Astrophysicists believe that these small moons, which are clustered in three bands, are the remnants of three massive moons which were broken apart by collisions with other bodies in space. Because of that small moon's orbit, it may be eventually be destined for an crash. Others including the oddball are "pro-grade" moons travelling with the planet's spin. They named it Valetudo, after a daughter of Jupiter and the Roman goddess of hygiene and personal health. Most big objects in the solar system all orbit the Sun and spin in the same sense (the motion is counterclockwise when looking down from above the Earth's north pole). Yet it's orbiting in the same direction as the planet, against the swarm's traffic. Those retrograde moons are then further grouped into three distinct sections. Only the two innermost planets in the solar system, Mercury and Venus, have none.
The new moons were first glimpsed in 2017, using a telescope based in Chile and operated by the National Optical Astronomical Observatory of the United States.
Gareth Williams, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director at the International Astronomical Union's Minor Planet Center, predicted that "there aren't any bigger objects undiscovered out there" around Jupiter.
"So, the whole process took a year", Williams, who calculated the orbits of the new moons, said in a statement.
"This is an unstable situation", Sheppard said.
It's further away than the prograde moons, taking around one and a half years to orbit around the planet. Instead, its path takes it into the realm of the nine moons that orbit in the opposite direction. Even though a dozen new moons is a pretty good haul, Sheppard expects that more searching will turn up even more moons. Given a provisional name Valetudo (the great-granddaughter of Jupiter in mythology), it orbits prograde, but is also so far from Jupiter that its orbit is solidly among the retrograde moons!
These new moons probably formed in a place in our solar system known as the giant planet region, which is between the asteroid belt, dominated by rocky asteroids, and the Kuiper belt, dominated by icy comets. "We could choose our field of observation to be very close to Jupiter, so we could look for things moving at Jupiter's rate-foreground objects, moving quite fast", while still on the hunt for relatively slower-moving objects in the fringes of the solar system, Sheppard says. The orbits of the new moons are marked with thicker curves.
The team had planned to use the observatory's Blanco four-meter telescope to scout for objects way out, beyond Pluto, and they also made a decision to train their gaze on Jupiter's neighborhood in the night sky.
Once they finish running and analyzing the simulations, the team plans to publish the results in early 2019.
Finding a moon that tiny, he noted, means it must have formed after all the gas and dust had cleared.
Over the weeks following full opposition, Jupiter will reach its highest point in the sky four minutes earlier each night, appearing as a bright, star-like object.