<p>Our understanding of orbits is not based simply on astronomical observation, it's based on physics as well.</p>

<p>In ancient times, we didn't have a very complete understanding of the physics of the universe. Philosophers (as what passed for scientists were called then) had many preconceived notions (e.g. all motion in the universe was either straight or circular), and they tried to fit their observations with them. It didn't feel like the Earth was moving, but they could see stars and planets moving, so they assumed the earth was stationary; to explain the complicated movements of planets they came up with things like epicycles.</p>

<p>Since then we figured out the heliocentric organization of the solar system (no more epicycles needed), then later that the solar system is part of a galaxy, which is part of a much larger universe, and scientists like Galileo, Newton, and Einstein refined our understanding motion and gravity. Astronomical observations on varying scales confirmed that these understandings were substantially correct (although when they got to galactic scales they discovered anomalies, leading to the notion of dark matter -- if this sounds like epicycles all over, read <a href="https://medium.com/starts-with-a-bang/what-astronomers-wish-everyone-knew-about-dark-matter-and-dark-energy-d13eada60ea0" rel="nofollow noreferrer">What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy</a>).</p>

<p>In addition to this, our observational technology has advanced. So even though we're observing from a single vantage point, we're able to detect motion in multiple directions. For instance, the Doppler effect can be used to detect motion to or away from the Earth (this is one of the techniques used to detect exoplanets, and it should be far more accurate for the Sun because it's so much closer). So we can map the precise motion of the Sun, and compare this with the expected motion around the barycenter with the known planets. If there were a significant difference, we'd go looking for an explanation; there might be another planet we haven't discovered (Neptune and Pluto were both discovered by looking for the sources of disturbances in other orbits), or some as-yet-unknown physical effect (e.g. General Relativity explained the anomalous orbit of Mercury).</p>

CC BY-SA 4.0
2019-12-15T09:20:21.710Z
A distant planet would not produce a measurable Doppler effect (or demonstrate otherwise). A search for "planet 9" continues. If it were at 1000au and 10 Earth masses, the barycentre would be 6 solar radii from the Sun. The Sun would orbit this barycentre every 32,000 years at 2.8 cm per second.
ProfRob
CC BY-SA 4.0
2019-12-16T04:20:16.743Z
The Doppler effect is more precise the faster an object is moving towards or away from us. For example, Barnard's Star has a high blueshift because it is currently getting closer to the solar system very quickly. Since the Earth's movement in the solar system is mostly perpendicular to the direction of the Sun, this doesn't help us much.
CJ Dennis
CC BY-SA 4.0
2019-12-14T20:52:49.857Z
2019-12-14T20:52:49.857Z
Barmar
<p>The barycentre of the whole solar system is inside the Sun roughly half the time, and outside the other half.</p>

<p>From <a href="https://en.wikipedia.org/wiki/Barycenter#Inside_or_outside_the_Sun?" rel="nofollow noreferrer">Wikipedia's article</a>:</p>

<blockquote>
  <p>To calculate the actual motion of the Sun, only the motions of the four giant planets (Jupiter, Saturn, Uranus, Neptune) need to be considered. The contributions of all other planets, dwarf planets, etc. are negligible. If the four giant planets were on a straight line on the same side of the Sun, the combined center of mass would lie about 1.17 solar radii or just over 810,000 km above the Sun's surface.</p>
</blockquote>

<p>The associated image shows that the barycentre is not stationary in regards to the Sun, but moves. More properly, the Sun moves around the barycentre. Sometimes the barycentre is almost at the centre of the Sun and sometimes it's about twice the radius of the Sun from its centre.</p>

<p><a href="https://i.sstatic.net/Flc0q.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Flc0qm.png" alt="enter image description here"></a></p>

<p>For the barycentre to be much further from the centre of the Sun than currently believed, there would have to be an undiscovered Uranus-to-Jupiter mass planet in the solar system close enough to the Sun to make a difference. This hypothetical body would have an effect on the rest of the solar system. This is how some of the outer planets were discovered: some orbits were being noticeably perturbed by <em>something</em>. The search for the <em>something</em> led to the discovery of another planet.</p>

<p>If it turns out that Planet Nine exists and is discovered and is a super-Earth with a mass around five to ten times that of the Earth (compared with 14.5 for Uranus and 17 for Neptune), it would be so far away that its effects on the barycentre would be negligible. The four giant planets are much closer, and much more massive, so have a much more noticeable effect.</p>

CC BY-SA 4.0
2019-12-15T09:11:18.493Z
Last paragraph is incorrect. $m_p a_p = M_{\odot} a_{\odot}$. The further away the planet is, the greater it's effect on the barycentre position.
ProfRob
CC BY-SA 4.0
2019-12-15T10:36:14.710Z
@RobJeffries - can you explain. It sounds like you are saying gravity has a greater effect the further the planet is :-)
Rory Alsop
CC BY-SA 4.0
2019-12-15T18:23:19.180Z
@RoryAlsop the formula I have given is how you calculate the centre of gravity. It also features in the correct, but still not accepted, answer by antispinwards. This answer is plain wrong and yet has attracted 4 upvotes. At least one since I made my comment.
ProfRob
CC BY-SA 4.0
2019-12-15T18:30:03.677Z
@RobJeffries - I think you still need to explain. My understanding is that the further away a mass is, the **less** effect it has. ie any planet outside the solar system will have negligible effect.
Rory Alsop
CC BY-SA 4.0
2019-12-15T19:24:01.260Z
Yeah, I understand the physics well - I studied Astrophysics as part of my degree course at University, however Rob's sentence does not say what you just said :-)
Rory Alsop
CC BY-SA 4.0
2019-12-15T22:39:07.870Z
I see how the barycentre of a _two_ body system increases by distance, but not for a system with more than two bodies. Otherwise we should consider all the dust, other stars, the black hole at the centre of our galaxy, other galaxies, etc.
CJ Dennis
CC BY-SA 4.0
2019-12-16T07:32:01.547Z
For a system with multiple bodies, calculate where the centre of mass is and then add in the extra body and see how much it moves the barycentre.  1. We should consider dust in the solar system, but it is negligible. 2. Other stars and galaxies aren't part of the solar system, so don't affect our calculation of the solar system barycentre.
ProfRob
CC BY-SA 4.0
2019-12-15T05:25:15.333Z
2019-12-15T05:25:15.333Z
CJ Dennis
<p>The radius of the suns orbit around the barycenter of the solar system is often measured to be roughly the radius of the sun. Based on the same type of bias that has made humans miss things about their solar system because they are looking at it from the vantage point of Earth orbiting inside the system, is it possible that the suns orbit around the barycenter is actually much larger?</p>

CC BY-SA 4.0
2019-12-14T08:24:06.460Z
"bias that has made humans miss things about their solar system"  What kind of things do you have in mind.  What things have we missed?
James K
CC BY-SA 4.0
2019-12-14T08:31:10.323Z
You see bias because of vantage point all the time in all sciences. Geocentrism is one example. I think it was Marshal McLuhan who said "the fish does not know it's swimming in the ocean". If ancient cultures believed the Earth was flat (as animals have evolved mainly to navigate a flat surface, "place cells" adapted for that), another good example. In my opinion very characteristic of human condition, though maybe others see it differently.
Kornelia
CC BY-SA 4.0
2019-12-14T08:41:01.313Z
Those are both very old and pre-science beliefs.  Do you have anything more recent? Perhaps in the last 50 or 100 years.
James K
CC BY-SA 4.0
2019-12-14T08:47:51.577Z
Easiest to have broad consensus on historical examples. Science is (unless you consider all possible discoveries to have already been made, and in that case and if that is proven then I'd have to agree ) a process. The context was just, the ability to not see the forest for the trees. In case anyone agrees on there being such a bias.
Kornelia
CC BY-SA 4.0
2019-12-15T05:51:23.063Z
James K - A lot of things take a years to discover if we're wrong about them.  Lobotomy won the Nobel Prize in 1949, and was common well-into the 1970s, unfortunately the peer review only served to make more people lose brain functionality. Lysenkoism was state-sponsored into the 1960s. And we've got Cutter Laboratories to thank for anti-vax movement. Some of our current ideas will probably be wrong or missing key info, it would be nice to be 100% right about the world, but the next Thomas Edison will probably have a "Don't talk to me about X-rays" moment too.
IKM
CC BY-SA 4.0
2019-12-15T09:27:02.693Z
Unless you mean something other than "could there be missing, asymmetrically distributed mass in the solar system", then you should reword your question. There are lots of Q+A on the site about possible unseen planets.
ProfRob
CC BY-SA 4.0
2019-12-17T04:11:25.083Z
Can you name one or two examples of this that have happened in the last fifty years? " *... bias that has made humans miss things about their solar system because they are looking at it from the vantage point of Earth orbiting inside the system*" Without clarification it's very hard to know what you are really talking about here, unless you mean before modern science began. *Thanks!*
uhoh
<orbital-mechanics>
Could the barycenter orbit of our sun be greatly underestimated?
Kornelia
# Could the barycenter orbit of our sun be greatly underestimated?

(Asked by: username_0 on 2019-12-14)

The radius of the suns orbit around the barycenter of the solar system is often measured to be roughly the radius of the sun. Based on the same type of bias that has made humans miss things about their solar system because they are looking at it from the vantage point of Earth orbiting inside the system, is it possible that the suns orbit around the barycenter is actually much larger?

**Tags:** orbital-mechanics
> **Comments:**
> **username_1**: "bias that has made humans miss things about their solar system"  What kind of things do you have in mind.  What things have we missed?  
> **username_0**: You see bias because of vantage point all the time in all sciences. Geocentrism is one example. I think it was Marshal McLuhan who said "the fish does not know it's swimming in the ocean". If ancient cultures believed the Earth was flat (as animals have evolved mainly to navigate a flat surface, "place cells" adapted for that), another good example. In my opinion very characteristic of human condition, though maybe others see it differently.  
> **username_1**: Those are both very old and pre-science beliefs.  Do you have anything more recent? Perhaps in the last 50 or 100 years.  
> **username_0**: Easiest to have broad consensus on historical examples. Science is (unless you consider all possible discoveries to have already been made, and in that case and if that is proven then I'd have to agree ) a process. The context was just, the ability to not see the forest for the trees. In case anyone agrees on there being such a bias.  
> **username_2**: James K - A lot of things take a years to discover if we're wrong about them.  Lobotomy won the Nobel Prize in 1949, and was common well-into the 1970s, unfortunately the peer review only served to make more people lose brain functionality. Lysenkoism was state-sponsored into the 1960s. And we've got Cutter Laboratories to thank for anti-vax movement. Some of our current ideas will probably be wrong or missing key info, it would be nice to be 100% right about the world, but the next Thomas Edison will probably have a "Don't talk to me about X-rays" moment too.  
> **username_3**: Unless you mean something other than "could there be missing, asymmetrically distributed mass in the solar system", then you should reword your question. There are lots of Q+A on the site about possible unseen planets.  
> **username_4**: Can you name one or two examples of this that have happened in the last fifty years? " *... bias that has made humans miss things about their solar system because they are looking at it from the vantage point of Earth orbiting inside the system*" Without clarification it's very hard to know what you are really talking about here, unless you mean before modern science began. *Thanks!*  

---

## 2 answers

### Answer by username_5 on 2019-12-14. Score: 4

Our understanding of orbits is not based simply on astronomical observation, it's based on physics as well.

In ancient times, we didn't have a very complete understanding of the physics of the universe. Philosophers (as what passed for scientists were called then) had many preconceived notions (e.g. all motion in the universe was either straight or circular), and they tried to fit their observations with them. It didn't feel like the Earth was moving, but they could see stars and planets moving, so they assumed the earth was stationary; to explain the complicated movements of planets they came up with things like epicycles.

Since then we figured out the heliocentric organization of the solar system (no more epicycles needed), then later that the solar system is part of a galaxy, which is part of a much larger universe, and scientists like Galileo, Newton, and Einstein refined our understanding motion and gravity. Astronomical observations on varying scales confirmed that these understandings were substantially correct (although when they got to galactic scales they discovered anomalies, leading to the notion of dark matter -- if this sounds like epicycles all over, read [What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy](https://medium.com/starts-with-a-bang/what-astronomers-wish-everyone-knew-about-dark-matter-and-dark-energy-d13eada60ea0)).

In addition to this, our observational technology has advanced. So even though we're observing from a single vantage point, we're able to detect motion in multiple directions. For instance, the Doppler effect can be used to detect motion to or away from the Earth (this is one of the techniques used to detect exoplanets, and it should be far more accurate for the Sun because it's so much closer). So we can map the precise motion of the Sun, and compare this with the expected motion around the barycenter with the known planets. If there were a significant difference, we'd go looking for an explanation; there might be another planet we haven't discovered (Neptune and Pluto were both discovered by looking for the sources of disturbances in other orbits), or some as-yet-unknown physical effect (e.g. General Relativity explained the anomalous orbit of Mercury).

> **Comments:**
> **username_3**: A distant planet would not produce a measurable Doppler effect (or demonstrate otherwise). A search for "planet 9" continues. If it were at 1000au and 10 Earth masses, the barycentre would be 6 solar radii from the Sun. The Sun would orbit this barycentre every 32,000 years at 2.8 cm per second.  
> **username_6**: The Doppler effect is more precise the faster an object is moving towards or away from us. For example, Barnard's Star has a high blueshift because it is currently getting closer to the solar system very quickly. Since the Earth's movement in the solar system is mostly perpendicular to the direction of the Sun, this doesn't help us much.  

---

### Answer by username_6 on 2019-12-15. Score: 0

The barycentre of the whole solar system is inside the Sun roughly half the time, and outside the other half.

From [Wikipedia's article](https://en.wikipedia.org/wiki/Barycenter#Inside_or_outside_the_Sun?):

> To calculate the actual motion of the Sun, only the motions of the four giant planets (Jupiter, Saturn, Uranus, Neptune) need to be considered. The contributions of all other planets, dwarf planets, etc. are negligible. If the four giant planets were on a straight line on the same side of the Sun, the combined center of mass would lie about 1.17 solar radii or just over 810,000 km above the Sun's surface.

The associated image shows that the barycentre is not stationary in regards to the Sun, but moves. More properly, the Sun moves around the barycentre. Sometimes the barycentre is almost at the centre of the Sun and sometimes it's about twice the radius of the Sun from its centre.

[![enter image description here](https://i.sstatic.net/Flc0qm.png)](https://i.sstatic.net/Flc0q.png)

For the barycentre to be much further from the centre of the Sun than currently believed, there would have to be an undiscovered Uranus-to-Jupiter mass planet in the solar system close enough to the Sun to make a difference. This hypothetical body would have an effect on the rest of the solar system. This is how some of the outer planets were discovered: some orbits were being noticeably perturbed by *something*. The search for the *something* led to the discovery of another planet.

If it turns out that Planet Nine exists and is discovered and is a super-Earth with a mass around five to ten times that of the Earth (compared with 14.5 for Uranus and 17 for Neptune), it would be so far away that its effects on the barycentre would be negligible. The four giant planets are much closer, and much more massive, so have a much more noticeable effect.

> **Comments:**
> **username_3**: Last paragraph is incorrect. $m_p a_p = M_{\odot} a_{\odot}$. The further away the planet is, the greater it's effect on the barycentre position.  
> **username_7**: @RobJeffries - can you explain. It sounds like you are saying gravity has a greater effect the further the planet is :-)  
> **username_3**: @RoryAlsop the formula I have given is how you calculate the centre of gravity. It also features in the correct, but still not accepted, answer by antispinwards. This answer is plain wrong and yet has attracted 4 upvotes. At least one since I made my comment.  
> **username_7**: @RobJeffries - I think you still need to explain. My understanding is that the further away a mass is, the **less** effect it has. ie any planet outside the solar system will have negligible effect.  
> **username_7**: Yeah, I understand the physics well - I studied Astrophysics as part of my degree course at University, however Rob's sentence does not say what you just said :-)  
> **username_6**: I see how the barycentre of a _two_ body system increases by distance, but not for a system with more than two bodies. Otherwise we should consider all the dust, other stars, the black hole at the centre of our galaxy, other galaxies, etc.  
> **username_3**: For a system with multiple bodies, calculate where the centre of mass is and then add in the extra body and see how much it moves the barycentre.  1. We should consider dust in the solar system, but it is negligible. 2. Other stars and galaxies aren't part of the solar system, so don't affect our calculation of the solar system barycentre.  

---

