Two of the newly discovered moons orbit much closer to Jupiter and have a prograde orbit, meaning that they orbit in the same direction as the planet. 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. The newly discovered moons' relatively small size - between 0.6 to 1.8 miles across - is probably why scientists did not know about them before. Some slipped in and out of view, complicating the task. "Collisions don't happen all that frequently, every billion years or so", said Sheppard.
In 2014, this same team found the object with the most-distant known orbit in our Solar System and was the first to realize that an unknown massive planet at the fringes of our Solar System, far beyond Pluto, could explain the similarity of the orbits of several small extremely distant objects.
Nine of the found moons are part of the distant retrograde group that orbit in the opposite direction of Jupiter's spin rotation, according to Carnegie Institution for Science.
Two new moons are closer in, go the right way, and take about an Earth year for one orbit.
"Our other discovery is a real oddball and has an orbit like no other known Jovian moon", Sheppard said. The name Valetudo has been proposed for it, after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.
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The curious find might shed light on how many of Jupiter's current moons were formed.
Because Valetudo's orbit crosses the orbits of some of the outer retrograde moons, it's possible that it suffered a head-on collision in the past.
"It's allowed us to cover the whole area around Jupiter in a few shots, unlike before, and we're able to go fainter than people have been able to go before", says Sheppard.
The researchers, from the Carnegie Institution for Science in Washington, DC, picked out one of the 12 moons as an "oddball".
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"Finding 12 is pretty surprising", said Scott S. Sheppard, an astronomer from the Carnegie Institution for Science, who led a team of researchers in the discovery.
Depending on what survives from any such collision, Jupiter may then have even more moons. The researchers even wonder if the crashes are responsible for the swarms of smaller Jovian moons we see today. If a moon circles in the same direction as a planet's rotation, that moon's orbit is called prograde.
"Valetudo is like driving down the highway on the wrong side of the road", said Sheppard. Nine others were detected as part of the outer retrograde moons.
Which raises the question of how long the tiny moon has left.
Valetudo is something of an oddball.
The survey looked for objects one kilometre and larger, so there is a chance that there are other, smaller "moonlets" in orbit around the giant Jupiter. The irregular satellites didn't form around Jupiter in the same way that the planets formed around the Sun, or the regular satellites around Jupiter, from a flat disc in a prograde orbit. The planet must have acted like a vacuum, sucking up all the material that was around it.Some of that debris was captured as moons.
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