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:
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.
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?
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.
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.>
The first stars to form after the Big Bang may have been larger, up to 300 M☉ or more, 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. 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. 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. 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.