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In an alternate universe, the nine realms of Norse mythology--Midgard, Asgard, Vanaheimr, Jotunheimr, Alfheimr, Hel, Nidavellir, Niflheim and Museplheim--are actual, real-life "Earths", in the sense that all of them have the exact same features listed below:

  • At least one large natural satellite, one-quarter the diameter and 1/80 the mass, to stabilize the parent's axial tilt
  • A predominantly iron core, meaning a strong, active magnetic field
  • An inner core measuring a radius of 760 miles
  • An outer core measuring 1367 miles thick (I don't know why Google doesn't have its width, which is what I really needed.)
  • A predominantly silicon crust
  • A predominantly silicon-magnesium mantle
  • Atmospheric pressure no greater than one bar
  • Clear, tangible evidence of life (albeit microbial at this point, but with the dark power of illegal multi-tiered terraforming, anything is possible.)
  • A crustal diameter anywhere between 7,520.8 miles ("Venusian") and 7,917.5 miles ("Terran"), with the sole exception of Jotunheim, who measures at a diameter of 4,212.3 miles ("Martian")
  • Mass ranging from 0.8 to one Terran, though Jotunheim has a mass almost 11% that of Earth's
  • Active tectonic plates

Now, having nine "Earths" requires a very wide habitable zone, in which liquid water is possible. So I have eliminated all the candidates in the search for all the star types that are both bright and long-lasting. Only two types remain:

  1. G-type main-sequence stars, or "yellow dwarves"
  2. K-type main-sequence stars, or "orange dwarves"
  3. A mix of both

You could have the Nine Earths orbiting giants that are hundreds of times more luminous than the sun, but larger stars have shorter lifespans. Red dwarves are small enough to live trillions of years, but they are unacceptably dim and constantly lash out deadly radiation.

So how many yellow dwarves, orange dwarves, or a mix of both, would be required to have the luminosity to have a habitable zone of either four or 14 astronomical units (372 or 1,302 million miles)?

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  • $\begingroup$ The former, yes. $\endgroup$ – JohnWDailey Aug 11 at 3:46
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    $\begingroup$ Natural or built system? $\endgroup$ – Ash Aug 11 at 12:24
  • $\begingroup$ Note that a large moonbis not necessary for axis stability allowing habitability. Earths axis currently oscillates by about 5 degrees. It will go up to 20 to 40 without it. This isn't too bad if the oscillation is focused on the poles. $\endgroup$ – TheDyingOfLight Aug 12 at 4:44
  • $\begingroup$ @John W. Dailey I added a bit to my answer at the end. $\endgroup$ – M. A. Golding Aug 23 at 2:43
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It turns out there is a simpler solution to the problem posed by the OP's question. Again this is based on the work of Sean Raymond.

This is based on a paper by Smith and Lissauer (2010) which looked at the number of planets that can share the same orbit.

It turns out there is a stability limit for the number of planets that can be spread along the same orbit. The planets must be evenly spaced and there must be at least 7 on one orbit (not a typo: at least 7!). The limit is simple: the planets sharing the same orbit must be separated by at least 12 Hill radii in distance along the orbit. This is different from before, where we were looking at the distance between orbits.

Smith and Lissauer ran simulations with not 2 or 3 but 42 Earth-mass planets sharing the same orbit! That is the maximum number of Earths that can fit along Earth’s present-day orbit. And guess what? It’s perfectly stable for billions of years. Over the last couple weeks I ran my own N-body simulations and they match perfectly. By the hammer of Thor, it really works!

If the number of Earth-mass planets sharing the same orbit must have a minimum of seven and have no more than a maximum number of forty-two (42), then this more than meets the criteria for where to place the Nine Norse Realms.

Nine equally spaced Earth-mass planets can share the same orbit around a sunlike star. Each planet can be accommodate one of the Nine Realms. Sean Raymond calls this an Ultimate Engineered Solar System, so astronomical engineering on a grand scale will be required to assemble it.

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  • $\begingroup$ But are 12 Hill diameters wide enough to prevent tidal locking? How do these same-orbit marriages even work? Because I've never seen a mention of this in either History Channel's "The Universe" or Science Channel's "How the Universe Works". And I don't quite understand that last paragraph. $\endgroup$ – JohnWDailey Aug 11 at 14:25
  • $\begingroup$ @John W. Daily The paper by Smith and Lissauer is only 9 years old, being published in 2010. Thus it is possible that the writers of the programs you mention have never heard about it. The last paragraph is clear. There are many things that can exist, but which are extremely unlikely to form though natural processes. The more extreme solar systems in Sean Raymond's site are called engineered solar systems because they would have to be built by highly advanced civilizations instead of forming naturally. $\endgroup$ – M. A. Golding Aug 11 at 16:03
  • $\begingroup$ @M.A.Golding But why taint the results, therefore "cheat", by messing up the planets instead of sticking to terraforming? $\endgroup$ – JohnWDailey Aug 11 at 18:36
  • $\begingroup$ @JohnWDailey The 12 Hill radii have nothing to do with tidal locking. They are basic requirement for the gravitational stability of the co-orbiting planets. Each planet is a single astronomical object. Tidal locking will not occur in this set-up. You are confusing the scenario with the co-orbiting binary planets in Raymond's Ultimate Solar System 1. This scenario is his Ultimate Engineered Solar System. Follow the link and have fun there. The TV programs won't mention set-ups like these because they ain't natural. $\endgroup$ – a4android Aug 12 at 2:11
  • $\begingroup$ @JohnWDailey RE your comment MAGolding. The planets aren't messed up. Terraforming isn't necessary. Each of the 9 co-orbiting planets will operate as if it was a normal Earth orbiting the Sun. Each one has 8 identical Earthlike buddies sharing the same orbit. $\endgroup$ – a4android Aug 12 at 2:14
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SHORT ANSWER:

You can have your nine worlds sharing a single orbit around their star, as suggested in an answer by a4android. Or you could have your nine worlds having nine different orbits in their habitable zone around their star.

Nobody know what the absolute maximum number of planetary orbits within the circumstellar habitable zone of a star is.

It seems very probable that stars with no planets within their circumstellar habitable zones are many times as common as stars with one planet each within their circumstellar habitable zones, stars with one planet each within their circumstellar habitable zone are many times as common as stars with two planets each in their circumstellar habitable zones, stars with two planets each within their circumstellar habitable zone are many times as common as stars with three planets each in their circumstellar habitable zones, and so on, with systems with more planets within their habitable zones becoming rarer and rarer and rarer.

So even if it is possible for a star to have nine planets in its habitable zone that would be extremely rare; maybe one star out of a million, or one star out of a billion, or one star out of a trillion, or one out of some other vast number. The odds against any one specific star having nine planets in its circumstellar habitable zone would be, to use an apt term, astronomical.

But the universe is so vast, and the numbers of stars so astronomically vast (possibly infinite), that if it is possible for a star to have as many as nine planets in nine different orbits in its circumstellar habitable zone, there probably would be some stars with as many as nine planets each in nine separate orbits in their circumstellar habitable zones.

LONG ANSWER:

HOW MANY SEPARATE PLANETARY ORBITS CAN THERE BE IN THE HABITABLE ZONE OF A SINGLE STAR?

long ago, before any exoplanets were found orbiting other stars, Earth's solar system was the only known example of a solar system.

There was a time when scientists couldn't understand why the Sun and other stars emitted radiation and light. There is a story that at one scientific conference an angry astronomer punched a geologist, because the geologist insisted that geology proves that the Earth was many times older than astronomical and physical calculations showed was the maximum possible time that the Sun could shine for.

Between about 1920 and 1950, brilliant scientists calculated how stars produce energy by thermonuclear fusion within their super dense and super hot cores.

And people began to realize the implications for the possible habitability of exoplanets if they existed. For example, as I remember, Robert A.Heinlein mentioned that spectral class G stars would be the best for having habitable planets in juvenile science fiction novels like Starman Jones (1953) and Time for the Stars (1956).

As far as I know the first extended discussion of the suitability of various spectral classes of stars for having habitable planets was Stephen Dole's Habitable Planets for Man (1964, 2009).

https://www.rand.org/pubs/commercial_books/CB179-1.html1

On pages 49 to 52 Dole discusses the spacing of the planets in our solar system. He calculates that each planet's gravity interferes with the orbits of other bodies nearby, so that each planet has a forbidden zone where no other planet could form or have a stable orbit, based on the mass of the planet and the planet's distance from the Sun.

According to Dole's data, it might be possible to have more potentially habitable planets in the habitable zone of the Sun, orbiting in the habitable zone and in the spaces between the forbidden zones of Venus and Earth, or between the forbidden zones of Earth and Mars, or between the forbidden zones of Mars and Jupiter.

It is obviously perfectly possible for a solar system to form without any planets of the suitable size in the habitable zone, or to form with only one planet of suitable size in the habitable zone - like our solar system. But it seems to me that on rare occasions a star exactly like the Sun might have about 1.5 to 2.0 times as many planets of suitable size in it's habitable zone if the forbidden zones of those planets almost touch.

So depending on the size of the Sun's habitable zone, and on whether there are believed to be one, two, or three planets of suitable size in the Sun's habitable zone, a star exactly like the Sun could have zero, one, or two planets of suitable size within its habitable zone, and possibly as many as three, four, five, or even six.

No doubt solar systems with more planets of suitable size in a star's habitable zone will be more rare, so that each increase in the number of suitably sized planets in the habitable zone will have a significant decrease in the percentage of star systems having that number.

And of a course planets of a suitable size in the habitable zone of a star would not necessarily be habitable for either native life or for humans. Earth, for example, is of suitable size and in the habitable zone of the Sun, but did not develop a breathable for humans atmosphere until it was billions of years old.

Dole discusses the age of a planet that is habitable for humans on pages 61 to 63, and concludes:

In general, it is probably safe to say that a planet must have existed for 2 or 3 billion years, under fairly stable conditions of solar radiation, before it has matured enough to be habitable.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a473471.pdf2

On pages 67 to 72 Dole discusses the characteristics of a planet's star. If a planet's star has to remain on the main sequence and emit a fairly steady amount of light for at least 3 billion years, it has to have no more than 1.43 times the mass of the Sun and a spectral Class F2 or later (Class F2 to F9, G Class stars, K class stars, M class stars). And to avoid having a planet become tidally locked to its star the star should have a mass at least 0.72 times the mass of the Sun, or spectral class K1.

So Dole concluded that stars with habitable planets in their habitable zones would probably range from class F2 to F9, G0 to G9, and K0 to K1.

And the more massive and brighter stars have more distant inner and outer limits of their habitable zones, and thus broader habitable zones. And the forbidden zones of planets would be relatively smaller in the broader habitable zones, making room for more potentially habitable planets possibly orbiting in the habitable zone.

So I long ago concluded that a solar system with the maximum possible number of planets in its habitable zone would probably be one with a spectral class F star as the primary that the planets orbit. So what is the widest possible circumstellar habitable zone?

Nobody knows.

The Wikipedia article Circumstellar habitable zone has a table listing various calculations of the inner or outer edge, or both, of the Sun's habitable zone.

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates3

And you will note that they vary widely in how broad or narrow they calculate the Sun's circumstellar habitable zone is. Some of the calculations may be for planets habitable for Humans, as Dole's were, and some may be for planets habitable for life but not for humans, which could explain some but not all of the radical differences.

The most commonly used definition of the inner and outer edges of the Sun's habitable zone is by Kasting et al. in 1993, with a conservative habitable zone from 0.95 to 1.37 Astronomical units (AU) and an optimistic habitable zone from 0.84 to 1.67 AU. So the outer edge of Kasting's conservative habitable zone is 1.442 times as distant as the inner edge, and the outer edge of Kasting's optimistic habitable zone is 1.988 times as distant as its inner edge. Kasting's conservative habitable zone is 0.42 AU wide and his optimistic habitable zone is 0.83 AU wide.

It is estimated that the habitable zone of a relatively hot F0 star would extend from about 2.0 AU to 3.7 AU and between 1.1 and 2.2 AU for a relatively cool F8 star.4

https://en.wikipedia.org/wiki/F-type_main-sequence_star5

So a class F2 or F5 star could have a significantly broader circumstellar habitable zone than a class G2 star like the Sun.

HOW MANY SEPARATE PLANETARY ORBITS CAN THERE BE IN THE HABITABLE ZONE OF A DOUBLE STAR?

And long ago I decided that a binary star with two very similar class F stars orbiting together could have a wider circumstellar habitable zone than just one of them, with their planets orbiting around both of the stars instead of just one.

Since the intensity of a light source varies according to the square of the distance, an object 1.4142 units as far away from a light sources will receive half as much light as an object 1.0 units from the source. So if the light source becomes twice as bright, an object 1.4141 units from it will receive as much light as an object 1 unit from the source did previously.

So the circumstellar habitable zone around a closing orbiting pair of twin stars will be 1.4142 times as wide as the habitable zone around only one of them would be. So the habitable zone around a close pair of F0 stars would about 3.4142 to 5.2354 AU, 1.81834 AU wide, and one around a closely orbiting pair of F8 stars would be about 1.5556 to 3.1112 AU, 1.5556 AU wide. That is 3.7038 to 4.3293 times the width of Kasting's conservative habitable zone and 1.8742 to 2.1907 times the width of Kasting's optimistic habitable zone.

So depending on the maximum possible number of Earth sized planets that could possibly orbit in Kasting's conservative or optimistic habitable zones, perhaps two to three, and possibly more, a close pair of twin class F stars might have room for three to twelve, or possibly more, Earth sized planets each with its own separate orbit, in their combined circumstellar habitable zone.

And so for a long time I imagined that a close binary of F class stars would be the type of star system most likely to have the maximum possible number - whatever that number was - of suitably sized and potentially habitable planets in the circumstellar habitable zone.

HOW MANY SEPARATE PLANETARY ORBITS CAN THERE BE IN THE HABITABLE ZONE OF A TRIPLE OR QUADRUPLE STAR?

But suppose that the maximum possible number of separate planetary orbits in even the combined circumstellar habitable zone of two Class F stars would be only five. How could one get as many as nine separate planets in separate orbits in a single star system then?

By having four class F stars in the star system, with two binary stars each orbited by a number of planets, and the two binary stars orbiting each other at a much greater distance than the planets orbited. Then one could have five planets orbiting one pair of stars in their combined circumstellar habitable zone (plus other planets not in the zone), and four planets orbiting the other pair of stars in their combined circumstellar habitable zone (plus other planets not in the zone), for a total of nine planets in the habitable zones (plus other planets outside the zones).

But if a writer wants or needs for story reasons to have all nine planets orbit around one star or group of stars in the circumstellar habitable zone, having the nine planets orbit two different pairs of stars will not do. It would be necessary to bring all four stars, both pairs, close enough together that planets can orbit in the combined circumstellar habitable zone of all four stars.

If there are four F class stars of (almost) identical luminosity in the center of the system, their combined luminosity will be four times that of a single one of them, and thus the inner and outer edges of their combined circumstellar habitable zone will be twice as far as those of a single one of those stars would be.

Is is possible for planets to orbit in the combined circumstellar habitable zone of four stars?

Calculations show that in a binary (two star system) it is sometimes possible, depending on various factors, for planets to have stable orbits around one of the stars, called S-type orbits, or for planets to have stable orbits around both of the stars, called P-type or circumbinary orbits. In some binary systems it would even be possible for planets to have stable S-type orbits around each of the stars with other planets having stable P-type or circumbinary orbits much farther out around.

The first known planet with a P-type or circumbinary orbit, Kepler-16 b, orbits around Kepler-16 A & B, at a distance of about 0.704 AU, which is about 3.2 times their separation of about 0.22 AU.

On the other extreme, FW Tauri AB b orbits at a distance of about 150-300 AU from FW Tauri AB, which orbit each other at a distance of about 11 AU.

Multiple star systems are usually structured with at least one pair that orbit very closely. When there are two pairs of stars the distance between each pair is usually tens or hundreds of times the distance between the stars in either of the pairs.

Assuming that an F class star is about 1,000,000 miles or 1,609,344 kilometers in diameter, and each pair of F stars is separated by 5 times their diameter, 5,000,000 miles or 8,046,720 kilometers, and the two pairs of stars are separated by 5 times the separation of each pair, and so the two pairs would be 25,000,000 miles or 40,233,600 kilometers apart.

A separation of 25,000,000 miles or 40,233,600 kilometers would be about 0.268945004 of an AU.

since it is estimated that:

It is estimated that the habitable zone of a relatively hot F0 star would extend from about 2.0 AU to 3.7 AU and between 1.1 and 2.2 AU for a relatively cool F8 star.

And since the combined circumstellar habitable zone around a quadruple system of identical stars would twice the inner and outer radii of a single star of that magnitude, such a combined circumstellar habitable zone would have an inner radius between about 2.2 AU to 4.0 AU and an outer radius between about 4.4 AU and 7.4 AU, depending on the exact luminosities of the class F stars.

An inner edge of the combined habitable zone between about 2.2 to 4.0 AU would be about 8.1801 to 14.8729 times the separation of the two pairs of stars. So if the two pairs of stars are separated by a very short distance, close to the minimum possible I think, planets at the inner edges of their combined habitable zone should have stable orbits.

So a quadruple star system of spectral class F stars is the system that I suggest has the absolute maximum possible sized habitable zone while remaining reasonably plausible.

DO MORE MASSIVE OR LESS MASSIVE STARS HAVE MORE ROOM FOR PLANETARY ORBITS IN THEIR HABITABLE ZONES?

It may be noted that a small change in the mass of a star will create a much larger change in the star's luminosity. A star with twice the mass of the Sun will have more than twice the luminosity of the Sun. A star with half the mass of the Sun will have less than half the luminosity of the Sun.

The intensity of a star's gravity will fall off with the square of the distance. The intensity of a star's light received on a planet will also fall off with the square of the distance. But since the luminosity of the star will increase or decrease much faster than the mass of the star changes, planets orbiting in the habitable zone of low luminosity stars should be in much stronger gravitational fields than planets orbiting in the habitable zones of high luminosity stars.

And I am not sure which will reduce the width of a planet's forbidden zone more and make it possible for more planets to orbit in the habitable zone.

Would more massive and brighter stars have room for more stable planetary orbits in their habitable zones, or would less massive and dimmer stars have room for more stable planetary orbits in their habitable zones?

SOME RECORDS OF EXOPLANET SYSTEMS

By now, over 4,000 exoplanets have been discovered in other star systems, and sometimes two or more planets have been discovered in the same system. In some planetary systems planets are space similarly to the way they are in our system, but in other systems the planets discovered so far are spaced far more widely, or far more closely, than in our solar system.

DISTANCE BETWEEN PLANETARY ORBITS

In the Kepler-70 system, Kepler-70c orbits about 0.0016 AU or about 240,000 kilometers farther out than Kepler-70b.

During closest approach, Kepler-70c would appear 5 times the size of the Moon in Kepler-70b's sky.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes6

The narrowest habitable zone for the Sun, calculated by Hart et al. in 1979, is between 0.95 AU and 1.01 AU from the Sun, or 0.06 AU wide. 0.0016 AU goes 37.5 times into 0.06 AU, so there is room for 38 planetary orbits separated by 0.0016 AU within Hart's habitability zone.

The much more widely used habitability zone calculated by Kasting et al. in 1993 has a conservative zone from 0.95 to 1.37 AU, 0.42 AU wide, and an optimistic zone from 0.84 to 1.67 AU, 0.83 AU thick. 0.0016 AU goes 262.5 times into Kasting's conservative zone and 518.75 times into Kasting's optimistic zone.

This questions requires that each planet has a natural satellite similar to the Moon. Since the Moon orbits Earth at an average distance of about 384.399 kilometers, the spacing between planetary orbits should be at least about five or ten times that much so that planets don't perturb each others's moons too much, and thus at least 1,942,995 or 3,843.990 kilometers - 0.012988119 to 0.02695486 AU.

So there should be room for about 3 to 5 separate planetary orbits with that spacing in Hart's zone about 0.06 AU wide, about 17 to 33 separate planetary orbits in Kasting's conservative habitable zone about 0.42 AU wide, and about 31 to 64 in Kasting's optimistic habitable zone about 0.83 AU wide.

RELATIVE SPACING OF PLANETARY ORBITS

The Kepler-70 system has the smallest spacing in kilometers or AU between planetary orbits. But what about the relative spacing of planetary orbits, the ratio between the semi-major axis of one planet's orbit and the semi-major axis of the next planet's orbit?

The Kepler-36 system has the smallest relative spacing of planetary orbits.

Kepler-36b and c have semi-major axes of 0.1153 AU and 0.1283 AU respectively, c is 11% further from star than b.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes6

To be precise, according to the figures given in that list, Kepler-36c is about 0.013 AU farther out than Kepler-36b, and has a semi-major axis that is about 1.1127 times that of Kepler-36b.

Hart's habitable zone for the Sun has an outer edge that is only 1.0631 times as distant as the inner edge. Thus it could have only one planetary orbit in it, if the minimum relative spacing of planetary orbits is 1.1127 times.

Kasting's conservative habitable zone for the Sun has an outer edge that is 1.4421 times as distant as the inner edge, and Kasting's optimistic habitable zone for the Sun has an outer edge that is 1.9880 times as distant as the inner edge.

If the minimum relative spacing of planetary orbits is 1.1127 times, there could be four planetary orbits in Kasting's conservative habitable zone, and seven planetary orbits in Kasting's optimistic habitable zone.

If the minimum relative spacing of planetary orbits is 1.1127 times, the minimum ratio between a ninth planetary orbit and the first planetary orbit would be about 2.349766 - the ninth planetary orbit would have a semi-major axis at least 2.349766 times that of the innermost planet.

Note that absolute size of a star's habitable zone doesn't make any difference to the number of planetary orbits that could be within it if that number of orbits is determined by the relative spacing of the planetary orbits.

The only ways to find room for nine planetary orbits with the habitable zone of a star would be to use a minimum relative spacing less than 01.1127 and/or to have a wider relative habitable zone, with an outer edge that is relatively farther out compared to the inner edge, at least 2.349766 times.

The table in the Wikipedia article Circumstellar habitable zone lists a number of papers that calculate different inner and outer limits of the Sun's habitable zone. Thus various combinations could produce much narrower or wider habitable zones for the Sun.

the four innermost inner edges are give as 0.912, 0.87, 0.75, and 0.38 AU, while the four outermost outer edges are given as 1.70, 2.0, 2.4, and 10 AU. the various combinations of them give ratios of outer edges to inner edges of 1.8640, 1.9540, 2.2666, 4.4736, 2.1929, 2.2988, 2.6666, 5.2631, 2.6315, 2.7586, 3.2000, 6.3157, 10.9649, 11.4942, 13.3333, and 26.32157.

And it is possible that if the minimum spacing of planetary orbits is determined by relative orbital ratios and not by distance, the minimum orbital ratio will be less than 1.1127, and that planetary orbits can have smaller ratios than 1.1127.

It is also possible that the minimum spacing of planetary orbits is determined by distance, and thus many stars would habitable zones wide enough for nine separate planetary orbits and even for several times nine separate planetary orbits. So finding out how to calculate the minimum possible spacing of planetary orbits would be useful.

NEED FOR LARGE MOONS?

The original question desires that all nine Earth like planets in the habitable zone should have large moons. Unfortunately the best present theory for the origin of Earth's Moon is that it is a cosmic accident, the result of a planetary collision of a specific and probably rather rare type. So if the accepted theory of the Moon's formation is correct, all nine improbable Earth like planets in a star's habitable zone would also have experienced highly improbable collisions that created large moons for them, greatly multiplying the improbability.

This multiplication of the improbability would probably be equally vast if the nine Earth like planets share a single orbit or have nine separate orbits.

Suppose for example, that an Earth like planet has only a 0.10 probability of having a large moon. Then the probability that two Earth like planets would have large moons would be 0.01, the probability of three would be have 0.001, the probability of four would be 0.0001, the probability of five would be 0.00001,the probability of six would be 0.000001, the probability of seven would be 0.0000001, the probability of eight would be 0.00000001, the probability of nine would be 0.000000001.

Does an Earth like planet need to have a large moon for habitability? Nobody knows.

https://www.npr.org/2011/11/18/142512088/is-a-moon-necessary-for-a-planet-to-support-life7

https://www.space.com/12574-moonless-earth-life-habitable-alien-planets.html8

https://www.astrobio.net/news-exclusive/earths-moon-may-not-critical-life/9

NINE PLANETS SHARING A SINGLE ORBIT

As some answers have stated, it is possible to have several planets, between seven and forty two, of equal mass and equally spaced, sharing a single planetary orbit, as suggested by a4android.

As a4android says, that is suggested in Sean Raymond's website, PlanetPlanet https://planetplanet.net/the-ultimate-solar-system/10 Which in turn is suggested by a paper by Smith and Lissauer (2010).

http://adsabs.harvard.edu/abs/2010CeMDA.107..487S4

And such a planetary configuration might be impossible to happen naturally, in which case a highly advanced civilization would have had to have built the planets and/or moved them into their orbits sometime in the past. That civilization might have been one of the societies presently living on the nine worlds, or a civilization that was active millions or billions of years before the time of the story, depending on the needs of the story.

And some of the other orbital configurations I have suggested in my answer might also be so very improbable that a planetary system with those configurations might have to have been constructed by a highly advanced civilization sometime in the past.

ADDED 08-22-2019

Someone has calculated how many more or less Earth-like planets could orbit in separate orbits within the circumstellar habitable zone of a star. And the answer seems to be up to five, according to the sources.

https://www.space.com/34555-how-many-planets-fit-inside-one-habitable-zone.html11

However, the calculations were said to have been made for a red dwarf star. And the article doesn't say whether the maximum number would smaller, the same, or larger for more massive G type and F type stars.

Assuming the maximum number of stable planetary orbits with the habitable zone of any star would be five, any attempt to have nine habitable planets in one solar system would have to have at least two stars, or put all the planets equally spaced along one single planetary orbit in the habitable zone.

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  • $\begingroup$ Absolutely delightful to see a comprehensive examination of possible stellar configurations & habitable zones. One additional factor you might consider is the time on the main sequence of F stars. They are close to being too short-lived for the probable of intelligent life (with Earth as a benchmark). Definitely +1. $\endgroup$ – a4android Aug 12 at 13:05
  • $\begingroup$ a4android F type stars are close to being too short-lived for the probability of oxygen atmosphere multi celled life, and intelligent being. But just barely on the edge. If Earth is typical, it should be rare, but possibly not unknown, for F type stars to have planets that have developed as much as needed within the lifetimes of those stars. Any system with nine planets habitable for Earth humans, as I think is the goal, would be very rare and improbable. $\endgroup$ – M. A. Golding Aug 12 at 18:45
  • $\begingroup$ "Very rare and improbable", my thoughts exactly. It's been a while since I looked at F type stars. I have a vague memory of realizing, I think, F6 stars last 4.5 billion years on the main sequence. Enough to evolve life & develop a technological civilization, but in face of stellar catastrophe. That said, your F type star multiple system is a nifty solution to the question. I appreciate the effort required for your answer. I salute you for it. $\endgroup$ – a4android Aug 13 at 5:23
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The answer is one. Your system of nine earthlike planets only need to be in orbit in the habitable zone of a single sunlike star. However, the habitable zone will be roughly one astronomical unit from the primary star. It simply isn't necessary to have more than one G-class dwarfs nor is it necessary to have the habitable zone wider than one astronomical unit.

Basically this problem has been solved by the astrophysicist Sean Raymond as part of his project to build the ultimate solar system. Your hypothetical solar system only requires a cut-down version; one that is far from ultimate.

But why skimp. Let's look at the ingredients for making Sean Raymond's Ultimate Solar System 1, and see if can easily fit in the Nine Norse Realms based on earthlike planets. In fact, there will be extra planets which can be whatever the OP desires them to be.

Let’s look at our ingredients.

A star a little smaller than the Sun (50-70% of the mass of the Sun).

Worlds (planets or moons) between half and twice the size of Earth, with between one-tenth and ten times as much water as Earth.

Any other accessories that we might need (e.g., gas giant planets).

We can arrange these ingredients in any way we choose. Our goal is to pack as many worlds as we can into the habitable zone. We can use our ninja tricks (co-orbital planets and moons). But the system must remain stable for billions of years. It won’t do us any good if there is a dynamical instability that sends a bunch of habitable worlds falling onto the star.

Now let's see what Sean's Ultimate Solar System 1 will look like and how many planets it will hold.

ULTIMATE SOLAR SYSTEM 1. Let’s only include Earth-sized worlds. No gas giants. As we saw, the number of planets we can cram into the habitable zone depends on how big they are. Bigger, more massive planets need to be more widely spaced. And for maximum orbital compactness we want planets that are all the same size. I’m going with planets that are half the mass of Earth (about 80% as big as Earth). Planets of this size definitely satisfy the criteria for habitability. I guess I’m a little nervous that the very smallest planets in our chosen mass range might be borderline habitable. And we don’t want to end up with a system full of Marses!

The diagram below provides an image of what this will like schematically.

enter image description here

Our first ultimate Solar System. Each orbit around the star (thick gray line) contains two pairs of binary Earths in a co-orbital (Trojan) configuration. The green shaded area represents the habitable zone.

We can fit six stable orbits into the habitable zone. Each orbit has two sets of binary Earths. These are Earth-sized planets with Earth-sized moons. Each binary planet is in a Trojan (co-orbital) configuration with another binary planet, separated by 60 degrees on their orbit around the star.

Six orbits. Two binary planets per orbit. Two planets per binary. That makes 24 habitable worlds in a single system!

If we take Sean Raymond's Ultimate Solar System 1, there are 24 earthlike planets. More than enough room to find locations for Nine Norse Realms and have fifteen other earthlike planets available for anything else. perhaps, a Valhalla or two?

Considering ultimate solar systems aren't likely to form naturally, but since what we're talking about is the home of the goods, only Norse gods admittedly, then surely they can move in their mysterious ways to move exactly the right number of co-orbiting planets they need for all their realms, with a few spare for good measure.

In conclusion, if the star only needs to be in range of 50% to 70% of solar mass, then a single K class star should be able to fill the bill.

Acknowledgement: This answer would not be possible if it wasn't for the brilliant and imaginative work of Sean Raymond who is a professional astrophysicist. His website is one that should be frequented by all good worldbuilders wanting to build interesting and exotic solar systems and planets.

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    $\begingroup$ I have no idea why this was flagged as low-quality. It may or may not be a good answer (I'm not going to vote on it as it's not my area of expertise) but it's a full answer. If you think an answer is wrong then downvote it. Answers should only be deleted if they are incomplete, not answering the question, or inappropriate in some way. This answer is none of those things. $\endgroup$ – Cyn says make Monica whole Aug 11 at 13:51
  • $\begingroup$ You're asking for a lot of tidal locking, which is NOT Earthlike at all. $\endgroup$ – JohnWDailey Aug 11 at 13:53
  • $\begingroup$ @JohnWDailey An interesting point. The planets would be Earthlike, within the criteria of your question, but they would have much longer days and nights. Very interesting climates too. Kindly refer to my other answer which IMHO is a better solution. $\endgroup$ – a4android Aug 11 at 14:07

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