Mutually Orbiting Objects

Pluto and Charon orbiting each other
Pluto and Charon orbiting each other

This gif I found on the Planetary Society’s website shows Pluto and its largest moon, Charon, mutually orbiting each other.

Their description says:

This animation contains 13 images taken over the course of one Pluto rotation between April 12 and April 18, 2015. During this period, New Horizons was roughly 110 million kilometers from Pluto. Stacking and deconvolution has been used to increase the resolution of the images over the raw data, revealing Pluto’s spotty surface. The brightness variations make Pluto appear lumpy, but it is actually round.

Stacking and deconvolution. Stacking means they took several pictures and put them all together, one on top of the other, to get a better composite. I understand that. I don’t know what deconvolution means. You can tell I’m not  graphics person.

These mutually orbiting dots can tell us a lot.

For a grainy picture of two moving spots of light, this visualization actually gives us a lot of information. For instance, it shows us that Pluto is spotty. It takes about six earth days to rotate once on its axis. During that time, Charon orbits it several times.

But what immediately grabbed my attention is the way the little world dances with its moon.

We usually think of a satellite orbiting its primary. In this case, it would be Charon, the tiny dot, orbiting Pluto, the larger dot. But we’ve always known this is not really the case. The two objects orbit each other. More accurately, they both orbit their common center of mass (or center of gravity).

This is the principle that was first used to find extrasolar planets; and it’s still used, along with other methods.

Other star systems are so far away that we can’t really see the movement, but the Doppler effect is used to infer any wobble as the star moves toward us and away from us. As it moves toward us, the light is blue shifted a little; away from us, it is red shifted. A complex wobble could indicate more than one planet. This same measurement can often tell us how many planets there are, their approximate masses, and something about their orbits.

This is amazing, because a star is usually so much more massive than any planets orbiting it that the common center of mass will actually be inside the star itself. While the planets orbit in wide ellipses, the stars themselves just wobble very slightly like toy tops beginning to run down. This is often true of planet/moon systems as well. For this reason, until a couple of decades ago, it was commonly believed that we would never be able to detect this wobble. Now, of course, it is a routine–though time consuming–procedure. It can take years to observe enough doppler data to determine the number of planets in a system.

In the case of Pluto and Charon, this dance is visible to us because both objects are so near the same mass.  (Back in the days when Pluto was a planet, they have even been referred to by some astronomers as a double planet.)

This is the first time I’ve seen this principle visualized so well.

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