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Is a solar system where the center is a planet possible? Where would the heat on it and its moons come from? Would life of any kind even be possible? Would everything be pitch-black, or would some light come from stars?

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    $\begingroup$ Just wanted to point out that a solar system by definition has a star or stars at its center. $\endgroup$
    – Shrike
    Oct 1 '20 at 18:14
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    $\begingroup$ Might be theoretically possible if the planet sits right at the gravitational center of a pair of stars... not sure this is stable, though, and it wouldn't have much of a night. $\endgroup$
    – Matthew
    Oct 1 '20 at 18:15
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    $\begingroup$ Is it required that there be any kind of sun/star in this system at all? $\endgroup$
    – cowlinator
    Oct 2 '20 at 1:33
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    $\begingroup$ See @Ash's answer, but basically a star and a planet orbit around a common point (the en.wikipedia.org/wiki/Barycenter) so rather than one or other being in the centre the question could usefully be rephrased based on whether the barycenter could be within a planet rather than within a star. There is, I believe, a "hard SF" story where a planet turns out to have a neutron star travelling through tunnels inside it, in which case the barycenter is technically inside both. $\endgroup$ Oct 2 '20 at 10:19
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    $\begingroup$ I think the big problem is gravity, a planet with much more gravity than earth life would be crushed to death. That puts strict limits on the size of your planet, and that put strict limits on the total size of your solar system. Maybe microbes would be possible at high gravity, but not complex life. Even if life could manage gravity 10x earth's gravity a planet 10x the mass of earth still wouldn't support much of a solar system. It would be more like moons. You would need orders of magnitude more mass and thus gravity to have a solar system. $\endgroup$
    – cybernard
    Oct 3 '20 at 14:07
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What you are describing is a rogue planet with moons. This is a planet that does not orbit any star, either having been ejected from its original solar system, or never having belonged to a solar system in the first place. This would not be aptly described as a solar system, since there is no star, but you could have a rogue planet with moons orbiting it. A rogue planet could perhaps be described its own system, as it does not belong to any solar system.

Rogue planets are naturally dark and cold, since they do not receive significant insolation from any star. Starlight, though weak, is still light however, so the planet would not be entirely pitch black. Any heat would have to come from geothermal or radioactive activity within the planet itself, as there is no significant external source of energy like the Sun. Rogue planets that don't have their own internal sources of energy will be dead. I wouldn't say it's impossible for life to exist on a rogue planet, but I'd expect the odds to be much lower simply due to far lower abundance of available energy.

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    $\begingroup$ If however, a rogue planet passed near a normal solar system which hosted a K1 or higher civilization and if that civilization had no FTL capability, it might colonize such a planet as an exploration method or as insurance against any event which might destroy their home solar system. Such a colony would need K1 level energy generation, artificial heat and light to thrive long-term, but given such resources and history, a rogue could host intelligent life. $\endgroup$ Oct 1 '20 at 18:56
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    $\begingroup$ @HenryTaylor Agree, I think life colonizing a rogue planet could be as or more likely than life developing there independently. $\endgroup$ Oct 1 '20 at 19:04
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    $\begingroup$ @HenryTaylor Civ>K1, agreed. A rogue planet's probably not a good exploration vessel unless you're at least K2. It's probably only a couple hundred km/s interstellar, aimed into a random location in the Outer Dark, and might not swing on another star for millions of years. I'd probably want to run my exploration vessels up to at least a moderate c-fraction, and steer them toward places where I want to go. And if you can do that with a planet... $\endgroup$
    – notovny
    Oct 2 '20 at 0:14
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    $\begingroup$ @notovny, the result of your K2-level navigable exploration planet still fits with what the original question is asking for, a planet with a couple moons, moving alone through space. Whether the occupants can control its journey or not, or whether they even remember that they were once in control is interesting but not crucial to the question's intent. If an author wants a dark sunless world on which to set their story, it is possible. $\endgroup$ Oct 2 '20 at 1:40
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    $\begingroup$ Have an upvote for almost tricking me into "correcting" a "typo" and teaching me a new word, 'insolation'. $\endgroup$
    – Ton Day
    Oct 2 '20 at 3:27
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A single big planet and a single big star orbiting around? No.

Wikipedia has a list of star extremes. This star is the smallest, 7% the mass of our sun. (So about $7*10^{28}$kg)

This planet is the largest, 20 * times the mass of jupiter (So about $3.7*10^{28}$kg)

These are very close in mass, they'll orbit around a point 2/3rd the distance from the sun to the planet.

But can we get a planet with a star orbiting around it?

Yes There are some precise configurations that have a massive planet in the centre and a big sun orbiting around it.

The simplest would be 3 massive planets, and a tiny sun. The tiny sun is twice the weight of each planet, and all 3 planets are the same weight. The tiny sun and 2 of the big planets share an orbit, with the two planets close together. The forces should cancel out, leaving the big planet in the barycentre of the system.

enter image description here

Note this isn't called a solar system - technicality.

Life on the planet will be very similar to if the sun was in the centre and the planet orbited around it. Ironically, if they're aware of normal solar systems; It may take them a while to realise the sun isn't the centre, actually)

This system is highly unlikely to occur naturally - perhaps a supernova blew out a chunk of gas cloud, forming a ring of gas, which formed the star and 2 gas giants? It's a bit of a stretch. Perhaps the bodies were precisely caught in the right way, or perhaps aliens built it. This would also not be stable for millions of years, it would decay over time.

Can we do it slightly more stable?

If we allow the planet to move, but meet the "at the centre" requirement by having nothing closer to the centre than the planet, we can have a slightly more stable system by having the big planet in a tight multi-body orbit around nothing (ie "at the centre"), with the sun in orbit around that mutual centre.

enter image description here

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    $\begingroup$ "It may take them a while to realise the sun isn't the centre, actually"... this seems unlikely, as humans once believed the sun revolved around the earth (n.b. Ptolemy's model). They would need to somehow have an a priori notion of Galileo's model. Humans with knowledge of Terran science magically teleported to the planet's surface might make such a mistake, but an indigenous people, or even humans ignorant of Galileo's model, most likely will jump to the conclusion that happens to be correct in this case. $\endgroup$
    – Matthew
    Oct 1 '20 at 18:54
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    $\begingroup$ The scenario in the diagram looks inherently unstable. As soon as the central planet gets perturbed even slightly, it will drift toward whatever body is closest. Unless the forces from the two orbiting planets and the star are perfectly balanced at all times, it'll be a runaway feedback loop as the central planet gets pulled toward the edge more and more strongly. Such a configuration would be unlikely to occur in nature, and would fall apart fairly quickly if it did. $\endgroup$ Oct 1 '20 at 19:19
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    $\begingroup$ This system stands upon the edge of a knife. Stray but a little, and it will fail, to the ruin of all. $\endgroup$
    – cowlinator
    Oct 2 '20 at 1:35
  • $\begingroup$ The second one might be a bit more stable, but I suspect that if you had a trio of brown dwarves sitting within the habitable zone of a red dwarf, they'd probably be inside their Roche limits and they'd promptly proceed to tear each other apart to form a new red dwarf star. $\endgroup$
    – nick012000
    Oct 2 '20 at 7:08
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    $\begingroup$ Forming such a system would be tricky... stars are the start of the process, and matter clumps together to form planets - at which time they're positively tiny compared to the star, so they naturally orbit around it. You'd need some very fluky external influence to knock the star and planets around to form such a system, but given the billions of stars in the universe, there's probably one a bit like this somewhere ;-) $\endgroup$ Oct 2 '20 at 13:20
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Huge gas giant rotating fast.

auroras of io

Behold the glowing sky of Io!

https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA01637

This eerie view of Jupiter's moon Io in eclipse (left) was acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter.

Io has different types of aurora. They are produced by interactions with Jupiter. Jupiter has a tremendous magnetic field and given off loads of radiation in the form of charged particles. Ultimately I think the energy fueling this is the rotational momentum of the Jupiter and maybe the residual heat of condensation when it was formed.

Your central planet is a colossal gas giant, 20 times the size of Jupiter. Its great mass and fast rotation generate huge magnetic fields. Your moons also have magetic field and atmospheres - these moons are the size of Earth. They also have magnetic fields which they need to protect them from the radiation emitted by their giant. The charged particles splash against the magnetic fields of the moon, lighting the sky just as Jupiter's particles light the skies of Io.

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Just yeet out a Gas Giant

So in general, you could have such a System. If you look at the Jupiter System, it is pretty much its own "Solarsystem". You got a bunch of Moons around a Central Object with a shi´tton of debris flying around.

This could just from natrually in the middle of nowhere. One might argu that such a System is just like or one. Only smaller and with a failed star at its center.

But how do you get light ?

Good question. Id say since the Gas Giant is the middle of nowhere, it probably wont emit any light by itself. If it would, it is a Mini Star. So one way of getting a bit of light would be to have a Planet VERY close to the Gas giant. In fact so close that it melts dou to the tidal forces. But even this would be really dark. Not to mention that such a close Planet would just fall into the Gas Giant in a very short amount of time.

You could try to go all Meta and have a lifeform on the Gas Giant that is Bioluminescent for some reason. Depending on the sizes of the Gas Giant that may create enough light for something. But i am not quiet sure why any life form would decide to go that way in the Darkness of Interstellar Space. Maybe because of some Aurora but even that is a real streatch because, where does the Aurora come from ?

But i would still assume that a Bioluminescent Gas Giant is probably your best bet of getting any amount of light. Even if it still is almost nothing. Such a Gas Giant would probably by hardly any more bright than or Moon.

Your main source of Energy in such a System isnt light anyways. Its the tidal forces. And the first life on earth really didnt need light so it might very well still start. But i dont see how life would get complex if every Moon around the Gas Giant is a frozen Ice ball.

The Stars in the night sky would be pretty though.

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This is not a realistic scenario, but may work for a sci-fi universe.

Imagine a rogue planet with some orbiting moons out in the darkness of interstellar space. Also imagine some alien race that can construct low-mass (less than a star) wormhole-like portals.

They construct a pair of such portals, one in orbit of a star far away, and one in orbit of the rogue planet. The mass of the portal is not enough to cause the planet to orbit around it. The portal also lets heat, light, and radiation flow through. This may be enough to allow some life (seeded by the alien race) to develop on the planet.

From the perspective of beings on the planet, they see the "sun" rise and set like we do.

Maybe it is part of a Truman Show-like experiment for the aliens.

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Oversize "Metal"-heavy Brown Dwarf

I suppose in theory you could somehow have a planet that was massive enough to be a red dwarf but which was predominantly elements heavier(*) than hydrogen and helium. So the core of the planet would be hot enough and have a high enough pressure to fuse hydrogen, but it's choked by carbon and heavier elements which it can't fuse, pushing hydrogen and helium up to less-dense levels of its mantle. It would still probably be able to fuse deuterium above the core, though, which would basically leave it as an oversize brown dwarf.

You could then pair it with a minimal-mass red dwarf and have what would effectively be a binary system with the brown dwarf as the larger of the two primary bodies. Smaller planets and moons could then be added as required, either orbiting one or the other of the two dwarfs as moons or orbiting the system barycenter as planets in their own right.

Just to be clear, though, this would require enormous amounts of heavy elements. There would be the equivalent of multiple Jupiters or several thousand Earths in the core of your brown dwarf.

(* - to be clear, when I say "heavy" or "metal", I'm using the astronomer's definition of "anything heavier than helium". So, carbon, oxygen, nitrogen, and on up to mostly iron at the top end.)

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