How to Create the Galactic Habitable Zone

Question

Our galaxy consists of a wide number of stars, anywhere from 100 to 400 billion stars, with just as many number of what are known as "habitable zones", in which liquid surface water is possible. In other words, the Milky Way is overrun with millions if not billions of "Goldilocks"--not too hot in which the water evaporates into vapor, nor too cold in which the water freezes into ice.

But what about a galaxy that has only one supermassive habitable zone?

In this scenario, a hyperadvanced alien intelligence has created a galaxy entirely from scratch. The tiny one on the bottom right is our own, Photoshopped for the sake of scale. At the center of the galaxy is a black hole one trillion times the mass of our sun (Source: Black holes so big we don't know how they form could be hiding in the universe.) The big green doughnut is the habitable zone. Everything in this galaxy is artificial--the absence of nebulae, the detouring or detonating of comets and asteroids and even the lifespans of stars prolonged to last trillions of years, as long as red dwarves. Contained within the habitable zone are a whole slew of "seedworlds", a new trend in the speculative evolution subgenre in which a handful of Earth species of organisms have been put on an extraterrestrial body, be it a planet or even a moon, as was the case of the first seedworld, Serina. How many worlds does the habitable zone hold? Well, in a manner similar to Serina, each one of them is seeded with one particular species of plant or animal as the planet's "hero", alongside whatever species of plant, animal, alga or fungus that that particular species needs for sustenance. The worlds are broken down as follows:

• 2,063 worlds in which the heroes are extant amphibians classified under the IUCN Red List from Vulnerable to Critically Endangered, with an additional 42 being seeded by amphibian that have been extinct since the Holocene
• 4,328 worlds in which the heroes are extant invertebrates classified under the IUCN Red List from Vulnerable to Critically Endangered, with hundreds of others being seeded by invertebrate species that have been extinct since the Holocene
• 1,481 worlds in which the heroes are extant dinosaurs classified under the IUCN Red List from Vulnerable to Critically Endangered, with many others being seeded by dinosaur species that have been extinct since the Holocene
• 2,343 worlds in which the heroes are extant fish species classified under the IUCN Red List from Vulnerable to Critically Endangered, with many others being seeded by fish species that have been extinct since the Holocene
• 1,244 worlds in which the heroes are extant mammal species classified under the IUCN Red List from Vulnerable to Critically Endangered, with many others being seeded by mammal species that have been extinct since the Holocene
• 989 worlds in which the heroes are extant reptile species classified under the IUCN Red List from Vulnerable to Critically Endangered, with a handful of others being seeded by reptile species that have been extinct since the Holocene
• 11,577 worlds in which the heroes are extant plant species under the IUCN Red List from Vulnerable to Critically Endangered, with an uncounted number of others being seeded by plant species that have been extinct since the Holocene

Also on the habitable zone are worlds in which the seedlist consists of every plant, animal, fungus, microbe and even soil of a particular ecoregion.

• 867 worlds seeded by extant terrestrial ecoregions, though that is just 1/18,207 of the total number of "Terrestrial Ecoregion Worlds", the rest being seeded by extinct terrestrial ecoregions from the Silurian to the Holocene
• 232 worlds seeded by extant marine ecoregions, though that is just 1/1624 of the total number of "Marine Ecoregion Worlds", the rest being seeded by extinct marine ecoregions from the Ediacaran to the Holocene
• 426 worlds seeded by extant freshwater ecoregions, though that is just 1/5112 of the total number of "Freshwater Ecoregion Worlds", the rest being seeded by extinct freshwater ecoregions from the Silurian to the Holocene

However, none of the lifeforms of these seeded worlds would survive in a habitable zone spanning light-years and parsecs, which would make the revolution last thousands if not millions of years. So within the habitable zone are thousands if not hundreds of thousands of co-orbital solar systems consisting of smaller stars that are themselves orbiting the black hole at the center. The stars can be among these things:

• Neutron stars 214% the mass of our sun that rotate once every hour. (Considering that the slowest neutron star we've found rotates once every 23-and-a-half seconds, this seems suspiciously artificial.)
• Brown dwarf stars 90 times more massive than Jupiter, creating a gravitational influence so wide that any habitable world orbiting it might not have to worry about tidal locking. (However, I don't know if brown dwarves can reflect light like our moon or a gas giant.)
• Binary systems in which brown dwarves orbit red dwarves 51% the mass of the sun for extra light.

Now the ultimate question is--through artificial means, what would create the singular galactic habitable zone in the first place?

Answer 1

Your artificial galaxy has a carefully-managed Active Galactic Nucleus, or quasar.

Typical quasars are around 25 trillion times brighter than the Sun, but examples up to 600 trillion times brighter than the Sun have been found. Natural quasars are not a great source of stable illumination for inhabited worlds, because their brightness can vary enormously and unpredictably as more or less material falls into the accretion disk--but that's not a problem if the whole galaxy is artificial. The Alien Space Bats just have to ensure that there is a steady supply of material feeding into the black hole at all times (or, heck, they could intentionally modulate it to create galaxy-wide seasons) to maintain the desired luminosity. It it's a lot more efficient than using the same fuel supply to build stars anyway!

At 600 trillion solar luminosities, the habitable zone for an Earthlike world will extend between about 194 to 581 light years (assuming a solar habitable zone of 0.5 to 1.5AU; adjust accordingly if you prefer different assumptions about the range of habitable insolations). That's not huge-galaxy-sized, but it's plenty of space to fit your mere tens of thousands of worlds.

But, remembering that this is all engineered, you can do better! The light of a quasar is distributed spherically, and in fact shines brighter away from the equatorial plane than in the equatorial plane, so there's a lot of power being wasted if you don't want to distribute your worlds spherically. If you want to maximize the size of a habitable annulus, you will want to redirect that power. So, suppose that the Engineers build a fleet of light-pressure-supported mirrors above and below the plane of the galactic core to reflect axial light back into the plane of the galaxy. Supposing that we restrict the light to, say, a 10 degree-wide beam; that will make the radial light about 17 times brighter, which will push your habitable zone out to between 800 and 2,395 light years.

That's still not as big as your diagram implies, but it's still an absolutely gargantuan habitable area.

If you really want a single-source habitable zone that's wider than our entire galaxy... well, aside from being overkill for your purposes, I don't see a plausible way to actually achieve that with known physics.

Answer 2

If that green ring in the picture has to be a continuum of habitable zones, you are quite out of luck: each individual star has a ring shaped habitable zone around it, and making a continuum out of rings will end up putting the stars so close that they will gravitationally collapse into each other, resulting in a massive black hole which, as far as I know, is rather uninhabitable.

The closest you can get is to put a lot of brown dwarf star close to each other.

The overlapping radiation coming from a brown dwarf and its prime neighbors might help in having a corridor of habitability, made of overlapping rings, which is not a continuum. And their limited mass should not mess up too much with the orbits of the planets.

However the long term stability of such a set up looks problematic.

Answer 3

Exercise for the science-based fans: estimate what is the radiative power required for a source to create an "insolation" of 1.2kW/sqm on an Earth placed at 194 ly from the source. Extra points awarded for putting the value in perspective ("swimming pool" units need not apply).

Constants:

• 1ly = 9.461e+15m
• Earth radius R_⊕= 6,371e+3m
• Earth cross section: A_⊕ = π R_⊕² = 1.275e+14m²
• Mass of Sun: M☉ = 2e+30kg

Assumption: conservation of energy flux outside the source, what goes in an imaginary volume is either used or must go out (i.e. no contribution from reflection of other "surfaces")

At 194ly from the source, the Earth cross-section will inscribe a solid angle of

A_⊕/(4 π (194ly)²) = 1.275e+14 / 4.231e+37 = 3.011e-24 steradians

Similarly, at 581ly, the Earth's cross-section will inscribe a solid angle of 3.358e-25 steradians

Model: isotropic emission (spherical symmetry)

1. Earth at 194ly

To receive 1.2kW/sqm at 194ly, the source will need to emit a total power of 1.2kW*(4*π steradians)/(3.011e-24 steradians) = 5.008e+24kW = 5.008e+27W.

If that energy were to be obtained from total transformation of mass to energy (E = mc²), to achieve it one will need 5.008e+27/(3e+8)² kg/s = 5.565e+10 kg/s.

At this rate, the source will totally consume the Sun in 3.594e+19 seconds = 1.14e+12y or about 82 times the age of the Universe

2. Earth at 581ly

• the total necessary emitted power = 4.492e+28W
• total rate of mass to energy required = 4.991e11 kg/s
• total time to consume Sun's mass = 4.008e+18 second = 1.27e+11 years = 9.224 times the age of Universe

Now, there may be a small problem with the "zone of habitability" though.

If Earth receives the optimum 1.2kW/sqm insolation at 194ly, the planets at 581ly will receive 0.1337 kW/sqm. Which is definitely not habitable, it's the insolation of a point between Jupiter and Saturn.

If Earths receives the optimum 1.2kW/sqm insolation at 581ly, the planets at 194ly will receive 10.76kW/sqm, which is the insolation at 1/3 distance between Mercury and Sun. Ouch.

Apart from this small problem, Earth will by much better to sell the Sun if they guarantee a direct transformation in energy without losses. So the questions are where do we sign up and when do we start to move?

Answer 4

Actually there are much smaller and simpler ways to create a habitable zone with space for the specific numbers of planets you mention.

The first sets of planets you mention total 24,025. The second set of planets you mention total 18,339,945. So presumably they would all be identical to Earth, and have a total surface area equal to that of 18,363,974 Earths.

Earth has a surface area of about 510,065,623 square kilometers or 196,937,438 square miles.

So the surface area of 18,363,974 Earths would be about 9,366,831,800,000,000 square kilometers or 3,616,553,900,000,000 square miles.

An artificial cylindrical space habitat that was 1 kilometer in diameter and 31.672631 kilometers long would would have an inner surface area of 1,000 square kilometers.

So it would take "only" 9,366,831,800,000 of such artificial space habitats to have a total surface area of 9,366,831,800,000,000 square kilometers. And each space habitat wuld have an ineer surface area which was relatively much greater compared to the mass of the habitat than the surface ara of a hhabitable palnet would be compared to the masd of the planet. Thus you could build 9,366,831,800,000 such artificial space habitats using less mass than it would take to build 18,363,974 Earth duplicates.

Similarly, an artificial habitat 1 mile in diameter and 31.672631 miles long would have an inner surface of 100 square miles. So it would take "only" 36,165,539,000,000 of such habitats to have a total surface area of 3,616,553,900,000,000 square miles. And it would take much less mass to build them than to build 18,363,974 Earth duplicates.

Possibly you might want to build each habitate with the same surface area as the entire planet Earth.

A habitat 10 miles in diameter by 316.72631 miles long would have a surface area of 10,000 square miles. So a habitat 10 miles in diameter by 6,237,526.82 miles long would have a surface area equal to Earth's.

A habitat 100 miles in diameter by 3,167.2631 miles long would have a surface area of 1,000,000 0square miles. So a habitat 100 miles in diameter by 623,752.682 miles long would have a surface area equal to Earth's.

A habitat 1,000 miles in diameter by 31,672.631 miles long would have a surface area of 100,000,000 square miles. So a habitat 1,000 miles in diameter by 62,375.2682 miles long would have a surface area equal to Earth's.

A habitat 10,000 miles in diameter by 31,672.631 miles long would have a surface area of 1,000,000,000 square miles. So a habitat 10,000 miles in diameter by 6,237.52682 miles long would have a surface area equal to Earth's.

Of course a cylindrical habitat could be built with many concentric levels. If some plants and animals would be happy with a ceiling only 100 feet above, a habitat could have 100 levels supported by tree like pillers for a total height of 10,000 feet or 1.8939 miles. That could be used to reduce the diameter of the cylinder to only 100 miles, for example.

If you think that building space habitats with the surface area of the planet Earth is thinking big, it would use a tiny fraction of the total mass of the Earth to build one such habitat with the same surface area as the Earth, so it is a much smaller project than building millions of Earth sized planets.

Andof course such a project is not the largest construciton project imagined by science fiction writers.

Larry Niven, who created one such mega project in Ringworld wrote an article about such vast imaginary mega projects called "Bigger than Worlds", Analog Science Ficiton/Science Fact, March, 1974.

http://www.isfdb.org/cgi-bin/title.cgi?133302

I note that a Dyson Sphere would have a surface area of about a billion Earth's on it's inner surface.

An Alderson Disc could have a surface area of about 320,000,000 to 3,731,000,000 times Earth's surface area.

Sean Raymond has a blog, PlanetPlanet, with a section called the Ultimate Solar System for designing plausible star systems with as many habitable worlds as possible.

https://planetplanet.net/2018/06/01/the-million-earth-solar-system/

And his largest systems have up to a million planets orbiting in a single star system.