“If you look at the four bright moons of Jupiter, the Galilean satellites, they are called regular moons, and all the giant planets have regular moons,” says Brett Gladman, a Canadian astronomer at the University of British Columbia and one of Ashton’s colleagues. Share the latest discoveries about Saturn. “And their moons, orbiting in the equatorial plane of the planet, just like the rings around the planet. They are thought to have formed in orbit, a flat disk of gas and dust that formed around the giant planets… in the same way our planets formed in orbit around the sun.”
The accepted wisdom, Gladman says, is that if moons form from planets close to them, they will stay close and orbit around their equatorial planes — just as planetary rings do.
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But it turns out that some moons don’t follow this rule book. Planets also have irregular satellites whose orbits do not follow a predictable path around the host planet’s equatorial plane. Their orbits are more elliptical and steeper, and they range farther from the planet, often in a different direction than the planet’s path around the sun. Many of them are also smaller.
For decades, lunar investigators on Earth had to use photographic plates to try to find evidence of the solar system’s moons. The smaller they are and the more irregular their orbit, the more difficult they are to observe. But in the 1990s and 2000s, digital photography suddenly changed the way people like Gladman could locate themselves. Charge-coupled device (CCD) sensors in digital cameras were much more sensitive to light than imaging plates or emulsions, which meant that much fainter objects could be detected.
However, a new problem arose. CCD sensors were small, so the field of view they could capture was very limited. “The giant planets are very big, and the area around them where you can orbit the planets — instead of orbiting the sun — is very large,” Gladman says. And when seen in the sky, it was large enough that no one could catch it with CCD cameras.
“In 1997, I discovered the first two moons near Uranus with a camera and it was difficult, but it worked,” Gladman says. He adds that the planet’s relative distance from Earth made for a relatively limited field of view for research.