Galactic Plane
Contrary to ridiculous claims, we will not pass through the central plane of the galactic disk in 2012.

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A typical bogus illustration of "Crossing the Galactic Plane".

Some authors are claiming that the Sun will pass through the galactic plane in 2012. See the illustration to the right, for example. Authors also tend to confuse the galactic plane with the "Galactic Equator".

Our place in the galaxy

First, a bit of background: The Milky Way divides the night sky into two roughly equal hemispheres. This indicates that the Solar System lies close to the galactic plane1. However, "close" is a relative term. The Solar System is moving in the general direction of the star Vega. It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year)

As the solar system orbits the galactic core, it wobbles or ‘bobs’ up and down through the disk of the galaxy, in kind of a sine-wave shaped path. This occurs because the mass of the galaxy is actually spread out into a thin disk. When a solar system is on one side of this disk the gravity of the disk will pull on the solar system, speeding it up until it passes through the center of the galactic plane. Once the Solar System reaches the other side, the gravity of the disk pulls on the solar system, slowing it down, until it comes to a stop, and begins a return trip. All of this happens while the systems are all orbiting the center of gravity of the Galaxy, every 33 million years or so during the complete 225-250 million year orbit2.

Above the plane

We are currently 'above' the plane (to the galactic North) by about 75 to 101 light years3. Some researchers have suggested that when the Sun ends up above the disk of the galaxy, that we will not be as well protected by the magnetic field of the galaxy itself, and would therefore be exposed to more intense cosmic radiation4. Based on the period of oscillation this would not appear to be due for another 10 million years or so.

Passing through the galactic plane

But, what will happen when it passes through the center of the disk? Some have passed on the idea that a passage through the galactic plane may trigger some extra bombardment. Their theory is that as we pass through the densest part of the plane, gravitational forces from the surrounding giant gas and dust clouds dislodge comets from their paths. The comets plunge into the solar system, some of them colliding with the earth5.

Are we due?

Based on the solar system's current position (27 +/- 4 pc, or 75 to 101 light years)6 above the galactic plane and the fact that we are currently moving away from it7, with an estimated passage through it about 3 million years ago, we are not due to re-encounter the galactic plane for quite some time. We won’t be back through the disk for another 30 million years!8 I don’t think Microsoft Outlook goes out that far.


We have shown that the Solar System passes through the galactic plane every 33 million years or so, and that we last passed through it about 3 million years ago. We are therefore not due to pass through it again in 2012.

1. Bahcall, John N.; Bachall, Safi. (22 August 1985). "The Sun's Motion Perpendicular to the Galactic Plane" Nature 316, 706 - 708 doi:10.1038/316706a0
2. Bash, Frank. (1986). The present, past and future velocity of nearby stars - The path of the sun in 10 to the 8th years The galaxy and the solar system (A87-34101 14-90). Tucson, AZ, University of Arizona Press, 1986, p. 35-46.
3. McFadden, Lucy-Ann; Weissman, Paul Robert; Johnson, Torrence V.; 2007 Encyclopedia of the solar system, Academic Press, 2007. ISBN 0120885891, 9780120885893
4. Medvedev, Mikhail V. & Melott, Adrian L. 2007 Do extragalactic cosmic rays induce cycles in fossil diversity? University of Kansas. (accessed 2009-06-03)
5. Science Daily. 2008-05-02. Sun's Movement Through Milky Way Regularly Sends Comets Hurtling, Coinciding With Mass Life Extinctions. (accessed 2009-06-03)
6. Wikipedia. 2009. Milky Way (accessed 2009-06-03)
7. Wikipedia. 2009. Milky Way - Sun's location and neighborhood (accessed 2009-06-03)
8. EarthSky. 2011. Will the Earth pass through the galactic plane in 2012? [] (accessed 2011-01-25)


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