Twelve, to be exact.
A team led by Scott S. Sheppard from Washington DC's Carnegie Institution for Science first spotted the moons in 2017 while on the hunt for a possible massive planet beyond Pluto. This gave the team a unique opportunity to search for new moons around Jupiter in addition to objects located past Pluto, according to the statement.
They were using a more powerful telescope than ever before, allowing the team to peer in at higher resolutions, across a wider field of view than other observations in the past. They are about one to two kilometres (miles) across, said astronomer Gareth Williams of the International Astronomical Union's Minor Planet Center. Observations were partly obtained at CTIO, NOAO, which are operated by the Association of Universities for Research in Astronomy, under contract with the NSF.
Europa, one of Jupiter's larger moons, casts a shadow on the planet's surface. The moon can be seen moving relative to the steady state background of distant stars. Jupiter is not in the frame and is off to the upper left. As for the number of satellites that orbit Jupiter, Williams said he dreads "to think how many objects are in the hundred-meter range".
"I like the serendipity of it as well - it also highlights how you can use observations looking for one thing to also look for others".
Jupiter has several different types of moons. These regular satellites consist of an inner group of four moons that orbit very closely to the planet and a main group of four Galilean moons that are Jupiter's largest moons.
An illustration showing how the orbits of the newly discovered moons (bold) fit into the known orbital groupings of the Jovian moons (not bold).
Most moons, including Earth's, have prograde orbits.
Finally, there are retrograde moons. They are prograde moons, meaning that they orbit in the same direction as Jupiter's rotation. Seven fall in a farther out "retrograde" cluster (red) rotating against the planet's spin, their opposing path kickstarted when ancient moons collided with comets, asteroids, or other moons. The "oddball" with the proposed name Valetudo orbits in the prograde, but crosses the orbits of the planet's outer retrograde moons.
With current technology, as well as the next generation of telescopes that will have even more capabilities, Sheppard believes that they could definitively say whether Planet Nine exists in the next few years. That means a head-on collision could occur. The astronomers are now running computer simulations to determine how the ancient moons fragmented.
"Over a human lifetime, it doesn't happen". By contrast, retrograde moons were probably objects that once were wandering around the solar system and got snared by Jupiter's gravity.
Interestingly, the astronomers believe that this is what happened in the past. "These objects are probably some intermediate-type composition, half-rock and half-ice, something like that".
The findings are another piece of the puzzle of the formation of our solar system.
The moons are remnants of what was out there, born in the disc of gas and dust around Jupiter after the planet formed and then captured and pulled into Jupiter's orbit. "These objects started orbiting Jupiter, instead of falling into it".
"This moon is going down the highway the wrong way", Sheppard said.