Galileo discovered the first four of Jupiter's moons, all huge, in 1610. The smallest moon is just over a half-mile across, while the largest is about three miles in diameter. Its powerful gravitational pull allows it to capture large passing objects that then collide with each other, forming dozens of new, smaller moons.
The astronomers were not intentionally searching for new Jovian moons when they began observing.
Not only that, but when the orbital characteristics (shape, tilt, and so on) are compared, these nine retrograde moons seem to fall into three groups; that implies that each group used to be a single moon that got smashed somehow, possibly a collision with another moon-sized body. He and his team have been photographing the skies with some of today's best telescope technology, hoping to catch sight of this mysterious ninth planet.
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.
The discovery means Jupiter, the oldest and largest planet in the solar system, has more moons than any of the other seven. The new moons are faint, so researchers haven't been able to spot features on their surfaces or clues to what they're made of. However, during the hunt, researchers caught glimpse of the moons back in March of 2017 but needed to confirm whether they were actually locked in orbit with Jupiter. But now put a auto moving in the wrong direction in that lane, and things will go downhill fast. Jupiter happened to be in the same field of view, so they also looked for any as yet unknown moons. Two of the newly discovered moons are in the inner circle of moons and orbit in prograde, or in the same direction as Jupiter's rotation. They are part of a larger swarm of moons orbiting a long distance out from Jupiter.
This, as you can imagine, has the potential to end poorly for our oddball friend and at least one of the other moons that are heading in the opposite direction.
This moon, now called Valetudo, moves in a prograde motion, though it is slightly inclined compared to the orbits of the other moons.
Due to their sizes - just 0.6 to 1.9 miles (1 to 3km) in diameter - these moons are more influenced by surrounding gas and dust.
The oddball moon, along with two other new moon discoveries, orbit in the prograde - or same direction as the planet s rotation. That's a lot of moons. They spotted the objects, but didn't immediately know if they were moons or just asteroids passing near the planet.
The oddball could be the last remaining remnant of a once-larger moon that gave rise to the retrograde retinue during previous smash-ups.
The so-called "oddball" has such a unique orbit that it is at risk of smashing into the other moons - a cosmic collision that could risk wiping the space rocks out.
Our solar system is already teaming with almost 200 moons, and we just got 12 more to add to the list.
Following this giant gas planet, Saturn's moon count is 61.
The last of the team's discoveries is the strangest of them all.
SCOTT SHEPPARD: So we could also search for Jupiter moons while looking for things that are well beyond Pluto. Sheppard's girlfriend came up with a name for it: Valetudo, the great-granddaughter of the Roman god Jupiter. But why hygiene? Sheppard says it comes from an inside joke with his girlfriend.