"This is an unstable situation", Sheppard said.
Elucidating the complex influences that shaped a moon's orbital history can teach scientists about our Solar System's early years. Instead, scientists will have to wait for a future spacecraft, either flying past Jupiter or orbiting it.
The researchers were searching for the proposed "Planet X" or "Planet 9", which astronomers believe exists and could account for the way distant objects orbit in a similar manner.
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.
Jupiter's moon Valetudo (pointed out with orange bars) moves relative to background stars in these images taken with the Magellan Telescopes at the Las Campanas Observatory. If moon circles a planet in the opposite direction of a rotating planet, that orbit is retrograde.
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These new moons aren't exactly record-breakers in terms of size. The orbits of 9 other small Jovian moons are yet unknown. Seven of them travel in remote orbits more than 20 million kilometers away from Jupiter, and in the opposite direction from the planet's rotation. In 2017, the group reported two additional Jovian moons. They then used telescopes in Chile, Arizona, and Hawaii to confirm the existence of the moons and their orbits around Jupiter, a process that required many follow-up observations over the past year. If the collisions had happened earlier, the moons would likely have interacted with dust and gas leftover from forming Jupiter and been dragged into the planet.
The tiny moon is likely to be named Valetudo after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.
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.
The team think this small "oddball" prograde moon could be the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings during past head-on collisions.
Two previously undetected moons were found closer in, part of a group that orbits in the same "prograde" direction as Jupiter's rotation. And astronomers have just announced the discovery of a dozen more.
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Scientists classified the findings as 11 "normal" outer moons, and another that they are calling an "oddball" for its unusual orbit.
Most moons, including Earth's, have prograde orbits. "By looking at these outer moons", he said, "we can get an insight into what the objects were like that ended up forming the planets we see today".
"This simulation takes a few months to run and we expect the answer is between about 100 million years and 1 billion years, which is long in human time but not all that long in astronomical time", Sheppard said. "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. We already have a classification for dwarf planets.
This screencapture shows the prograde moons of Jupiter, including the two newest discoveries. So they were likely formed after they had dissipated.
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