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Nefer-Ankhe

Hypothetical World

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I have a theory for a hypothetical world, however I was wondering if the concepts work and if so how.

Instead of the world orbiting the sun, the sun would orbit the world, therefore the worlds stays in one place, however still spins.

This world has two moons, one which orbits the world and the other which orbits the larger moon.

I have been informed that in order for a sun to orbit a world, the world would need to be larger than the sun, having a greater gravitational force. Therefore the sun must be smaller and in closer proximity to the world, in order to have efficient amount of light and radiation to sustain life on the planet. However is it at all possible a sun can orbit a planet, if so...

How large would the sun have to be?

How far from the world would it have to be?

If the sun is orbiting the world and the world is spinning at the same time, will there be normal days and nights?

Would the years be longer or shorter?

Just out of pure curiosity.

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Amidst a double star system the orbit of planets would be interesting. Would their orbits regularly change from sun to sun? And could moons also be juggled back and forth between the planets?

I think a moon could have its own sattellite as well as its own rings. Such sights would be breathtaking.

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Posted (edited)

There are others here with a far better grasp of orbital science than me, but here's a few tips to get you started.

I have a theory for a hypothetical world, however I was wondering if the concepts work and if so how.

Instead of the world orbiting the sun, the sun would orbit the world, therefore the worlds stays in one place, however still spins.

Problem there is that the less massive object will always orbit the more massive one unless other forces intervene, eg a third object (but see below)... and of course mass is pretty much what determines whether you are a star or a planet. You need to be over a certain amount of mass before the forces at the centre of the object are sufficient for the fusion reactions to light up.

This world has two moons, one which orbits the world and the other which orbits the larger moon.

3-body systems like that are inherently very unstable and the system is likely to be very shortlived - the lesser body may jump from orbiting one object to the other, or simply plunge into one or the other as its orbit gets perturbed over time and the two bodies fight over it.. unless.. the objects are so far apart and the moon so small as to not be significantly affected by the second object (that is the situation with the Sun-Earth-Moon system - the moon is small and so near to us that the system remains stable) - refer to my comments to taniwha below.

If you add in your initial suggestion that the planet revolves around the star, then you are really pushing this beyond any science I can imagine...

I have been informed that in order for a sun to orbit a world, the world would need to be larger than the sun, having a greater gravitational force.

Greater gravitational force means more massive, not larger, so size isn't the issue here...

I think the rest of your questions are predicated on this being possible, and as far as I am aware, it is not, sorry. Happy to be corrected by more lateral thinkers, though..!

to taniwha, yes, they can.. And as long as the secondary moons are sufficiently far away from any other perturbing bodies, then yes, 'sub-moons' are possible - you might want to look up the concept of the Hill Sphere..

Added - this is a cool discussion of these issues:

http://www.universetoday.com/109666/can-moons-have-moons/

Edited by ChrLzs
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On further thinking and a bit of googling, I guess it is possible to have a large and very massive but now dead star (like a brown dwarf) that is being orbited by a slightly less massive but still-burning star. Maybe..... But normal star formation theories don't really offer a way for that to happen, and no such systems have been observed or detected, as far as I am aware. I guess it depends on how much fantasy you wish to have in your story - I'm assuming you are writing something?

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Creating something more or less.

My initial idea of having smaller moon 2 orbit larger moon 1, evidently will not work, as already mention, will result in a collision course.

Is a sun orbiting a planet possible?

Still remains the issue for me, as to how the nights/days will turn out. I would have posted a layout of my Hypothetical world, however I don't know how to copy it from Microsoft Word to this particular forum(?).

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...Is a sun orbiting a planet possible?

Still remains the issue for me, as to how the nights/days will turn out. I would have posted a layout of my Hypothetical world, however I don't know how to copy it from Microsoft Word to this particular forum(?).

As ChrLsz pointed out, the object with less mass orbits the object with more mass. And as far as we can tell, for an object to be a sun it needs to be a lot more massive than any planet.

What's the least-mass star possible? According to this article from the Bad Astronomer (http://www.slate.com/blogs/bad_astronomy.2.html) it would be about 7-8% of the mass of our Sun. Our Sun is about 330,000 times the mass of the Earth. So a hypothetical smallest possible star would still have something like 30,000 times the mass of the Earth.

Even if you replaced the Earth with a super-Earth which had 100 times the mass of the Earth, the smallest possible star would still have 300 times the mass of that super-(uncomfortable)-Earth.

But would such a pip-squeak star orbit that super-Earth? No. The best way to think of it is to imagine two ball tied together with a piece of strong fishing line. Throw them up in the air so that they spin around each other. If the balls are the same mass they'll spin around a point halfway between them. But if one ball is lighter than the other it will tend to spin around the heavier ball. By the time you get to a mass differential of 300, the movement of the heavier ball will be negligible compared to that of the lighter ball. And that's what would happen in this case, even with the lightest possible star and a planet even far more massive than the Earth.

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On further thinking and a bit of googling, I guess it is possible to have a large and very massive but now dead star (like a brown dwarf) that is being orbited by a slightly less massive but still-burning star. Maybe.....

First point, a brown dwarf is a low mass "failed star" sitting somewhere between a massive gas giant and a small "true" star.

There are hypothetical objects called black dwarfs which are cold, dead, white dwarf stars. The problem with this is that the lower the mass of the star the longer it burns and the universe just isn't old enough to have any black dwarfs yet.

A problem with Charlzs' scenario is the idea of a "large and very massive but now dead star". The thing with massive stars is that they die not with a whimper but a bang, a really big bang. Massive stars die as supernovae, leaving behind neutron stars or black holes. Whilst these remnants of a once mighty star may orbit a living star they do not really resemble planets.

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I have a theory for a hypothetical world, however I was wondering if the concepts work and if so how.

Instead of the world orbiting the sun, the sun would orbit the world, therefore the worlds stays in one place, however still spins.

This world has two moons, one which orbits the world and the other which orbits the larger moon.

I have been informed that in order for a sun to orbit a world, the world would need to be larger than the sun, having a greater gravitational force. Therefore the sun must be smaller and in closer proximity to the world, in order to have efficient amount of light and radiation to sustain life on the planet. However is it at all possible a sun can orbit a planet, if so...

How large would the sun have to be?

How far from the world would it have to be?

If the sun is orbiting the world and the world is spinning at the same time, will there be normal days and nights?

Would the years be longer or shorter?

Just out of pure curiosity.

If the planet were more massive that the star how would you suppose it would avoid starting nuclear fusion and becoming another star? Planets don't actually orbit stars. Both revolve around a common center of mass.
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Star planet systems orbit around a common center of mass. However this isn't as interesting as say a star rotating around a planet. When a planet gathers enough mass it begins to fuse hydrogen in it's core. Once fusion starts it becomes a star. So by definition no planet can have a mass greater than the mass of its star, resulting in the planet ALWAYS orbiting the star, correct? and there is no way around this whatsoever?

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If you want a real ``star'', then yes - it is impossible for a star to orbit a planet.

If you just want a luminous object you could have a large planet with a substantial amount of radioactive elements in the core and a thin crust (so the planet generates its own heat), and have a smaller moon with a very radioactive core, and a surface of diamond, glass, water, ice (or some other suitable transparent material) such that Cherenkov radiation provides light.

I seriously doubt something like this would occur naturally, but it is the most plausible physically possible situation I can think of that sort of matches your requirements.

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Posted (edited)

It is important to mention that, if we look at Solar system for example, The Sun has almost all of the mass of Solar System and since every star actually forms in similar way its logical to expect that any star at its birth will take almost all of the surrounding material for it to be formed. Leftover material is what makes other objects in system and since new star has taken almost all of the material from its surroundings at its birth - there cant possibly be enough material left for anything with more mass then the central star to be formed.

I am not sure but the Sun has almost all of the mass of Solar system, maybe only few percentages of mass is what makes everything else in solar system.

[edit]

Just checked, the Sun has 99.86% percentage of Solar system mass, amazing :) Its only 0.14% that makes every planet, moon, asteriod in asteroid belt, oort cloud...

Edited by Sir Smoke aLot

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Its only 0.14% that makes every planet, moon, asteriod in asteroid belt, oort cloud...

And most of that is contained within Jupiter. It has 2.5 times the mass of all the other planets combined.

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Star planet systems orbit around a common center of mass. However this isn't as interesting as say a star rotating around a planet. When a planet gathers enough mass it begins to fuse hydrogen in it's core. Once fusion starts it becomes a star. So by definition no planet can have a mass greater than the mass of its star, resulting in the planet ALWAYS orbiting the star, correct? and there is no way around this whatsoever?

Correct. It's a result of two fairly well understood concepts - gravity, and how stars work.

In the case of gravity, expecting the more massive star to orbit the less massive planet would be like dropping a leaf on one end of a see-saw and expecting it to launch you off the other end.

In the case of how stars work, stars must be at least a certain mass. That mass is much larger even than Jupiter, the largest planet in our Solar System.

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And most of that is contained within Jupiter. It has 2.5 times the mass of all the other planets combined.

I didn't know that our guardian is so massive. I mean, Saturn is not so far in size when compared to Jupiter but then Jupiter has got to be so dense that its kinda miracle that it hasn't formed as something other then gas giant. Need to read more about it.

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Posted (edited)

Saturn is not so far in size when compared to Jupiter

Planets actually don't get much larger than Jupiter. As their mass increase so, obviously, does gravity. The greater the gravity the more the planet will try to contract and the greater internal pressure becomes. So the end result is that planets can be much more massive than Jupiter but not much larger.

but then Jupiter has got to be so dense that its kinda miracle that it hasn't formed as something other then gas giant.

Massive though Jupiter is, it is a long way short of being anything other than a gas giant. It would need to be a least 13 times more massive to be a brown dwarf and at least 75 times more massive for it's core to ignite and become a true star.

Edited by Waspie_Dwarf
I really should know the difference between "there" and "their".
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