The orbits of the 9 newly discovered retrograde moons of Jupiter are shown here.
All of these new moons are very small, only about one to three kilometers across (a kilometer is 0.6 miles). Two of the newly discovered moons were found among these prograde moons, and take a little less than a year to go around in their orbit once.
This means Jupiter has at least 79 moons, the highest number of any planet in our solar system.
The twelfth moon has already earned a proposed name, despite being only about two-thirds of a mile in diameter. The outer moons are clustered in at least three groups based on their distances from Jupiter and the angles of their orbits, and astronomers think these moons are fragments of three larger objects that were captured by Jupiter's gravity and later broken up by collisions - though whether that was with passing comets, rogue asteroids, or other moons is unclear.
They take about two years to orbit Jupiter, which is the largest planet in the solar system.
The 2 new regular prograde moons join 15 other previously discovered prograde satellites that typically orbit Jupiter in about an Earth year or less.
Astronomers Find a Staggering 12 New Moons Orbiting Jupiter
Due to their sizes-one to three kilometers-these moons are more influenced by surrounding gas and dust.
A moon is defined as any object, regardless of size, that orbits a planet, not the Sun.
As a result, head-on collisions are much more likely to occur between the "oddball" prograde and the retrograde moons, which are moving in opposite directions. Sheppard says "Valetudo is like driving down the highway on the wrong side of the road". "Head-on collisions would quickly break apart and grind the objects down to dust". Ganymede, Jupiter's largest moon and the largest in the Solar System, the diameter is approximately 5,300 kilometers (about 3,300 miles). That moon, named Valetudo after Roman goddess of health and hygiene, is also in the outer swarm but moves in the opposite direction of its peers.
The discovery, which was confirmed by observations made with several other telescopes, delighted other astronomers."I love this result", Jackie Faherty, an astronomer with the American Museum of Natural History in NY who was not part of the team, said in an email. As the team noted in its announcement, "Elucidating the complex influences that shaped a moon's orbital history can teach scientists about our Solar System's early years".
During its full opposition the planet was brightest in Britain on May between 9:30pm and 4:30am BST while in the United States it peaked on May 9 between 1:10am and 6:20am ET.
They would be able to tell the difference between Jupiter and the objects around it versus the distant solar system objects because any objects around Jupiter would be moving at the same rate as the gas giant.
"Jupiter was well-placed in the sky to kill two birds with one stone", Sheppard said. Scientists were using the camera to look for a large but elusive planet beyond Pluto, dubbed Planet X or Planet Nine.
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