<blockquote>
<p>Could there be a planet at the barycenter between two or more stars revolving around each other?</p>
</blockquote>
<p>No.</p>
<p>The best case two star scenario is two stars of equal mass. In that case, the barycenter is midway between the two stars and coincides with the L1 Lagrange point. The L1 Lagrange point is metastable. Another name for metastable is unstable. Think of it as a very sharp pencil standing straight up. In theory, a pencil can be stood straight up. In practice, it falls over in a very short time.</p>
<p>If one of the two stars is more massive than the other the barycenter isn't even metastable. The barycenter is closer to the more massive star than it is to the less massive star, which in turn means the gravitational acceleration toward the more massive star is greater than it is to the less massive star. The object at the barycenter will orbit the more massive star at a rate greater then the two stars orbit one another. The less massive star will be a mere perturbation.</p>
<p>The same applies to more than two stars. While there are theoretical balanced on the tip of a standing up pencil that are metastable, these points are a space of measure zero. In other words, there is zero chance of this happening.</p>

CC BY-SA 4.0
2020-08-16T05:13:15.763Z
The James Webb telescope is to be placed at the L-point between the Earth and Sun. Why would that be stabler?
Giovanni
CC BY-SA 4.0
2020-08-16T06:18:10.263Z
@Giovanni The JWST will be in a halo orbit around the L2 point, which puts the Earth between it and the Sun, and it will need to spend a few m/s of delta-V per year to stay near that point. Once the station keeping fuel runs out, it will remain in a solar orbit, but it won't stay near L2.
notovny
CC BY-SA 4.0
2020-08-16T07:23:44.910Z
@notovny Thank you. Though if I understand you correctly, the JWST will be on the other side, where the Earth is between them, not the telescope between the Earth and Sun?
Giovanni
CC BY-SA 4.0
2020-08-16T11:19:35.967Z
@Giovanni - The vicinity of the Sun-Earth L1 point is a good location for a spacecraft that observes the Sun, but not so good for observing the universe. The vicinity of the Sun-Earth L2 point is a good location for a spacecraft that observes the universe. Note that I wrote "vicinity of". The linear Lagrange points (L1, L2, and L3) are metastable. An immense amount of fuel would be needed to have a spacecraft stay at these points. It turns out that there are pseudo orbits about these points that are much less unstable in the sense that much less fuel is needed for stationkeeping. (continued)
David Hammen
CC BY-SA 4.0
2020-08-16T11:25:36.963Z
The JWST will be placed in a halo orbit (one of three different kinds of pseudo orbits) about the Sun-Earth L2 point. The way to visualize these pseudo orbits is to imagine a frame of reference that rotates with the Earth's orbit about the Sun and that stretches and contracts as the Earth moves further from and closer to the Sun. The position of the Sun and Earth are fixed from this perspective. From this perspective, objects in one of these pseudo orbits (halo orbits, Lissajous orbits, and Lyapunov orbits) appear to orbit one of the Lagrange points.
David Hammen
CC BY-SA 4.0
2020-08-16T11:28:55.683Z
Stationkeeping occasionally needs to be performed to keep such spacecraft in the vicinity of the Lagrange point about which it is orbiting. When the JWST runs out of stationkeeping fuel it will fall away of its halo orbit and orbit the Sun.
David Hammen
CC BY-SA 4.0
2020-08-16T12:04:22.113Z
@DavidHammen I see, but how is L2 even possible if it's on the other side of Earth? Is it because the Sun has so much more gravity than Earth that there's a Lagrangian point on the other side? And wouldn't L2 be unstable also due to the Moon's gravity?
Giovanni
CC BY-SA 4.0
2020-08-16T12:09:15.577Z
@Giovanni - A good place to start is a web search on the "circular restricted three body problem", or CR3BP for short.
David Hammen
CC BY-SA 4.0
2020-08-16T01:43:31.527Z
2020-08-16T01:43:31.527Z
David Hammen
<p>No.  Such an arrangement is at best &quot;metastable&quot;.  That is, although there are periodic solutions to the three body problem (stable orbits) an infintesimal perturbation (eg the proverbial butterfly flapping its wings) will push the system off the stable orbit and into chaos.  Getting a planet to remain at the barycentre is like trying to balance a pencil on its sharpened point.</p>
<p>With two bodies, each orbits around the barycentre.  But with three bodies, the bodies don't orbit around the three-way barycentre. And a planet placed near the barycentre of two stars will not tend to remain in orbit around that point.</p>
<p>The Lagrangian point L1 is also at best metastable. Satellites that orbit the sun at the Earth-Sun Lagrangian point need to fire their engines and do regular &quot;station keeping&quot; to keep them from drifting away.</p>
<p>The L4 and L5 points can be stable. Bodies at the L4 and 5 points are called &quot;Trojans&quot;.  However no Trojan exoplanets are known.  A Trojan planet would see the two stars separated by (a variable amount averaging at) 60 degrees</p>

CC BY-SA 4.0
2020-08-15T12:13:05.787Z
But what if the planet was at the very barycenter between two stars of equal mass? I mean like the barycenter of the system being in the center of the planet's core. Would this really be unstable?
Giovanni
CC BY-SA 4.0
2020-08-15T12:56:33.207Z
One way to see why it’s unstable is to think about what happens if the planet is pushed a tiny bit one way or the other. If the gravitational pulls of the two stars are exactly equal, then a perturbation that pushes the planet even a tiny bit one way will cause the pull of one star to now be stronger than the other, and the planet will keep going in that direction.  You might say “but I will keep it at the center - I won’t allow it to be perturbed!”   But that’s impossible unless you are in an empty universe; there are always tiny perturbations from other stars.
Eric Jensen
CC BY-SA 4.0
2020-08-15T11:49:23.177Z
2020-08-15T11:56:00.003Z
2020-08-15T11:56:00.003Z
James K
<p>In binary star systems, could there be a planet the stars revolve around, with eternal day on all sides?</p>
<p>1st scenario: Imagine a binary system consisting of two Sunlike G5V stars of 1 solar mass each, orbiting each other, and at the barycenter between them is a planet (which would make the planet itself revolve at the same horizontal speed around its axis, making each sun appear always above the same location on the planet). If the stars revolved around the planet both sides would be illuminated similarly. I don't see why it should be impossible. A planet that once was the outermost one in orbit around one of the stars got ejected from its orbit by the other star's gravity, migrating into the barycenter between them.</p>
<p>2nd scenario: Imagine there's a planet at the Lagrangian point between Alpha Centauri A and B. If the stars revolved around the planet both sides would be illuminated similarly. Would <em>that</em> be possible?</p>
<p>Do situations like those above occur or was such even observed?</p>

CC BY-SA 4.0
2020-08-15T11:45:02.003Z
@antispinwards I expanded my question.
Giovanni
<orbit><binary-star><lagrange-point>
Could there be a planet at the barycenter between two or more stars revolving around each other?
Giovanni
# Could there be a planet at the barycenter between two or more stars revolving around each other?

(Asked by: username_0 on 2020-08-15)

In binary star systems, could there be a planet the stars revolve around, with eternal day on all sides?

1st scenario: Imagine a binary system consisting of two Sunlike G5V stars of 1 solar mass each, orbiting each other, and at the barycenter between them is a planet (which would make the planet itself revolve at the same horizontal speed around its axis, making each sun appear always above the same location on the planet). If the stars revolved around the planet both sides would be illuminated similarly. I don't see why it should be impossible. A planet that once was the outermost one in orbit around one of the stars got ejected from its orbit by the other star's gravity, migrating into the barycenter between them.

2nd scenario: Imagine there's a planet at the Lagrangian point between Alpha Centauri A and B. If the stars revolved around the planet both sides would be illuminated similarly. Would *that* be possible?

Do situations like those above occur or was such even observed?

**Tags:** orbit, binary-star, lagrange-point

> **Comments:**
> **username_0**: @antispinwards I expanded my question.  

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## 2 answers

### Answer by username_1 on 2020-08-16. Score: 3 | Accepted answer

> Could there be a planet at the barycenter between two or more stars revolving around each other?

No.

The best case two star scenario is two stars of equal mass. In that case, the barycenter is midway between the two stars and coincides with the L1 Lagrange point. The L1 Lagrange point is metastable. Another name for metastable is unstable. Think of it as a very sharp pencil standing straight up. In theory, a pencil can be stood straight up. In practice, it falls over in a very short time.

If one of the two stars is more massive than the other the barycenter isn't even metastable. The barycenter is closer to the more massive star than it is to the less massive star, which in turn means the gravitational acceleration toward the more massive star is greater than it is to the less massive star. The object at the barycenter will orbit the more massive star at a rate greater then the two stars orbit one another. The less massive star will be a mere perturbation.

The same applies to more than two stars. While there are theoretical balanced on the tip of a standing up pencil that are metastable, these points are a space of measure zero. In other words, there is zero chance of this happening.

> **Comments:**
> **username_0**: The James Webb telescope is to be placed at the L-point between the Earth and Sun. Why would that be stabler?  
> **username_2**: @username_0 The JWST will be in a halo orbit around the L2 point, which puts the Earth between it and the Sun, and it will need to spend a few m/s of delta-V per year to stay near that point. Once the station keeping fuel runs out, it will remain in a solar orbit, but it won't stay near L2.  
> **username_0**: @username_2 Thank you. Though if I understand you correctly, the JWST will be on the other side, where the Earth is between them, not the telescope between the Earth and Sun?  
> **username_1**: @username_0 - The vicinity of the Sun-Earth L1 point is a good location for a spacecraft that observes the Sun, but not so good for observing the universe. The vicinity of the Sun-Earth L2 point is a good location for a spacecraft that observes the universe. Note that I wrote "vicinity of". The linear Lagrange points (L1, L2, and L3) are metastable. An immense amount of fuel would be needed to have a spacecraft stay at these points. It turns out that there are pseudo orbits about these points that are much less unstable in the sense that much less fuel is needed for stationkeeping. (continued)  
> **username_1**: The JWST will be placed in a halo orbit (one of three different kinds of pseudo orbits) about the Sun-Earth L2 point. The way to visualize these pseudo orbits is to imagine a frame of reference that rotates with the Earth's orbit about the Sun and that stretches and contracts as the Earth moves further from and closer to the Sun. The position of the Sun and Earth are fixed from this perspective. From this perspective, objects in one of these pseudo orbits (halo orbits, Lissajous orbits, and Lyapunov orbits) appear to orbit one of the Lagrange points.  
> **username_1**: Stationkeeping occasionally needs to be performed to keep such spacecraft in the vicinity of the Lagrange point about which it is orbiting. When the JWST runs out of stationkeeping fuel it will fall away of its halo orbit and orbit the Sun.  
> **username_0**: @DavidHammen I see, but how is L2 even possible if it's on the other side of Earth? Is it because the Sun has so much more gravity than Earth that there's a Lagrangian point on the other side? And wouldn't L2 be unstable also due to the Moon's gravity?  
> **username_1**: @username_0 - A good place to start is a web search on the "circular restricted three body problem", or CR3BP for short.  

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### Answer by username_3 on 2020-08-15. Score: 7

No. Such an arrangement is at best "metastable". That is, although there are periodic solutions to the three body problem (stable orbits) an infintesimal perturbation (eg the proverbial butterfly flapping its wings) will push the system off the stable orbit and into chaos. Getting a planet to remain at the barycentre is like trying to balance a pencil on its sharpened point.

With two bodies, each orbits around the barycentre. But with three bodies, the bodies don't orbit around the three-way barycentre. And a planet placed near the barycentre of two stars will not tend to remain in orbit around that point.

The Lagrangian point L1 is also at best metastable. Satellites that orbit the sun at the Earth-Sun Lagrangian point need to fire their engines and do regular "station keeping" to keep them from drifting away.

The L4 and L5 points can be stable. Bodies at the L4 and 5 points are called "Trojans". However no Trojan exoplanets are known. A Trojan planet would see the two stars separated by (a variable amount averaging at) 60 degrees

> **Comments:**
> **username_0**: But what if the planet was at the very barycenter between two stars of equal mass? I mean like the barycenter of the system being in the center of the planet's core. Would this really be unstable?  
> **username_4**: One way to see why it’s unstable is to think about what happens if the planet is pushed a tiny bit one way or the other. If the gravitational pulls of the two stars are exactly equal, then a perturbation that pushes the planet even a tiny bit one way will cause the pull of one star to now be stronger than the other, and the planet will keep going in that direction.  You might say “but I will keep it at the center - I won’t allow it to be perturbed!”   But that’s impossible unless you are in an empty universe; there are always tiny perturbations from other stars.  

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