Earth’s Moon is the fifth largest satellite in the Solar System, making it one of the larger objects, period: only Io, Callisto, Ganymede, Titan, and the eight planets are larger. None of the dwarf planets designated by the International Astronomical Union (IAU)—Ceres, Eris, Haumea, Makemake, or Pluto—are bigger, and the Moon is significantly more massive than any of them as well, due to its dense interior. The Moon’s history is tied to Earth’s, while the Moon’s tidal pull on Earth churns the oceans and atmosphere. Earth’s tidal force on the Moon slowed its rotation over time until it always presents the same face to us, and the Moon has slowed Earth’s rotation as well—in the very distant future (probably beyond Earth’s real lifetime), the planet will also present the same face to its satellite. Earth and Moon surely make a double world.
However, an even truer double world is the pair Pluto and Charon. (Charon is pronounced either CAHRon or SHARon, depending on how Greek you want to be.) Pluto is most famous of late for the quasi-controversy over its planetary status, which I will not get into in this post. (My own views on the subject are pretty well-documented.) However, Pluto is a very interesting object, not least because of its busy satellite system; Charon is the largest by far, but there’s also Nix, Hydra, and a fourth unnamed moon. We don’t know a lot aboutany of these bodies: Pluto is too small and too far away for our telescopes to resolve its surface features, and the moons are even smaller. While Pluto was discovered in 1930, Charon wasn’t found until 1978, when James Christy (astronomer at the US Naval Observatory) determined Pluto to have a lump protruding from its side. The full analysis revealed the lump was a moon, of course; Nix, Hydra, and P4 were found within the last 10 years.
Nix and Hydra are somewhere between about 30 and 120 kilometers in diameter (you can see how imprecise our data are!), and P4 is even smaller. Charon, on the other hand, is about 1200 kilometers across, which is more than half of Pluto’s 2300 km diameter! Charon is significantly less dense than its host world, though, indicating a greater amount of ice in its makeup. Pluto is rockier, and weights in at about 10 times the mass of Charon, but it is still far less dense than Earth’s Moon. All four satellites probably formed when another body struck Pluto in the early days of the Solar System, breaking chunks of ice and rock off. Astronomers postulate this based on how the moon’s orbit: they are all regular satellites, orbiting roughly in the plane of Pluto’s equator and in the same direction Pluto rotates.
In fact, just as the Moon presents one face to Earth, Charon presents one face to Pluto—and Pluto reciprocates! If you lived on Pluto (somehow surviving average temperatures just 44 degrees above absolute zero) on the opposite side to Charon, you’d never see the moon at all. Additionally, Pluto and Charon are similar enough in mass that they are in mutual orbit: they both circle around an empty point in space known as the barycenter, the average position of the total mass in the system. To see what I mean, watch the video I made below.
We’re seeing the Pluto-Charon system from a point in the plane of Pluto’s orbit around the Sun. The larger gray sphere is Pluto, the smaller gray sphere is Charon, and the red dot is the barycenter. Everything in this video is to scale: the relative sizes of Pluto, Charon, and the orbits are accurate, so you can see how close the two worlds are. Since Charon’s orbit is aligned with Pluto’s equator, you can also see how tilted Pluto’s axis is compared to the plane of its orbit—nearly as dramatic as the tilt of Uranus, the planet that orbits nearly on its side.
I called Earth and the Moon a double world too, so let’s do a comparison. For one thing, the Moon is a lot farther away from Earth, relatively speaking, so to make everything clear, I’ve doubled the diameter of both worlds to make the video below. However, everything else is accurate: Earth moves very slightly around the barycenter (represented here as a red line), which lies beneath Earth’s surface. Another interesting difference is that the Moon’s orbit isn’t aligned with Earth’s equator, and also is slightly tilted from the plane of Earth’s orbit around the Sun. I’ve marked Earth’s axis with a white line, so you can see how this works.
Of course, the slight misalignment between the Moon’s orbit and Earth’s orbit is what leads to the pattern of eclipses we see; if the orbits were in the same plane, we’d get solar eclipses every new Moon, and lunar eclipses every full Moon.
Every world in the Solar System, whether we call it a “planet” or not, was shaped by its detailed history. Collisions, proximity to other worlds, location relative to the Sun, and many other factors created the worlds we see. Pluto and Charon are no different, and when the New Horizons probe arrives in 2015, we will be able to trace more of the natural history of the edges of our Solar System.