Saturn has taken over from Jupiter as host to the most moons in the solar system after astronomers spotted 20 more lumps of rock orbiting the ringed planet. It brings the number of Saturnian moons to 82, surpassing the 79 that are known to orbit Jupiter, its larger, inner neighbour. “It’s exciting to find them,” said Scott Sheppard, an astronomer who led the work at the Carnegie Institution for Science in Washington DC. “These moons are very far away from the planet.” Each is about three miles across.
The scientists discovered the moons when they set algorithms to work on decade-old images captured from the powerful Subaru telescope on Mauna Kea in Hawaii. By comparing images taken over hours and days, the algorithms distinguished between stationary stars and galaxies and moons that hurtled around the planet. At the birth of the solar system, vast amounts of dust and gas circling the sun coalesced into the eight known planets. But Sheppard believes that shortly after Saturn formed, more than 4bn years ago, passing asteroids and comets were captured by the planet’s gravity and have circled it ever since. Depending on the angle of approach, comets and asteroids straying too close to Saturn in the early solar system would have become locked into radically different orbits around the planet. Only three of the new moons have so-called prograde orbits, meaning they circle Saturn in the same direction that it rotates. The other 17 are in retrograde orbits, meaning they orbit the planet backwards. One is the most distant moon ever spotted from the planet.
The outer moons of Saturn fall into three broad families according to how they orbit the gas giant. Two of the new prograde moons appear to belong to a group that swings around Saturn at an angle of about 46 degrees. The moons, named after Inuit mythology, may once have belonged to one far larger moon that broke apart in the distant past. The new retrograde moons appear to belong to another group named after Norse mythology and are also thought to be fragments of a much bigger parent moon that was smashed to pieces in the solar system’s violent past. “This kind of grouping of outer moons is also seen around Jupiter, indicating violent collisions occurred between moons in the Saturnian system or with outside objects such as passing asteroids or comets,” Sheppard said.
The discovery was announced on Monday by the International Astronomical Union’s Minor Planet Center.
Credit: Ian Sample for The Guardian, 7 October 2019.
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