. As the matter got close to the black hole it would be accelerated by the black hole's intense gravity and it would heat up and emit light - the light emitted by infalling matter is one method used to detect black holes. And naturally, they would want to calculate the trajectories of the infalling matter which would produce the most light and stellar wind from the black hole.

A hypothetical intermediate-mass black hole with a mass of about 1,655.7 times the mass of the Sun would be right to give a framework around it at a distance of 1 AU with a surface gravity of 1 g.

Since in this example the framework would have a radius of about 23,481 times the radius of Earth, it would have a surface area of about 23,481 squared times the surface area of Earth, or about 551,357,361 times the surface area of Earth. If only 1 millionth of the surface was the solid framework, that would still be 551.357 times the surface area of Earth. If only 1 thousandth of the surface was the solid framework, that would 551,357.361 times the surface area of Earth.

And of course, other sizes of such a type of artificial world could be designed.

• My knowledge of astronomy and astrophysics does not go beyond basic concepts, so it will take some time for me to process your answer and understand it well. I will definitely have some questions and hope you can spare some time to answer them. Commented May 30, 2021 at 21:41
• I am not very smart, please, bear with me. Am I correct in understanding that: 1. Your 3d proposal suggests placing a non-rotating framework (mega-structure) around an intermediate-mass black hole at a distance of 1 AU; 2. The solar sails tie the pieces of the framework together and at the same time keep it 'floating' in the same place; 3. The black hole needs to be regularly 'fed' with some mass from outside the system to produce radiation and light? Commented Jun 1, 2021 at 6:19

Not a Planet

But a Dyson Ring or a Dyson Sphere would give you the area except isn't a planet.

Unless the people were capable of space travel, they wouldn't really be able to see the difference except it's never night.

• The mighty god Habdwavium can arrange night. What about the climate? Commented May 30, 2021 at 0:02
• Earth like. Can be anything you desire Commented May 30, 2021 at 6:49

Put a tarp over a brown dwarf.

Hurricane-affected regions of the U.S. are living proof that there is almost nothing that you can't manage with a proper tarp, and Sun-sized planets are no exception. Now, let's start with the surface gravity of the Sun being 28 times that of Earth. This means that for a Sun-sized planet we need a star with 1/28 = 0.036 times the mass of the Sun, or something like 36 Jupiter masses in the ad hoc metric system. A little less, but who's counting - a brown dwarf is up to 80 Jupiter masses.

Point being, you have your construction fleet fly around the brown dwarf laying down blue tarp. It should only take a little bit of atmospheric pressure beneath to hold it up, so long as you do something to support it at the edges where atmosphere could get out around it. The outside doesn't literally have to be blue, but the inside does have to be a darn good mirror to prevent any light passing that way. You want it to be like a thermos holding the heat in. A material other than plastic is advisable. Once you've sewn that up nicely, deuterium burning keeps the brown dwarf getting hotter and hotter. A brown dwarf is a failed star, but that doesn't mean it fails to burn deuterium! You let out the brown dwarf's clothing now and then, because all that built up heat will expand the star. It keeps getting larger until it is the size of the Sun. Then you can let some heat escape in places, and quit feeding it so much deuterium.

How do you keep the blue tarp from melting? I can't explain it. How do you keep the inside of a tokamak from melting? It's nuclear physics, complicated engineering of magnetic fields. And space boats at the ready with patches, for the occasional foul up. The brown dwarf turns into a massively expanded ball of plasma, like a mini mini Wolf-Rayet star; atop the plasma (once brown dwarf atmosphere, now hotter) you have the Tarp; atop the Tarp, you have a comfortable temperature and atmosphere and moderately deep soil ... just keep making your magnetic protective barrier against heat transfer from the plasma more and more effective, somehow, so that you can press down harder on the Tarp and pile a thicker planetary surface on top of it.

As others have said, you need a lot more than sun-sized to get the sort of travel times you are talking about. However, my solution scales reasonably well:

You have some sort of hollow unobtainium sphere or forcefield or the like. On top of this you have a 4,000 mile thick layer of typical planet stuff. (Yes, not a coincidence that that's Earth's radius.) You have a "planet" with 1g at the surface. Put enough radioactives at the bottom and you can have a geologic cycle similar to Earth's, although obviously each plate will be a far smaller percentage of the total. No magnetic field but at Earth size at least (the size I've played with the numbers) the problem will be with the solar wind sticking, not with losing it.

Illumination comes from some sets of stars in rosettes orbiting the planet. I haven't figured out any way to get the weather even across the planet but you can have plenty of Earthlike area. Each rosette contains a number of stars equal to it's orbital period in days--while multiple stars will be above the horizon as they get far enough away they'll fade out anyway.

The hottest points will be where rosettes cross. Under any reasonable arrangement this will be scorched. Beyond that we have the warmest areas being under the rosettes, cooling off as you get farther away.

You can make the Earthlike climate be under the rosettes (and making rings around the intersections) which makes for the easiest travel, or make things a bit warmer under the rosettes, giving more total habitable area but cutting it up into chunks where you have to cross the hot areas to go between them.

Note that this system is not long-term stable for two reasons:

1. While a rosette of stars can be stable they will tug on each other where the rosettes cross. Stationkeeping will be needed.

2. Stellar wind will hit the planet and stick, adding hydrogen to the atmosphere. That's going to have to be pulled out somehow or the oxygen slowly turns to water.

Let's try logically thinking this through.

There are two cases (at least): the earth is spherical, or, the earth is a different shape (cylindrical, toroid...).

Let's start by considering what difference shape makes

If the earth is spherical/spheroid, then there is only one option for an orbiting sun, because all paths are the same. If it's long and thin, then there are different orbits - it could have a shorter solar path and a very long other dimension. If its toroid, then more complex paths are possible and not all paths are topologically the same. We'll ignore this for now and come back to it.

The way it works

Let's look at what makes an earthlike planet earthlike, and what a larger earth requires. This'll be a thumbnail sketch only.

Gravity: A larger earth will need a way to keep comparable surface gravity. It need not be exactly the same, but too much less it loses atmosphere, and too much more, life becomes challenged and probably severely modified, and retention of dense gases/heating becomes a problem. So however large your earth, you need to ensure its average density is such that surface gravity is a reasonable comparable level. (This is easy, name your chosen size and surface gravity, calculate your density, done.)

Diurnal cycle/surface temperature: you need a day/night cycle for earthlike climate. But unless you want permanent areas of day and night, hot and cold, and a few twilight zones, you need a flow of heat to average things out. The earth does that by daily rotation, and by having just enough heat from the sun to power it, and that's the easiest way to do it.

For (almost) any given size, we can arrange a sun that rotates around it in a given daily period. Even if earth is huge, it can be made narrow in one dimension, like an egg timer or cylinder, to shorten the path. But we need the sun to be able to rotate in a given time, and that means one or more of

• a rotating earth
• a short-ish solar path around a huge non rotating earth
• a dense sun moving very fast around a huge non rotating earth
• multiple suns?

We can mix, match and calculate these, and adjust the earths shape to match, then adjust its density to get gravity right, etc.

I believe that these, together, can be used to get an earthlike planet, with some handwaving, but that's how id calculate the details.

Consider Iain Banks' Culture series' Orbitals:
https://en.wikipedia.org/wiki/The_Culture#Living_space

"One of the main types of habitats of the Culture, an orbital is a ring structure orbiting a star as would a megastructure akin to a bigger Bishop ring. Unlike a Ringworld or a Dyson Sphere, an orbital does not enclose the star (being much too small). Like a ringworld, the orbital rotates to provide an analog of gravity on the inner surface. A Culture orbital rotates about once every 24 hours and has gravity-like effect about the same as the gravity of Earth, making the diameter of the ring about 3,000,000 kilometres (1,900,000 mi), and ensuring that the inhabitants experience night and day. Orbitals feature prominently in many Culture stories."