Jupiter already had the most moons in the Solar System, but now scientists have discovered twelve new ones bringing the total up to 79. But they realized in March previous year that Jupiter would pass through the part of the night sky they wanted to search.
But why are scientists just now finding these moons?
Over the weeks following full opposition, Jupiter will reach its highest point in the sky four minutes earlier each night, appearing as a bright, star-like object.
Sheppard adds that the moons may be leftovers from early solar system objects.
For example, the discovery that the smallest moons in Jupiter's various orbital groups are still abundant suggests the collisions that created them occurred after the era of planet formation, when the Sun was still surrounded by a rotating disk of gas and dust from which the planets were born.
"This is an unstable situation", Scott Sheppard of Carnegie says in a statement. There is one other moon orbiting Jupiter that has the same rotation, but Valetudo orbits further away from the planet so its unorthodox movement is more baffling. According to their research, each moon takes about two years to circle the gas giant. They are thought to be the remnants of larger parent bodies that were broken apart in collisions with asteroids, comets and other moons. Astronomers believe that prograde moons form around a planet from the same cloud of dust and gas, which is why they tend to have closer orbits and matching rotation.
Jupiter's biggest moon, Ganymede, also has an underground salt ocean - although this ocean may be too salty to be habitable. But astronomers can't wait a year to make the follow-up observations, because the objects become lost or hard to properly associate with the same object, so it's necessary to track the candidate moons multiple times over the course of a year. The oddball also orbits in prograde, but at a greater distance than the other prograde moons. They named it Valetudo, after a daughter of Jupiter and the Roman goddess of hygiene and personal health. It is more distant (for good reason!), more inclined, takes longer to orbit Jupiter than its siblings, and crosses the outer retrograde moons.
It has most likely collided with other moons, breaking it down into the fragment it is today.
Sheppard and his colleagues speculate that Valetudo was probably once much larger, but was ground down, over the course of billions of years, as a result of collisions.
Diagram of the new moons split into different groups. The planet must have acted like a vacuum, sucking up all the material that was around it.
The moons had not been spotted before because they are tiny.
They did not find Planet Nine. They could be rock, ice or a mixture.
Even more, Valetudo is very weird, according to the scientists.
So why is this confirmation so hard? At the same time, they watched for Planet Nine or smaller, distant dwarf planets in the background.
"Jupiter is like a big vacuum cleaner because it's so massive", Sheppard said.
"Jupiter was well-placed in the sky to kill two birds with one stone", Sheppard said.