Famed astronomer Galileo Galilei discovered the first four moons of Jupiter way back in the early 1600s. More than 400 years later, astronomers are still finding moons orbiting the solar system’s largest planet. We’re not just talking about one or two stragglers, either. Astronomers from the Carnegie Institution for Science have spotted 12 new moons orbiting Jupiter, bringing the total to 79.
The Carnegie Institution team, led by Scott Sheppard, didn’t set out to find even a single new Jovian moon. The team’s goal was to scan the sky for evidence of a massive ninth planet in the outer solar system. Many scientists are on the same quest, believing that a ghostly gas giant out past the orbit of Neptune could explain a number of orbital quirks in other objects. Sheppard realized last year that the team’s search zone for a ninth planet overlapped with Jupiter’s orbit. So hey, why not look for some more moons?
Using the 4-meter Víctor Blanco Telescope in Chile, the team really hit the moon jackpot. This instrument has a large camera sensor, allowing Sheppard and his team to search the entire vicinity of Jupiter with just a handful of exposures. Just like that, Jupiter has 12 new moons., all of which have been confirmed by other telescopes.
Nine of the new moons are in a group of more distant moons in prograde orbits. That means the revolve around Jupiter opposite from the planet’s rotation. The other three are prograde, which orbit in the same direction. 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. A prograde satellite may simply have been floating through the solar system before being captured by Jupiter.
The team also discovered one particularly odd moon in the new batch. The moon Valetudo is in the retrograde cluster, but it orbits in a prograde direction and crosses the orbits of several other satellites. Sheppard’s team speculates Valetudo could be a remnant of a collision between one or more moons. Its small size backs that up. It’s only about one kilometer in diameter, by far the smallest of the newly discovered moons.
Some have wondered if perhaps that shouldn’t count as a moon. We already have a classification for dwarf planets. Maybe we need something similar for tiny moons. This question might come up more in the future, too. Sheppard expects there could be even more small moons lurking out there.
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