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The Wobbling Sun April 3, 2009

Posted by CosmicThespian in Tidbits, Toolkit.

In a previous post, we discussed how a planet and its star orbit a common center (the barycenter).  This causes the star to wobble in space as the planet orbits and tugs on it.  In most cases, astronomers searching for extrasolar planets use this wobble to infer the presence of another world rather than directly seeing the planet itself.

I came across a paper a couple of weeks ago: “Exoplanets – search methods, discoveries, and prospects for astrobiology” (B.W. Jones, 2008).  The paper is a summary of exoplanet detection methods written for astrobiologists, scientists who work at the crossroads of physics, astronomy, biology, geology, oceanography, chemistry, etc. to understand the cosmic origins of life and the prospects for the development of life elsewhere in the Universe.  One of the figures he uses in the paper I found to be interesting and relevant to the past several posts on this blog:

sunwobbleThe figure shows the wobble of our Sun over a span of 50 years viewed from 30 light years away.  Our Sun’s wobble is complicated; it’s driven mostly by the competing gravitational tugs from eight worlds!  The dots show the location of the Sun at different years (starting in 1975) while the solid line traces out the path of the wobble.   The dashed line shows what the Sun’s wobble would be if Jupiter were the only planet in our solar system.  As you can see, Jupiter is responsible for much of the Sun’s apparent excursion across the sky.  This is because Jupiter is the most massive planet in the Solar System – roughly 300 times the mass of the Earth and 2.5 times more massive than all the other planets added together!  All of that mass means that Jupiter is the most influential planet on our Sun’s motion with the other seven major worlds contributing various amounts of perturbation on top of that motion.

The disk in the upper right shows how large the Sun would be on this scale.  It’s interesting to note that the amount the Sun moves over 50 years is roughly the size of the Sun itself.

The numbers along the side and the bottom indicate the angular scale on the sky; this box is 2 milliarcseconds on a side. That’s 2 millionth’s thousandths of an arcsecond or half a billionth millionth of a degree!  That is incredibly small.  It’s roughly equivalent to the thickness of a dime seen from 140 km, or just over 80 miles, away!

What’s also important to remember is that this is seen from only 30 light years away.  The further away you get, the smaller the wobble will appear.  30 light years encompasses only our very local neighborhood; the Milky Way galaxy is roughly 100,000 light years across!  Looking at a star that far away is peering only 3 hundredths of a percent across the length of our Galaxy!!

Clearly,  when we start detecting the wobbles of stars within 30 light years of our Sun, we will have barely scratched the surface of what could be out there!

Edit 4/22: Because I forgot how the metric system works.  (Thanks to GaryC for that catch!)

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1. Carnival of Space #99 - April 19, 2009

[…] you need 1 Cup hot water (as hot as you can get from the tap) – and in that vein Christopher of the fairly new and piping hot Innumerable Worlds presents an analysis of the Sun’s wobble (What if we were looking back at ourselves from 10pc […]

2. GaryC - April 22, 2009

The numbers along the side and the bottom indicate the angular scale on the sky; this box is 2 milliarcseconds on a side. That’s 2 millionth’s of an arcsecond or half a billionth of a degree!

A milliarcsecond is one thousandth of an arcsecond.
A microarcsecond is one millionth of an arcsecond.

cosmicthespian - April 22, 2009

Yikes! Thank you for that catch. I can do basic metric manipulations, I swear!

3. Osolu - January 4, 2010

Look at http://www.sunorbit.net, there you could read it since 2002!

4. Pat Blewett - May 2, 2011

In the two body problem of the sun and jupiter;
the sun should wobble about a fixed point,the
center of mass. In turn jupiter should revolve
around the same fixed point. I don’t understand
the sketched circle, should it not be a fixed point?

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