Scientists were looking for objects on the fringes of the solar system a year ago when they pointed their telescopes close to Jupiter's backyard, according to Scott Sheppard of the Carnegie Institute for Science in Washington. They used the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in Chile, which just received a dark energy camera optimized to look for faint objects in the sky.
Nine of the new moons belong to an outer group that orbit Jupiter in retrograde, meaning they travel in the opposite direction to the planet's spin. "So, the whole process took a year".
They also have a retrograde orbit, or the opposite direction to the spin of Jupiter on its axis. If a moon circles in the same direction as a planet's rotation, that moon's orbit is called prograde.
They're calling one moon an "oddball" because of its unusual orbit.
Astronomers peering into the depths of the solar system in search of a presumed ninth planet far beyond Pluto happened to be looking past Jupiter during their observations and happened to discovery 12 new moons orbiting the giant planet.
It took a year for their orbits to be confirmed with a series of other telescopes in the United States and Chile.
Astronomers have proposed the name "Valetudo" for the oddball moon, after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene. Sheppard thinks it may be all that's left of a larger moon that crashed into one or more of the retrograde moons sometime in the past. In fact, of Jupiter's 67 previously discovered moons, the 33 outermost moons all have retrograde orbits.
With the moon's orbit set at an angle to the rest, this means that Valetudo doesn't take the riskiest path around Jupiter, but it does dive through the orbits of the retrograde moons, inviting a collision at some point.
Sheppard said the oddball moon is "likely Jupiter's smallest known moon, being less than 1 kilometer in diameter".
Before Sheppard's team conducted their survey, there were 69 known Jovian moons, but there's always been reason to believe there are quite a few more. "Head-on collisions would quickly break apart and grind the objects down to dust".
But the discovery might be short-lived because Valetudo faces destruction in head-on collisions. "They also are fragments of the early solar system before the planets were formed, which makes studying them important to learning about the solar system's history". Mars has two, Earth has one and Mercury and Venus have none. At the same time, they watched for Planet Nine or smaller, distant dwarf planets in the background.
This was at a time when the Sun was still surrounded by a rotating disc of gas and dust from which the planets were born.
If these raw materials had still been present when Jupiter's first moons collided to form its current clusters, the drag exerted on the smaller ones would have made them spiral inwards. The researchers even wonder if the crashes are responsible for the swarms of smaller Jovian moons we see today.