# FeaturedI Solar system's path in the Milky Way galaxy

1. Jun 26, 2017

### mathman

My understanding: Circuit is ~ 30 million years. Plane of solar system orbit is tilted from plane of milky way.
Is milky way plane crossed once or twice? What is error bar for 30 million? How much does dark matter density vary along solar system path?

2. Jun 26, 2017

### Bandersnatch

By 'circuit' do you mean 'orbital period'?

3. Jun 26, 2017

### CygnusX-1

The orbital period of the solar system around the Milky Way's center is about 230 million years.

The vertical period of the solar system--up, then down, then up again--is about 60 million years. That includes two crossings of the Galactic plane. The uncertainty is probably a few million years.

The reason the two periods are not equal the way they are for a planet in the solar system is because nearly all mass in the solar system is in the central object whereas the Milky Way's mass is spread out.

4. Jun 27, 2017

### snorkack

No, the reason they are not equal is that Milky Way has a disc.
The fact that Milky Way mass is spread out has the consequence of a third period being different. Namely the period of apside passages.

5. Jun 27, 2017

### mathman

If I understand you correctly, the solar system crosses the galactic plane about every 30 million years. Is this the same interval between all crossings or does it vary through the 230 million year orbit?

6. Jun 27, 2017

### CygnusX-1

The solar system is also a disk. But more than 99.9% of the mass is in the central object, the Sun. That's why the periods are equal.

If 99.9% of the Milky Way's mass were in Sagittarius A*--the central black hole--then the periods would also be equal. But it's not, and they're not.

The Sun also has a third period, in addition to its vertical period and orbital period. This is the radial period: how long the Sun takes to move from perigalacticon (its closest point to the Galactic center) to apogalacticon (its farthest point from the Galactic center) and back to perigalacticon. Because the Milky Way's mass is spread out, this period does not equal the orbital period or the vertical period. It is in between the two other periods--in other words, more than 60 million years but less than 230 million years.

If the Milky Way were spherical, it wouldn't make sense to talk about the vertical period, because there would be no disk. But the orbital period and the radial period would still differ from each other because the mass is spread out. This point is discussed on page 109 of Galactic Dynamics by James Binney and Scott Tremaine.

7. Jun 27, 2017

### CygnusX-1

Yes, there is a Galactic plane crossing every 30 million years, and it's the same interval throughout the 230-million-year-long orbital period.

8. Jun 27, 2017

### stefan r

Interactions with other stars would make the path vary. We also have some uncertainty caused by dark matter. What dark matter is and where exactly the dark matter is located will change the sun's motion. The spiral arms can also change while the sun makes an orbit. We cannot see the other spiral arms. Any "picture of the milky way" is really a picture of another galaxy with some similarity or it is an artist. Artists are also looking at other galaxies that might be similar. No one is expecting to be alive when humans see the far side and prove the picture is wrong.

A quick search on the internet came up with a range of numbers for the crossing period. Some estimates extrapolate from extinction events and craters.

9. Jun 28, 2017

### mathman

An article I read recently (American Scientist - latest edition) describes extinctions having a 26 million year period, somehow correlated with the 30 million year plane crossing. I am puzzled, since after two or three cycles they would be too far apart.

10. Jun 28, 2017

### Bandersnatch

What you should take away from this thread, is that there just isn't enough data to justify anything but very rough first order approximations. Certainly not enough to support such correlations as cited above.

For example, you'd only get a constant ~60 My period of oscillation if the galactic gravitational potential in the plane-normal direction was constant. It patently isn't, as densities of both baryonic and dark matter are expected to vary with time. But until people figure out how it changes, that simple picture of constant-period oscillations is all we've got.
This is an active area of research, with current GAIA mission being the main source of data for improved astrometry and potential modelling.

11. Jun 28, 2017

### stefan r

Yesterday I saw numbers ranging between 30 and 40 million years.

A 4 million year delay is not unreasonable. The oort cloud gets disturbed and then it takes a few thousand years for the comet to go through the solar system. It might make hundreds of passes before Jupiter/Saturn captures it into a lower orbit. Then it can wonder around for a long time before hitting anything.

12. Jun 29, 2017

### snorkack

Terrestrial system is also a disc, with 98,8 % of mass in the central object. But there does happen to be mass outside terrestrial system.
Neither the orbital plane of Moon relative to distant stars nor the apside line remains constant.

13. Jun 29, 2017

### CygnusX-1

A very different situation, because a very massive object--the Sun--is located close to the Earth-Moon system.

In the Milky Way, the periods would be equal if all mass were located in the central object. But it's not, and they're not.

14. Jun 29, 2017

### CygnusX-1

As I stated earlier, the period is uncertain by a few million years, so the difference isn't that troublesome.

It is true that over very long periods of time--billions of years--the periods will change, but over the course of a full Galactic orbit (230 million years) the change should be small. The main drivers of change in the solar system's orbit are probably giant molecular clouds, which can have as much as a million solar masses; when the solar system encounters such clouds, their gravity alters the orbit. Spiral arms may also play a role. And the Milky Way has certainly acquired mass during the Sun's 4.6-billion-year lifetime.

It has been known for more than a century that what we now know are young stars have more circular orbits around the Galaxy and stay closer to the Galactic plane than do older stars in the Milky Way disk. Young stars are thought to be born on fairly circular orbits in the Galactic plane; then giant molecular clouds and spiral arms scatter the stars into more elliptical orbits that take the stars farther from the Galactic plane.

This process is discussed on pages 93-100 of Ken Croswell's book The Alchemy of the Heavens: Searching for Meaning in the Milky Way. Croswell interviewed and quotes the two astronomers--Martin Schwarzschild and Lyman Spitzer, Jr.--who developed this idea. Amazingly, Schwarzschild and Spitzer predicted the existence of million-solar-mass clouds two decades before they were discovered! "We ourselves were a little bothered by that large number," Schwarzschild told Croswell. "It was quite a jump of faith."

Incidentally, the Sun has a more circular orbit and stays closer to the Galactic plane than does the average star of its age, so perhaps we haven't been as perturbed by giant molecular clouds and spiral arms as the typical solar-age star in the Galaxy.

15. Jun 29, 2017

### snorkack

If all mass were located in the central object, all three periods should be equal, yes.
Moon happens to have anomalistic month longer than sidereal... likely because there is a big mass outside the orbit.
Sun has anomalistic orbital period shorter than sidereal, because the mass distribution of Milky Way crosses Sun´s orbit. The anomalistic period of Sun must be shorter than 230 million years, but also necessarily longer than 115 million years. Guestimates are around 150 million years, but not sure what the observational bounds are.
If Milky Way were spherical then there might be no disc, but you could still define draconic period relative to any arbitrarily chosen plane. And the draconic period would then be equal to sidereal - even if the anomalistic period is much shorter. Actually, the same would happen if the disc were plainly marked, but the bulk of Milky Way mass were in spherical dark matter crown and also spherical bulge of old, relatively dim stars, with the disc being a bright but low mass wisp of gas and young bright stars - the draconic period should be close to sidereal, while anomalistic would be much shorter.
That it is the opposite - the draconic period at 60 million years is the much shorter one - suggests that either the disc is massive compared to dark matter crown, or else the dark matter is also a disc and not a sphere.

16. Jun 29, 2017

### mathman

The point I was trying to make is that after three periods, the difference is 12 million, almost half the period. This looks too big for correlation.

17. Jul 3, 2017

### anorlunda

I think that's very interesting. During one orbit, the solar system must pass through regions of higher star density. I wonder what the probability is of completing a full orbit without an interaction with another star strong enough to cause a significant change in its orbit. Since I lack a good definition of significant, I can't formulate that question properly.

How about this? Let me assume that any star passing through a globular cluster is "significantly" disturbed.
I presume that we know the density of globuar clusters at the galactic radius of today's solar system. What is the probability of a star plunging through the galactic plane (at our radius) of encountering a globular cluster?

Or perhaps there is a fractal measure for the galaxy as a whole. On one extreme, all stars follow stable periodic galactic orbits. On the other extreme it is all chaotic such that the word orbit becomes meaningless. Between those extremes is a fractal measure.

Arrrgh. I'm frustrated by my inability to articulate my question.

18. Jul 3, 2017

### BillTre

Going back to the 1980's, there has been a series of arguments that the Sun's movements through the galaxy have lead to periodic extinction events due to impactors with disrupted orbits.

19. Jul 3, 2017

### Staff: Mentor

https://arxiv.org/abs/1108.1570

The abstract illuminates some points made in the thread, and does not appear to support some others. The paper seemingly does not claim to be more than a simple report of what currently appears to be known. I'm not an astrophysicist so I cannot comment on the validity of the comments in the paper.
Hopefully someone else can.

20. Jul 3, 2017

### CygnusX-1

Interactions with individual stars do NOT cause the solar system's path to vary significantly: "in the Galaxy, stars lie so far from one another that they hardly ever exchange much energy."--The Alchemy of the Heavens by Ken Croswell, page 95.

You're getting warmer! To perturb the solar system's path, you need objects much more massive than individual stars. Globular clusters might seem like a good bet--until you realize there aren't very many of them, and most are in the Galactic halo, whereas the solar system is in the Galactic disk. So globular clusters won't do the job, because most stars in the disk never encounter one.

But there are massive objects located in the Galactic disk: giant molecular clouds. The most massive of these can have millions of times as much mass as the Sun, and their gravitational pull definitely perturbs the orbits of stars passing near them.