Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

In a recent paper (news release here) Lisa Randall and Matthew Reece propose that a dark matter disk coinciding with the galactic plane together with the solar system's oscillations through the galactic plane could explain the 35 million year periodicity in mass extinctions. They propose that the solar system passes through this dark matter disk in the galactic plane every 35 million years, disrupting bodies in the Oort cloud and causing some of them to collide with Earth. An image of the proposed cycles from the news release is below.

My question is: What mechanism is responsible for this 35 million year oscillation about the galactic plane? Is there a companion body to the sun? Are we orbiting around one of the arms of our galaxy? Is this a well known phenomenon or are they proposing the 35 million year oscillation as well as the dark matter disk?

enter image description here

share|improve this question
up vote 4 down vote accepted

The cause for the oscillations perpendicular to the galactic plane is the gravity of the non-spherical mass distribution (needed for a plane Kepler ellipse) in the Milky Way. Simplified, there is a dense galactic plane. The density is not exactly known; therefore there is some uncertainty (a few million years) about the precise oscillation period. Details see this article, subsection 3.3.

The idea of a correlation of mass extinctions with this oscillation isn't new, it originates back probably to about 1970, or earlier.

"The outer solar system probably does not contain a large gas giant planet, or a small, companion star", see this press release.

We are probably not orbiting around a spiral arm.

The dark matter disk is a hypotheses, an idea to investigate. Usually only a small fraction of this kind of hypotheses can be confirmed definitively later, most of them can be ruled out after some time, some remain unsolved, some can be refined to match observations.

share|improve this answer
the sun may have still a "very far" extra planet, or a "very very far" binary companion, but the distances for a certain mass object to be, are getting bigger and bigger, since the observational accuracy is getting higher and higher. Given that most stars are binary, the question than becomes "where is our suns companion", which is one of the big gaps in star formation theory: binary systems and why certain systems become binary and others do not – usethedeathstar Mar 19 '14 at 14:44
@usethedeathstar The sun may have been ejected from an open star cluster within a few million years after formation. The released kinetic energy may have bound binaries, which are now somewhere else in the Milky Way. – Gerald Mar 19 '14 at 18:00

Ok, it makes a lot of sense that the gravity of the disc of the Milky Way is pulling stars up and down as they go about their circum-galactic orbit. But this wouldn't explain why recent 3-D observations of the nearest stars using the FLAMES-GIRAFFE spectrograph on ESO’s Very Large Telescope and the IMACS spectrograph at the Las Campanas Observatory showed a definite wave-structure in the motions of stars orbiting the Galactic plane. In other words, most of the stars in the disc are following each other in a chain or train, as if they are bobbing up and down on a CURRENT. Which means they most certainly are oscillating on a current. And this was predicted in 1978!

Our Sun produces a magnetic field stretching out along the equatorial plane in the heliosphere. This field stretches throughout the Solar System where it is called the “Interplanetary Magnetic Field”. In 1965 John M., Wilcox and Norman F. Ness published their finding of the “Heliospheric Current Sheet” which showed that the Sun’s rotating magnetic field is constantly producing waves in the plasma of the interplanetary medium.

These waves form a “Parker Spiral” and are described in terms of an electromagnetic current but they are also mechanical waves which cause the planets themselves to oscillate up and down as they orbit the Sun. In 1978 Hannes Alfven and Per Carlqvist suggested that there is a similar “Galactic Current Sheet” carrying an electric current of 10^17 to 10^19 amperes through the plane of symmetry of the Galaxy.

Ok? That pretty much solves the mystery of the oscillation of the Milky Way's stars. But the problem here is that (ahem, cough cough) OUR SOLAR SYSTEM IS NOT PART OF THE MILKY WAY. In 1994 it was discovered that we are actually part of the Sagittarius Dwarf Elliptical Galaxy, or Sag-DEG for short, which is in a 500 million year POLAR ORBIT around the Milky Way.

Have you ever wondered why they say our Solar Apex is near Vega, but Vega itself is moving towards us almost twice as fast as we are moving towards it??? Well, in the late 80's it was discovered that nearly all of the stars orbiting the Milky Way seem to be "raining" down on our position. Which could only mean that our Solar System is moving upwards, out of the Milky Way. Sorry to inform you guys, but even though the stars of the Milky Way oscillate up and down during their 250 million year orbit, we aren't part of that dance. Our own path will take us high above the Galaxy, with a spectacular view at apogalacticon, and then back down again.

share|improve this answer
Hi! Thank you for your answer. We tend to encourage the use of references. It would be great if you could include those in your answer. You can typeset links using markdown like this [link]( We hope you find this site as amazing as we do! – harogaston Aug 3 '14 at 15:57
Provide references. – Rob Jeffries Dec 24 '14 at 14:29
-1 for the crap contained herein, such as the claim that we are part of the Sagittarius Dwarf Elliptical Galaxy. Wikipedia clearly states, "Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth" and "the main parent cluster is on the opposite side of the galactic core from Earth". From your answer, "Well, in the late 80's it was discovered that nearly all of the stars orbiting the Milky Way seem to be "raining" down on our position." is also clearly incorrect. – HDE 226868 Jul 15 '15 at 1:21

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.