<p>Planets don't orbit stars. Stars don't orbit planets.</p>

<p>Whenever there are two bodies bound by gravity, they are both orbiting their common center of mass. For example, both the Earth and the Moon orbit their common center of mass - but that's pretty close to the center of the Earth actually, so it seems like the Moon orbits the Earth.</p>

<p>For a star to seem to orbit a planet, that would mean the planet is much heavier than the star. As far as we know, that's impossible. Their common center of mass would be much closer to the star, so it would seem like the planet is orbiting the star, as usual.</p>

<hr>

<p>If you have two stars orbiting each other (actually, orbiting their common center of mass) very closely, then you could have a planet circling around both. If the planet is circling around too closely, its orbit would not be stable.</p>

<p>But if the planet's orbit is far enough from the pair of stars, the orbit could be stable a very long time. The minimum distance it at least 2x ... 4x the distance between stars, ideally much bigger. We have discovered such planets. Kepler-47c is a gas giant in the circumbinary habitable zone of the Kepler-47 system.</p>

<p>This wiki page has more details:</p>

<p><a href="https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems" rel="noreferrer">https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems</a></p>

<hr>

<p>EDIT: If you have a closely-bound binary star, with a planet right in between them, in the barycenter of the two stars, that's not a stable system. Any slight perturbation would pull the planet out of there, and then the net force would be pulling it further out. It would eventually fall into one of the stars. It's not self-stabilizing.</p>

<p>I can't even imagine any conceivable mechanism that would put the planet in there to begin with.</p>

CC BY-SA 3.0
2017-07-20T18:57:35.727Z
yes, that's why I added the quotations marks to the word orbit when I made the question. But you get the meaning. But I think you didnt answer if it's possible for a planet to be in the barycenter of the 2 stars.
Pablo
CC BY-SA 3.0
2017-07-20T19:14:47.763Z
@Pablo - I see now what you mean. I've made an edit at the end of the answer.
Florin Andrei
CC BY-SA 3.0
2017-07-20T19:17:11.407Z
I'll leave the question open for a while. If no one comes with a mathematical explanation of how much slightly a perturbation force would have to be to thrown a planet out of there (which nobody conceives how could it have reached there) I'll accept your answer.
Pablo
CC BY-SA 3.0
2017-07-20T19:25:39.397Z
Almost no math is needed. Planet is in barycenter: net force is zero. Planet is slightly out of barycenter (just random fluctuations would do that), net force is away from the barycenter, towards the closer star (because the star that's closer now exerts a greater force than the other star). That's the problem. The net force acting on the planet never points at the barycenter, it points away from it. So it never returns to equilibrium, but it runs away from it. The planet is like a bowling ball sitting on top of a round dome - eventually the slightest breeze will push it downhill.
Florin Andrei
CC BY-SA 3.0
2017-12-14T11:16:24.390Z
@FlorinAndrei **of course math is needed!** This is a three body dynamical problem. Your conclusion may be right but your explanation is not QED. You need to be careful using analogies of a simple potential surface running downhill. This famously fails when people try to explain why orbits near the triangular CR3BP Lagrange points L4 and L5 are stable even though the simplistically drawn potential energy surface is convex. The problem is that the pseudopotential in the rotating frame contains velocity dependent terms and some people set those to zero before plotting.
uhoh
CC BY-SA 4.0
2023-06-21T00:35:01.317Z
What if there are three planets orbiting in between two stars orbiting each other outside of that? Only one would be at the barycenter and some weird resonance could perhaps cause a shepherding effect.
Michael
CC BY-SA 3.0
2017-07-20T18:28:26.197Z
2017-07-20T19:13:57.963Z
2017-07-20T19:13:57.963Z
Florin Andrei
<p>I was watching a science fiction anime from the mid 70's. In it, there was a star orbiting a planet. Since this seemed impossible, and I had never heard of it, I looked on the internet to see if this could be possible in a very unlikely situation or in theory. </p>

<p>On <a href="https://www.quora.com/Is-it-possible-for-a-star-to-orbit-a-planet-Has-it-ever-been-observed" rel="nofollow noreferrer">a similar question on <em>Quora</em></a>, after many no, no, no, no answers from astronomers/astrophysicists, some of them with very detailed models and theories with different scenarios and equations, one answer read,</p>

<blockquote>
  <p>Absolutely.</p>
  
  <p>If there were two stars of identical mass orbiting each other in equilibrium, then you could throw a planet of any size at the barycenter of the two stars. Then optically you'd have not just one, but two stars orbiting a planet. The planet would be equidistant from the two stars at all moments in time, and would experience identical gravitational pull in two diametrically opposite directions and hence never deviate from the barycenter. This system would not be in a stable equilibrium though, and a passing celestial object of any mass could disrupt this system. Hence, this is probably something that is unlikely to be observed.</p>
</blockquote>

<p><strong>Is this possible?</strong></p>

CC BY-SA 3.0
2017-07-20T00:02:11.903Z
I'm not an astrophysicist so I can't give you a proper answer with math, hence this as a comment.  I'd bet that the answers you'd get here will be along the lines of "that is too unstable to last for any period of time. Even the slightest perturbation would eventually send the planet into one of the stars, or away from both of them".  I bet you your quoted answer is from a pure math, ignore-unnecessary-variables point of view, not a this-can-really-happen point of view. Related, you'll find this article interesting: https://spaceplace.nasa.gov/barycenter/en/
Cody
CC BY-SA 3.0
2017-07-20T12:13:29.550Z
Interesting question, as @Cody says the answer probably lies in some mathematical model of the 3 body problem as to how stable such an orbit would be.
Dean
CC BY-SA 3.0
2017-07-20T12:16:03.920Z
But as far as I know there are astrophysicts here, someone could answer that or it's too difficult? I honestly ask because I have no idea, it could be very easy to calculate or not since I dont have the knowledge I can't tell
Pablo
CC BY-SA 3.0
2017-07-20T12:47:06.173Z
Cody is correct.  You don't need to be an astrophysicist either.   A basic understanding of orbits will tell you that 2 large bodies orbiting a 3rd smaller one isn't stable because the central body would slip out of place and there's no force keeping it in the center but once off center, there would be forces to toss it about.   Like a pencil balancing on it's point in the wind.  It's not stable.  (if anyone wants to answer with pictures and saddle points, feel free), but that particular solution is clever, unrealistic and temporary.
userLTK
CC BY-SA 3.0
2017-07-20T12:49:36.517Z
is there any way to calculate how much time could spend in that unstable state? Because unstable in star terms could be many years
Pablo
CC BY-SA 3.0
2017-07-20T15:09:11.497Z
Not really, and the answer could probably range from 5 minutes to 100,000 years. There's too many unknowns and too many variables to say anything like that for certain. To use userLTK's analogy, you could stand a pencil precisely on it's point, but who's to say how long it will be until it falls? Can you account for all slight Earth tremors or slight gusts of wind? Besides, based on known formation processes, there's no natural way for a system like that to have ever formed. It'd have to be purposefully designed like that and who's going to do that?
zephyr
CC BY-SA 3.0
2017-07-20T15:12:47.207Z
Last question I made was about "rogue stars". I believe there is a theory I readed somewhere they are thrown away from galaxies in certain events. Cant a planet being thrown away by an event and be later captured by a binary star? (very unlikely, but considering the size of the known universe doesnt that become less unlikely?)
Pablo
CC BY-SA 3.0
2017-07-20T15:30:41.627Z
@sephyr it would be cool to know which magnitude of forces would be needed to perturbate such system to know if the example of the pencil is good or not
Pablo
CC BY-SA 3.0
2017-07-20T20:52:44.267Z
@Pablo that should probably be a separate question, but planets can get tossed about, lost by one star, and captured by another.   Any 3 or more body system makes both expelling (rogue) and capture possible.   Capture is less common and rogue planets are probably quite common, but capture of rogue planets and stars stealing planets from other stars both can happen when they pass close enough.
userLTK
CC BY-SA 3.0
2017-07-21T07:51:03.040Z
Related: http://www.scholarpedia.org/article/Sitnikov_problem cool GIF and then a lot of math...
uhoh
CC BY-SA 4.0
2023-06-21T00:31:48.557Z
If it can, and there are two other planets in the system, expect ferocious anvil headed creatures to come out to feed whenever the nearest planet eclipses one of the stars to cause a rare event of total darkness
Michael
<star><planet><orbit><binary-star><orbital-elements>
Can a binary star optically "orbit" a planet?
Pablo
# Can a binary star optically "orbit" a planet?

(Asked by: username_0 on 2017-07-19)

I was watching a science fiction anime from the mid 70's. In it, there was a star orbiting a planet. Since this seemed impossible, and I had never heard of it, I looked on the internet to see if this could be possible in a very unlikely situation or in theory.

On [a similar question on *Quora*](https://www.quora.com/Is-it-possible-for-a-star-to-orbit-a-planet-Has-it-ever-been-observed), after many no, no, no, no answers from astronomers/astrophysicists, some of them with very detailed models and theories with different scenarios and equations, one answer read,

> Absolutely.
>
> If there were two stars of identical mass orbiting each other in equilibrium, then you could throw a planet of any size at the barycenter of the two stars. Then optically you'd have not just one, but two stars orbiting a planet. The planet would be equidistant from the two stars at all moments in time, and would experience identical gravitational pull in two diametrically opposite directions and hence never deviate from the barycenter. This system would not be in a stable equilibrium though, and a passing celestial object of any mass could disrupt this system. Hence, this is probably something that is unlikely to be observed.

**Is this possible?**

**Tags:** star, planet, orbit, binary-star, orbital-elements
> **Comments:**
> **username_1**: I'm not an astrophysicist so I can't give you a proper answer with math, hence this as a comment.  I'd bet that the answers you'd get here will be along the lines of "that is too unstable to last for any period of time. Even the slightest perturbation would eventually send the planet into one of the stars, or away from both of them".  I bet you your quoted answer is from a pure math, ignore-unnecessary-variables point of view, not a this-can-really-happen point of view. Related, you'll find this article interesting: https://spaceplace.nasa.gov/barycenter/en/  
> **username_2**: Interesting question, as @Cody says the answer probably lies in some mathematical model of the 3 body problem as to how stable such an orbit would be.  
> **username_0**: But as far as I know there are astrophysicts here, someone could answer that or it's too difficult? I honestly ask because I have no idea, it could be very easy to calculate or not since I dont have the knowledge I can't tell  
> **username_3**: Cody is correct.  You don't need to be an astrophysicist either.   A basic understanding of orbits will tell you that 2 large bodies orbiting a 3rd smaller one isn't stable because the central body would slip out of place and there's no force keeping it in the center but once off center, there would be forces to toss it about.   Like a pencil balancing on it's point in the wind.  It's not stable.  (if anyone wants to answer with pictures and saddle points, feel free), but that particular solution is clever, unrealistic and temporary.  
> **username_0**: is there any way to calculate how much time could spend in that unstable state? Because unstable in star terms could be many years  
> **username_4**: Not really, and the answer could probably range from 5 minutes to 100,000 years. There's too many unknowns and too many variables to say anything like that for certain. To use userLTK's analogy, you could stand a pencil precisely on it's point, but who's to say how long it will be until it falls? Can you account for all slight Earth tremors or slight gusts of wind? Besides, based on known formation processes, there's no natural way for a system like that to have ever formed. It'd have to be purposefully designed like that and who's going to do that?  
> **username_0**: Last question I made was about "rogue stars". I believe there is a theory I readed somewhere they are thrown away from galaxies in certain events. Cant a planet being thrown away by an event and be later captured by a binary star? (very unlikely, but considering the size of the known universe doesnt that become less unlikely?)  
> **username_0**: @sephyr it would be cool to know which magnitude of forces would be needed to perturbate such system to know if the example of the pencil is good or not  
> **username_3**: @Pablo that should probably be a separate question, but planets can get tossed about, lost by one star, and captured by another.   Any 3 or more body system makes both expelling (rogue) and capture possible.   Capture is less common and rogue planets are probably quite common, but capture of rogue planets and stars stealing planets from other stars both can happen when they pass close enough.  
> **username_5**: Related: http://www.scholarpedia.org/article/Sitnikov_problem cool GIF and then a lot of math...  
> **username_6**: If it can, and there are two other planets in the system, expect ferocious anvil headed creatures to come out to feed whenever the nearest planet eclipses one of the stars to cause a rare event of total darkness  

---

## 1 answers

### Answer by username_7 on 2017-07-20. Score: 7 | Accepted answer

Planets don't orbit stars. Stars don't orbit planets.

Whenever there are two bodies bound by gravity, they are both orbiting their common center of mass. For example, both the Earth and the Moon orbit their common center of mass - but that's pretty close to the center of the Earth actually, so it seems like the Moon orbits the Earth.

For a star to seem to orbit a planet, that would mean the planet is much heavier than the star. As far as we know, that's impossible. Their common center of mass would be much closer to the star, so it would seem like the planet is orbiting the star, as usual.

---

If you have two stars orbiting each other (actually, orbiting their common center of mass) very closely, then you could have a planet circling around both. If the planet is circling around too closely, its orbit would not be stable.

But if the planet's orbit is far enough from the pair of stars, the orbit could be stable a very long time. The minimum distance it at least 2x ... 4x the distance between stars, ideally much bigger. We have discovered such planets. Kepler-47c is a gas giant in the circumbinary habitable zone of the Kepler-47 system.

This wiki page has more details:

<https://en.wikipedia.org/wiki/Habitability_of_binary_star_systems>

---

EDIT: If you have a closely-bound binary star, with a planet right in between them, in the barycenter of the two stars, that's not a stable system. Any slight perturbation would pull the planet out of there, and then the net force would be pulling it further out. It would eventually fall into one of the stars. It's not self-stabilizing.

I can't even imagine any conceivable mechanism that would put the planet in there to begin with.

> **Comments:**
> **username_0**: yes, that's why I added the quotations marks to the word orbit when I made the question. But you get the meaning. But I think you didnt answer if it's possible for a planet to be in the barycenter of the 2 stars.  
> **username_7**: @Pablo - I see now what you mean. I've made an edit at the end of the answer.  
> **username_0**: I'll leave the question open for a while. If no one comes with a mathematical explanation of how much slightly a perturbation force would have to be to thrown a planet out of there (which nobody conceives how could it have reached there) I'll accept your answer.  
> **username_7**: Almost no math is needed. Planet is in barycenter: net force is zero. Planet is slightly out of barycenter (just random fluctuations would do that), net force is away from the barycenter, towards the closer star (because the star that's closer now exerts a greater force than the other star). That's the problem. The net force acting on the planet never points at the barycenter, it points away from it. So it never returns to equilibrium, but it runs away from it. The planet is like a bowling ball sitting on top of a round dome - eventually the slightest breeze will push it downhill.  
> **username_5**: @FlorinAndrei **of course math is needed!** This is a three body dynamical problem. Your conclusion may be right but your explanation is not QED. You need to be careful using analogies of a simple potential surface running downhill. This famously fails when people try to explain why orbits near the triangular CR3BP Lagrange points L4 and L5 are stable even though the simplistically drawn potential energy surface is convex. The problem is that the pseudopotential in the rotating frame contains velocity dependent terms and some people set those to zero before plotting.  
> **username_6**: What if there are three planets orbiting in between two stars orbiting each other outside of that? Only one would be at the barycenter and some weird resonance could perhaps cause a shepherding effect.  

---

