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So I have a concept for a habitable alien planet that has 7 suns.

This planet is supposed to have a surface similar to Earth, it doesn't have to be habitable to humans but should be capable of hosting humanoid life.

Apparently, the maximum known amount of stars that a planet can orbit is 3. It is theorized that a planet can orbit more stars but none have been located in the Galaxy that fit such a criteria.

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    $\begingroup$ Nu Scorpii is the first place to start, but I'm not sure where you'd go from there. $\endgroup$ – Halfthawed Feb 23 '20 at 19:48
  • $\begingroup$ Huh.... I've never heard of Nu Scorpii before. Are there any other discovered star systems with more suns? $\endgroup$ – JordanTheCynic Feb 23 '20 at 20:38
  • $\begingroup$ Related: The Saga of Seven Suns by Kevin J Anderson has a planet just like you describe. The orbital dynamics are not elaborated on, but the planet is described as never having night. $\endgroup$ – Luke Feb 24 '20 at 21:56
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Short Answer:

It might, repeat might, be possible for a habitable planet to have seven suns in its sky. But such a solar system might have to have been created by a very powerful civilization.

Long Answer:

The original question asks:

So I have a concept for a habitable alien planet that has 7 suns.

This planet is supposed to have a surface similar to Earth, it doesn't have to be habitable to humans but should be capable of hosting humanoid life.

Apparently, the maximum known amount of stars that a planet can orbit is 3. It is theorized that a planet can orbit more stars but none have been located in the Galaxy that fit such a criteria.

What is a star?

A star is a giant spheroidal ball of plasma with fusion reactions at its core which emit strong amounts of electromagnetic radiation.

What is a sun?

A sun is The Sun, the star Sol that the planet Earth and some other planets, dwarf planets, asteroids, comets, etc. orbit. Or else it is a star that appears as a disc in the sky of a planet that orbits it closely enough, just as the Sun appears as a disc in the sky of the planet Earth.

Since there are two known star systems which apparently include seven stars, Nu Scorpii and AR Cassiopeiae, it is possible for a planet to orbit around one or more stars in a stars system which has seven stars.

https://en.wikipedia.org/wiki/Star_system#Septenary1

https://en.wikipedia.org/wiki/Nu_Scorpii2

https://en.wikipedia.org/wiki/AR_Cassiopeiae3

But if would be very unlikely for a planet to orbit around all seven stars in a septenary star system.

Star systems have hierarchical arrangements, with a pair of stars in them separated from other stars by distances which are several times the distance between the two members of the pair. So a quartenary star system, for example, usually has two close pairs of stars, with the paris separated by greater distances, or one close pair of stars and two single stars orbiting at greater distances.

Most multiple-star systems are organized in what is called a hierarchical system: the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on.12 Each level of the hierarchy can be treated as a two-body problem by considering close pairs as if they were a single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable5[13] Keplerian orbits around the system's center of mass,[14] unlike the unstable trapezia systems or the even more complex dynamics of the large number of stars in star clusters and galaxies.

https://en.wikipedia.org/wiki/Star_system#Hierarchical_systems4

Some multiple star systems are trapezia, which are not hierarchical, but tend to be unstable and to split into smaller systems relatively soon.

Trapezia are usually very young, unstable systems. These are thought to form in stellar nurseries, and quickly fragment into stable multiple stars, which in the process may eject components as galactic high-velocity stars.[28][29] They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula.[28] Such systems are not rare, and commonly appear close to or within bright nebulae. These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is called interplay.[30] Such stars eventually settle down to a close binary with a distant companion, with the other star(s) previously in the system ejected into interstellar space at high velocities.[30] Example of such events may explain the runaway stars that might have been ejected during a collision of two binary star groups or a multiple system. This event is credited with ejecting AE Aurigae, Mu Columbae and 53 Arietis at above 200 km·s−1 and has been traced to the Trapezium cluster in the Orion Nebula some two million years ago.[31][32]

https://en.wikipedia.org/wiki/Star_system#Hierarchical_systems4

A stable seven star systems would probably be hierarchical, which would mean it contained, one, two, or three close binary pairs of stars, and one to five single stars, all the stars and pairs orbiting the common centers of gravity. And the distances between pairs, or single stars, would be at least several times the distances between two close stars in a pair.

So if a planet orbits around a single star in a septenary system, the single star that it orbits can appear as a sun in its sky if it orbits close enough. But the other pairs and single stars will probably appear as very bright points of light in the sky, because they will be many times, possibly hundreds or thousands of times, as far away as the star that it orbits. So the other six stars will not look like suns.

So if a planet orbits around a pair of stars in a septenary system, the pair of stars that it orbits can appear as two suns in its sky if it orbits close enough. But the other pairs and single stars will probably appear as very bright points of light in the sky, because they will be many times, possibly hundreds or thousands of times, as far away as the pair of stars that it orbits. So the other five stars will not look like suns.

In a binary star system, with only two stars, there are two possible types of orbits that a possible planet could have, S-Type and P-Type.

Non-circumbinary planet (S-Type)

In non circumbinary planets, if a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.5 Whether planets might form in binaries at all had long been unclear, given that gravitational forces might interfere with planet formation. Theoretical work by Alan Boss at the Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars.6

Studies of Alpha Centauri, the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 au distance at closest approach (23 au mean), and both have stable habitable zones.2 A study of long-term orbital stability for simulated planets within the system shows that planets within approximately three au of either star may remain stable (i.e. the semi-major axis deviating by less than 5%). The habitable zone for Alpha Centauri A extends conservatively estimated from 1.37 to 1.76 au2 and that of Alpha Centauri B from 0.77 to 1.14 au2—well within the stable region in both cases.8

For a circumbinary planet, orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.

The minimum stable star to circumbinary planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.9

For example, Kepler-47c is a gas giant in the circumbinary habitable zone of the Kepler-47 system.

If Earth-like planets form in or migrate into the circumbinary habitable zone they are capable of sustaining liquid water on their surface in spite of the dynamical and radiative interaction with the binary star.10

The limits of stability for S-type and P-type orbits within binary as well as triple stellar systems have been established as a function of the orbital characteristics of the stars, for both prograde and retrograde motions of stars and planets.11

https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems5

Planets have been discovered in binary systems with S-Type orbits and others have been discovered with P-Type orbits as this list shows:

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes#Orbital_characteristics6

JordanTheCynic wrote:

Apparently, the maximum known amount of stars that a planet can orbit is 3. It is theorized that a planet can orbit more stars but none have been located in the Galaxy that fit such a criteria.

it is not certain whether they meant that no planet is known to orbit in a system with more than three stars, of if they meant that no planet is known to orbit around more than three stars.

If JordanTheCynic meant the first, there seems to be an exoplanet in a quartenary system with four stars. The planet Kepler 64 b or PH1, in the Kepler 64 system, has a circumbinary orbit, orbiting two of the four stars, while the other two stars orbit outside the orbit of Kepler 64 b.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes#System_characteristics6

If JordanTheCynic meant the second, I don't know of any known exoplanet that orbits more than two stars, even in systems o with three or four stars.

Of course it is possible for an exoplanet to have an orbit that is very wide. For example the exoplanet DT Virginis c orbits the star DT Virginis at a distance of 1,168 AU. An AU is an Astronomical Unit, the standard distance between Earth and the Sun. Dt Virginis, or Ross 458 AB, is a binary star, so DT Virginis c is the circumbinary planet with the widest known orbit.

If a planet orbited at the distance of DT Virginis c, there could be seven stars orbiting within 0.25 to 0.50 of that distance, within 292 to 584 AU of the center of gravity of the system. For example, there could be pairs of stars with separations of 0.2 to 0.3 AU. Two pairs could orbit at a distance of 3 AU, another pair could orbit at a distance of 30 AU, and a single star could orbit at a distance of 90 AU, and the planet in question could orbit at a distance of 450 AU and have a stable orbit.

But if a planet orbited the center of gravity at a distance of 450 AU, even the outermost star at 90 AU would never be closer than 360 AU from the planet, which would be so far away that star would appear as a bright point of light in the sky of the planet. It wouldn't look like a disc like "sun".

JordantheCynic wrote:

This planet is supposed to have a surface similar to Earth, it doesn't have to be habitable to humans but should be capable of hosting humanoid life.

If JordanTheCynic meant that the planet has to be habitable for carbon based lifeforms that use liquid water, and a biologically roughly similar to Earth lifeforms, and meant that the lifeforms have to include highly advanced multi celled lifeforms including intelligent beings, that makes the problem even harder and more complicated.

On Earth the first lifeforms appeared only a few hundred million years after the planet formed. But it took billions more years for the life to evolve into multi celled organisms and for large amounts of oxygen to accumulate in the atmosphere, making it breathable. So scientists believe that it usually takes several billions of years for a planet to become interesting for typical science fiction plots by becoming habitable for humans or similar lifeforms or for intelligent life to evolve.

And a planet's star has to have a rather stead luminosity for all those billions of years or else large changes in its luminosity and the temperature of a planet orbiting it will wipe out any life on that planet.

Astronomers have calculated how long different types of stars can remain on the main sequence and shine with a relatively steady and slow changing luminosity necessary for life.

As a result, Robert A. Heinlein's juvenile science fiction novels Starman Jones (1953) and Time for the Stars (1956) have scenes when the spectral types of stars likely to have habitable planets are discussed. The first edition of Habitable Planets for Man, Stephen Dole, (1964, 2007) discussed the spectral types of stars suitable for having habitable planets, restricting them to main sequence stars including some spectral type F, all spectral type G, and some spectral type K, with the possibility that spectral type M stars might possibly be able to have habitable planets depending on various unknown factors.

Science Marches On, as TV Tropes says, though their astronomy examples don't seem to mention the spectral types of stars suitable for habitable planets as an example.

https://tvtropes.org/pmwiki/pmwiki.php/ScienceMarchesOn/Astronomy7

So science fiction writers who assume that any type of star might possibly have a naturally occurring planet habitable for humans or advanced lifeforms similar to those on Earth are using scientific ideas which have been obsolete for fifty or sixty years.

Of course there is always the possibility that a highly advanced civilization could terraform and make habitable planets orbiting otherwise unsuitable stars, or move naturally habitable planets from suitable stars to unsuitable stars.

Thus the problem of having a fictional habitable planet orbiting around seven stars, or orbiting one or two stars in a system with seven stars total, and being close enough to all seven of those stars for them all to appear as disc like suns in its sky, is made worse by the restriction of suitable stellar spectral types to class F and smaller stars. All the really large types of stars, which can be seen as disc like suns at the greatest distances, are unsuitable to have habitable planets in the same star system.

Fortunately there is a blog called PlanetPlanet, with a section called Ultimate Solar System, with various posts or articles about creating imaginary solar systems with the greatest number habitable planets.

In one article, The Ultimate Engineered Solar System, a paper is mentioned that points out that many objects of identical mass can orbit in the same orbit if spaced at equal intervals around the orbit. This works for seven to forty two objects according to the paper.

https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/9

https://ui.adsabs.harvard.edu/abs/2010CeMDA.107..487S/abstract10

So one could have imagine a star system with a large central star and seven to forty two smaller stars orbiting it in a ring at the same distance, and with a habitable planet orbiting the central star either inside the orbit of the ring of stars or outside the orbit of the ring of stars. A planet in such an orbit would experience only minor fluctuations in temperature as it orbited.

But a central star massive enough to hold on to seven to forty two smaller stars in orbit around it would probably be far too massive to have naturally developing habitable planets.

And the question asks for seven suns in the sky, and such a system would have between eight and forty three suns in the sky, which is more than required. And the central sun might be far more luminous and sun like than the others.

If the central star was invisible or dark, and there were only seven smaller stars in the ring of stars, then the planet could have the required seven suns in the sky. If the central sun was a massive black hole, it wouldn't emit any light and it would be dark and wouldn't look sunlike.

So the solar system might resemble this one:

https://planetplanet.net/2018/03/21/asimov-kalgash-take2/11

or this one:

https://planetplanet.net/2018/05/30/the-black-hole-ultimate-solar-system/12

In having a central super massive black hole orbited by stars and planets.

And of course such a system would be rather improbable, so it would probably have to have been constructed by a highly advanced civilization instead of having formed naturally.

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I got your 7 suns right here.

https://astronomy.com/magazine/2012/02/watch-stars-orbit-our-galaxys-central-black-hole

stars around a black hole

Two groups of astronomers have separately tracked the orbits of tens of stars at the center of the Milky Way for about 20 years. These observations provide the strongest evidence that a supermassive black hole exists at that location. The scientists have obtained enough detail to determine the exact orbits of a handful of those stars — they move in ellipses (like the planets in our solar system) and thus are clearly orbiting a single object.

Your planet is orbiting a star which itself is orbiting a black hole. Other stars orbit this black hole as well, and are at varying distances from your planet and its sun. These other stars are visible in the sky and probably range in brightness from how Venus looks on Earth to the brightness of a full moon.

Of course these stars are much more distant from your planet than either Venus or the moon is from us. Usually. This might not be a stable orbital pattern.

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  • $\begingroup$ If the stars are orbiting instead of being orbited, would they still be called 'suns' or 'planets'? $\endgroup$ – Justin Thyme the Second Feb 23 '20 at 22:48
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    $\begingroup$ @JustinThymetheSecond - From the perspective of the planet the other stars would wander like planets, the wanderers. But they would shine with their own light, not reflected light. And they are big and fairly close. $\endgroup$ – Willk Feb 23 '20 at 23:16
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Yes and no.

This is even more extreme than Asimov's Nightfall planet Lagash, which only had six suns.

However, yes, you could have a seven suns system as long as your planet orbited one of the suns, and no other sun passes too near its one (otherwise it'd risk destabilizing the planet's orbit or even snatching the planet away).

There is the big problem of the stability of the seven suns system. Normally the stars would slingshot past one another, and within very little time one star would gather kinetic energy at the expense of the others, and be flung away into outer space. You can easily simulate this with any particle simulation system.

The trick to avoid this is that the binding energy of the system must reside for more than a specific threshold (which I don't remember, but really high) in one of the bodies. That is, six suns must orbit the seventh. This means that the seventh sun must be massive, which forces it to be large, hot, and short-lived. After relatively few tens of millions of years, it would go nova or even worse, a type II supernova. In all cases, goodbye habitable planet.

So your system would need to have come along a different way:

  • start with Willk's suggestion, a large black hole with six companion stars at a large distance.
  • enters the seventh sun, with our habitable planet. One advantage to this scenario is that the newcomer can have a different metallicity, and a completely different history.
  • a sufficiently long-range four-body interaction between the seventh sun and three of the six incumbents leads to a gravitational capture with reduced acceleration, to the point that the planet's orbit is not significantly perturbed (this, I fear, requires a relatively small seventh sun, so that its Goldilocks zone can be relatively safely close to its primary. Not sure it would work, but it's plausible)
  • and now you have your habitable planet with seven suns.

One requirement would almost have to be that during the capture, the orbital radius of the planet is increased slightly. This would allow the planet to survive the extra heat coming in whenever one of the other suns comes near.

Because here's the rub: while the planet would have, on paper, seven suns, six of them would stay well beyond the distance between our Sun and Pluto, which means that they would be visible not as suns, but as bright stars or small moons. This is one of many simulations of the Sun as seen from Pluto.

Sun seen from Pluto (artwork)

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  • $\begingroup$ From Pluto, I would certainly not see Neptune so big in the sky (more likely this is a Neptunian satellite other than Triton). Also, even if I saw a big Neptune near the Sun in the sky, its unlit face would show up as being black. $\endgroup$ – Victor Stafusa Feb 23 '20 at 22:53
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    $\begingroup$ @VictorStafusa I understand that's the artist's view of Charon, Pluto's satellite (more like a twin planet actually). However, the purpose was to show how the Sun might look at that distance - a really bright point around -20 magnitude, more or less. Your observation on the unlit face being black is of course absolutely correct. $\endgroup$ – LSerni Feb 24 '20 at 0:21

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