Catcher in the Argot

Saturday, December 31, 2011

Earth is not crossing the galactic plane in 2012 and the sky is not falling

Earth is not crossing the galactic plane in 2012 and the sky is not falling

For over two years I debated New Agers about the 2012 myths. The main believer was a charming guy by the name of Arrgy. I, and some other skeptics tried to dispel his fears about 2012, and in return we were insulted and called names.

It was a fascinating discussion, at least on the part of the rationalists who provided data and evidence to back their claims. It was also an interesting study in how New Agers and doom merchants ignore facts that don’t fit in with their beliefs.

In this post, I thought I’d share two interesting exchanges with astronomers that arose due to of my involvement in that thread.

(Here’s the full, 59 page thread for anyone who’s interested:

Arrgy believes, even after being repeatedly proved wrong, that:

  • Earth will cross the galactic plane on December 21, 2012;
  • Potentially Hazardous Asteroids (PHAs) are increasing as we approach the Galactic Plane, endangering Earth.

We are not crossing the Galactic Plane in 2012 and PHAs are not increasing due to said crossing.

Most scientists and astronomers agree with the following paper that we last crossed the Galactic Plane around 3 million years ago, and we will not cross it again for about 30 million years.

In March 2011 I asked a friend working as an astronomer in China about that paper. Here's what he said:

Basically the Sun orbits the Galactic centre along with the rest of the disk stars, but they all move up and down in a oscillating fashion. At the moment the Sun is a little above the nominal plane of the disk and heading upwards. The idea behind this woo is that the nominal plane of the disk is a dense collection of stars and debris that greatly increases the chance of collision as we pass through it.

In reality the plane is only defined by the relative density of stars, and the nominal plane is the turnover point in this density gradient. The scale height of the Galactic disk is about 100 parsecs for the youngest thin disk population and about 700 parsecs for the older thin disk, although there's a range of values for star of different ages. In simple terms, the density decreases in a smooth logarithmic way as you move away from the nominal plane.

The scale height is the vertical distance from the Galactic plane over which the density decreases by a factor of e, the natural logarithm base. So the density of stars on the nominal Galactic plane is only e times larger than the density a few hundred parsecs above the nominal plane. There is no sudden density spike. There is no dense collection of debris to pass through, we simply get a bit closer (on average) to the other stars as we pass through the plane.

What the paper means by the term Solar vicinity is the area a few parsecs around the Sun, i.e. at or near the nominal plane. The last time we crossed the plane was about 3 million years ago, and we won't cross it again for about 20 to 30 million years. This periodicity does not correlate with terrestrial mass extinctions.


In April 2011 I contacted the Mt Lemmon SkyCenter (which has discovered the most PHAs):

Dear Mt Lemmon SkyCenter

A friend of mine thinks Earth will be crossing the galactic plane in 2012,
but as far as I can make out, this will not happen for around 30 million

He also thinks that more Potentially Hazardous Asteroids are being
discovered because we are nearing the galactic plane. But my research
suggests that more PHAs were being discovered due to new observatories and technologies tackling the subject, and that the Mt Lemmon Survey, who has discovered the most, is now finding less, according to this:

Could you tell me:

When will we cross the galactic plane?

Are there more PHAs in existence because of Earth's position in the galaxy,
or are we just getting better at detecting them?

Many thanks for your amazing work, and in anticipation of a reply.

Here's the reply:

My name is Ed Beshore. The Catalina Sky Survey (which
includes the Mt. Lemmon Survey, the original Catalina Sky Survey, and
the Siding Springs Survey) are my responsibility, so Adam
thought it might be best if I tried to answer your questions.

While I can't give you a precise time of crossing the galactic
plane, I can say confidently it will not do so as soon as 2012.
Right now, the Earth is about +2 deg north latitude in galactic coordinates,
and the Earth takes about 250M years to rotate around the
galaxy - so if Earth is going to cross the plane next year, we have
to move it! I think your estimate of ~30 M years sounds more

The reason we are discovering more PHAs is because we only started
looking in earnest about 10 years ago. Catalina's 1.5 m reflector,
one of the largest telescopes routinely searching for hazardous
asteroids, only began its work in 2005. We are getting
better at the job, and larger telescopes are getting involved as
well, and that explains the surge in PHA discoveries. Indeed,
most of the PHAs have been in orbit in the inner solar system
for many hundreds of thousands of years, independent of our position
with respect to the galactic plane.

Finally, the Sun's (and the solar system's) orbit around the
center of the Milky Way is close to circular and stays close to
the galactic plane. Any forces from the galaxy
on our environs are pretty near constant, so something like
crossing the plane of the galaxy should be a pretty
hum drum event.

I hope this helps.

Kind regards,

Ed Beshore

Edward Beshore, Senior Staff Scientist
Lunar and Planetary Laboratory and the Steward Observatory


If you’re interested in other rational explanations of 2012 myths, I highly recommend the following website:

Have a great, superstition-free year!

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