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I'm trying to construct a habitable world which has a sun orbiting around it. I know a star can't orbit around a planet, because a star is much more massive than a single planet, which has been known here:

Can a habitable planet have mini-suns (i.e. solar satellites or glowing moons)?

But how about a star orbiting around MULTIPLE planets? For example:

  1. There is a multiple plantary system, which contains several planets without a sun, but they are very massive so that their total mass exceeds the lower mass limit of star

  2. Later, a very small star (just reached lower mass limit of star) is captured by the whole system above, which starts orbiting around the whole system

Is that possible?

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    $\begingroup$ en.wikipedia.org/wiki/Barycenter read, understand $\endgroup$ – Raditz_35 Jul 19 '17 at 9:07
  • $\begingroup$ You could probably have a binary system with stars that have more or less equal mass and put your planet at the barycenter or maybe at Lagrangian points. Exterior forces, though, may destabilize the system. $\endgroup$ – Reaper Jul 19 '17 at 15:27
  • $\begingroup$ Would an artificial planet work for your story? If so perhaps the planet is in fact some kind of Dyson sphere, with a star at it's centre. It's this star your second sun actually orbits around. To the unknowing inhabitants of the 'planet' it appears the sun orbits their planet. The possible issues are the size of such an object and maybe the gravity affecting it's inhabitants (I don't know enough to comment on what effects might be likely) $\endgroup$ – adaliabooks Jul 20 '17 at 16:44
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This is simultaneously both impossible and what already happens.

You need to remember that in space nothing really orbits anything else. By which I mean they are always orbiting their common center of gravity. Now in cases where one is much more massive such as our sun that common center of gravity is very close to the center of the much more massive body. In other words The Sun is getting pulled towards The Earth just as much as The Earth is being pulled towards The Sun. The only difference is the Sun is so much more massive that it only moves a tiny amount by comparison.

So to get what you are looking for then you need the cluster of orbiting planets to somehow be significantly more massive than a star so that the common center of gravity is not inside the star.

That sort of structure is going to be basically impossible to form, you just can't get enough mass from planets that still look anything like planets and which are in a stable situation.

Your best bet may be to have a neutron star or other stellar remnant in a binary system with the star. The planets can then be orbiting the neutron star and the other half of the binary would appear to be orbiting them (although again the common center of gravity would be somewhere between the neutron star and the star).

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    $\begingroup$ In the case when one celestial body is much more massive than the other, the center of mass is so close to the first that it is located inside said celestial body. Such is the case of Earth-Sun barycenter. Tim, feel free to add this to your A if you feel like. +1'd $\endgroup$ – Mindwin Jul 19 '17 at 13:50
  • $\begingroup$ @Mindwin Yes, that's what I meant by "very close to the center of the much more massive body" although your comment is rather more explicit :) $\endgroup$ – Tim B Jul 19 '17 at 14:35
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You have two options:

  1. A single massive planet (around 1 Jupiter) and several smaller ones (Earth or Neptune like).

In this case the Jupiter-like would act as center of mass for the system, and the other planets would orbit it. But then the arrival of a new kid in town, a star, would disrupt the system, as the star would in any case be more massive than the Jupiter-like. Result: scattered planets all around

  1. A bunch of Jupiter like planets.

In this case the bodies will initially orbit their common center of mass (which in case of more than 2 bodies is already a chaotic system). Again, the arrival of the star will simply mess up the system, slingshotting all the planets away.

To summarize, it is highly unlikely that such an encounter will settle down pacifically.

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  • $\begingroup$ What would the timeframe be for these disruptions? Would the whole system be torn apart in years, decennia, millenia? $\endgroup$ – Falc Jul 19 '17 at 7:51
  • $\begingroup$ @Falc, assuming the star is entering the system at velocity low enough to have chances of being captured or capture, it can happen within few years or at best few centuries, I guess. $\endgroup$ – L.Dutch Jul 19 '17 at 7:58
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    $\begingroup$ One can add that the odds of a bunch of Jupiters finding each other (I highly doubt they can be "born" in the same place without a star forming) and then a star arriving in just the right distance and momentum are "astronomical". One would need - let's be optimistic - 50 slightly-larger-than-Jupiters to catch your average brown dwarf $\endgroup$ – Raditz_35 Jul 19 '17 at 9:17
  • $\begingroup$ Keep in mind that even Jupiter (2.5 times as much mass as all the other planets put together) is still 1/1000th the size of the sun. You would need 1000 Jupiter sized planets just to have the barycenter be mid-way between them and the sun. $\endgroup$ – Tim B Jul 20 '17 at 16:30
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Definitely not as a natural configuration. In our solar system, the sun makes up 99.86% of the total mass. Even if you assume many Jupiter-sized planets (at 0.0009546 solar mass each) and a smaller star, getting it all to orbit around anything that isn't the star simply won't happen.

As a construct, it comes down to how far you're willing to go in allowing super-tech (or magic, depending on your genre) to create solar-level energy output from vastly sub-solar masses.

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You could have a star orbit a planet but the star would be an artificial satellite created to terraform the planet.

Stars need mass to kick off the fusion/ fission reaction. It's mass stops it from flying apart

To make a small artificial sun, you'd need a satellite capable of creating a large magnetic bottle or artificial gravitational field to compress and sustain the reaction and enough mass to power it all

Think a tokamak reactor reactor the size of the Death Star

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  • $\begingroup$ See also the pocket-sized stars in The Collapsium. $\endgroup$ – imallett Jul 19 '17 at 7:55
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As other commenters noted, you need to make star lighter than planets. But small body cannot have fusion reaction.

You can make light star based on fission. For example "star" from U-235 and radius about 5000 km would have temperature about 3000K, and it would be relatively stable for hundreds of millions of years. From 150 000 km afar that would provide the same amount of energy per m2 as Sun provides to Earth, thought it would be much colder(towards infrared) radiation.

Of course it is extremely unlikely that something like this would occur naturally.

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