Date: February 15, 2012

Title: Astronomy Word of the Week : Barycenter

Podcasters: Dr. Christopher Crockett

Organization: United States Naval Observatory


Description: Today’s astronomy word-of-the-week is ‘barycenter’ where we learn what see-saws can tell us about finding distant planets.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Sponsor: This episode of “365 Days of Astronomy” is sponsored in-kind by Universe Today, your source for space and astronomy news updates since 1999. Find us at


If you want to find a planet orbiting a distant star, it helps to first know something about see-saws.

While astronomers and philosophers have long mused on the possibility of other solar systems, the first planets confirmed to orbit a star other than our Sun weren’t found until 1992. The reason it took so long is that it’s really hard! When viewed from a distance of 10 light-years, the separation between the Sun and Jupiter is roughly equal to the thickness of a quarter viewed from 2 km away. What’s more, the Sun is roughly one billion times brighter than the giant planet. The distance between a planet and its sun combined with the colossal difference in brightness between the two means that, except in special cases, simply taking a bunch of pretty pictures of the night sky and hoping a planet pops out isn’t a terribly reliable way to find other solar systems.

Fortunately, nature has provided another way.

Most people, if asked, would say that the Earth orbits the Sun or that the Moon orbits the Earth. But the truth is, that’s not quite right. The notion that the Earth orbits the Sun is only approximately true. The Sun and the Earth actually orbit around each other. They swoop around a common center called the barycenter.

Imagine twin brothers who want to balance themselves on a see-saw. Where should they sit? It probably doesn’t come as a great surprise that the brothers should sit at equal distances from the center. Any other arrangement and the see-saw will tip one way or the other. A physicist would say that when the brothers have got the see-saw balanced, that their center of mass is over the pivot of the see-saw.

Now lets take a 200 lb uncle and his 50 lb niece. How should they sit? The uncle, being four times heavier, needs to sit four times closer to the center than his niece in order to achieve balance. To use some real numbers, if the niece is 4 feet away from the center, the uncle needs to sit one foot away on the other side. Using a physicist’s nomenclature once again, we can say that the center of mass of the uncle and niece is four times closer to the uncle than his niece.

What about two objects where one is 1000 times heavier than the other? Well, the center of mass would be 1000 times closer to the heavier object than the lighter one. Are you seeing a trend? However many times heavier one object is than another, that’s how many times closer the center of mass is to that object (or, to think in see-saw terms, that’s how many times closer that object needs to be to the center to make the whole thing balance).

So what do see-saws have to do with astronomy? Well, anytime you have two objects in orbit around one another they orbit around a location in space defined by their center of mass. When it comes to things in orbit, the center of mass and the barycenter are the same thing.

If you have two stars in orbit around one another, both with equal mass, they will be orbiting around a point exactly halfway between them. If one star is four times more massive than the other, they will orbit around a point that is four times closer to the more massive star. If one is 1000 times heavier…..well, you get the point.

However, when it comes to planets and stars, the difference in masses is enormous! The Sun is over 330,000 times more massive than the Earth! That places the Earth-Sun barycenter 330,000 times closer to the center of the Sun than the Earth. Given the 93 million miles between the two, that means the barycenter of the Earth-Sun orbit is only about 280 miles from the center of the Sun. But, wait! The Sun has a radius of nearly 700,000 miles! That means the point around which the Sun and Earth orbit is deep in the interior of the Sun – only 4 hundredths of a percent of the distance from the center of the Sun to its surface!

We can apply this idea of a barycenter to any two objects which orbit one another: moons and planets, binary stars, even systems of galaxies! The Moon and the Earth, for example, both orbit around a barycenter that sits about 1000 miles beneath your feet. The barycenter of icy Pluto and its largest moon Charon actually sits between the two, roughly 200 miles above the surface of Pluto. This, incidentally, has led some astronomers to argue that Pluto and Charon should be classified as a “double planet”. I think we all know how that turned out…

So, back to the original question: how does all this help astronomers find other solar systems? As the Sun tugs on the Earth, the Earth tugs back with an equal and opposite force. As our planet loops around on its orbit, it yanks our star around on a very small orbit. The Sun is orbiting around a barycenter located deep in its own interior, wobbling 280 miles to either side (for 560 miles of total wobble). And that is what astronomers look for: stars that are wobbling! A wobbling star is the sign post of a star sharing an orbit with something much smaller and fainter. We often don’t directly look for planets: we look for the effect a planet has on its sun.

In reality, though, the wobbling motion of our Sun is considerably more complex because the Earth isn’t the only thing orbiting around it. Our Sun has a family of eight major planets and each planet has its own barycenter with the Sun. But the locations of the planets are constantly changing. Since the distribution of mass around the Sun not constant, that means the barycenter of the entire Solar System is not in a fixed location but does some wandering of its own. Most of the Sun’s motion is driven by the great bulk of Jupiter, but the other seven planets each contribute their own part to our star’s complex dance.

The identification of wobbling stars isn’t just limited to the discovery of unknown solar systems. This works for any situation where a star has a much dimmer companion in its neighborhood: planets, other dim stars, even more exotic creatures like neutron stars and black holes. Nevertheless, the same principle behind getting a see-saw to work has led to the discovery of over 700 planets just in our tiny corner of the Galaxy.

And that number just keeps on growing.

For additional information including links and images, be sure to stop by the AstroWoW blog where we explore the Universe one word at a time. From the U.S. Naval Observatory, this is Christopher Crockett.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.