The planet Jupiter is a beast: Three-hundred-and-seventeen times the mass of the Earth, mostly made of metallic hydrogen, and at the center of an astonishing collective of orbiting natural bodies.
In fact, Jupiter’s satellites form a shrunken version of a full planetary system: from the tightly bound larger Galilean moons (orbiting in their Laplacian mean-motion resonances, akin to places like TRAPPIST-1) to the remarkable array of smaller moonlets that encircle this world out to more than 30 million kilometers.
These bodies circle Jupiter in anywhere from about 7 hours to an astonishing 1,000 days.
NASA’s Juno spacecraft captured this set of time lapse images of the large Galilean moons during the spacecraft’s approach in early 2016:
Until recently the cataloged satellites totaled 67 in number. But only the innermost 15 of these orbit Jupiter in a prograde sense (in the direction of the planet’s spin). The rest are retrograde, and are likely captured objects – other pieces of the solar system’s solid inventory that strayed into Jupiter’s gravitational grasp.
That population of outer moons is mostly small stuff, only a few are 20-60 kilometers in diameter, most are barely 1-2 kilometers in size, and increasingly difficult to spot.
Now astronomers Scott Sheppard, David Tholen, and Chadwick Trujillo have added two more; bringing Jupiter’s moon count to 69.
These additions are also about 1-2 km in size, and were spotted in images that were part of a survey for much more distant objects out in the Kuiper Belt. Jupiter just happened to be conveniently close in the sky at the time. The moons are S/2016 J1 and S/2017 J1, and are about 21 million km and 24 million km from Jupiter.
By themselves these small satellites don’t amount to much. But they are a vivid reminder of the sheer abundance of material out there in our solar system, and of Jupiter’s royal gravitational status.