This image series, taken by NASA’s New Horizons spacecraft in late January 2015, reveals the dwarf planet Pluto and its largest moon, Charon, orbiting their common center of mass. For the geeks among us (including yours truly), this is seriously cool. Technically speaking, a smaller body doesn’t orbit around a stolid larger body; they both orbit their common center of mass, called the barycenter. That holds even for the solar system: instead of the planets twirling around a stationary Sun, Sun and planets orbits a common center of mass. Currently that center of mass is inside the Sun, but it moves around as the planets move and should be outside the Sun in a couple of years.
Charon is about half Pluto's width and somewhere around one-eighth to one-tenth its mass, and with a center-to-center orbit of roughly 20,000 km, the center of mass for this system is actually above Pluto’s surface. And you can totally tell when you watch this time-lapse series! New Horizons’ Long Range Reconnaissance Imager (LORRI) took the images from January 25th to 31st as part of the effort to better pinpoint Pluto and Charon’s locations for the spacecraft flyby this July.
(And if you’re wondering where Pluto’s other four moons are, they’re too faint to be resolved at the spacecraft's distance — about 120 million miles, or 200 million kilometers.)
You can read more about it in the press release from Johns Hopkins University Applied Physics Laboratory.