Rings of the Solar System

Rings of the Solar System
The rings of Jupiter [Credit: NASA JPL / Cornell University]

Saturn and its splendid rings are one of the most exquisite sights in the Solar System. Although the planet had been known for thousands of years, the first sight of the rings wasn't until 1610. Galileo viewed Saturn through his small telescope, but didn't know what he was seeing. Saturn looked as though it had a big moon on either side of it.

Fifty years on, Christiaan Huygens suggested that the odd shape was due to a solid ring surrounding Saturn. It took a long time and good telescopes to confirm that there were rings, but they weren't solid. The Cassini spacecraft found over thirty rings, though there are fewer than ten major ones. The rings are mostly made of icy material. That's what makes them so reflective.

Although Saturn has the prettiest rings, it seems that rings are not uncommon.

No telescope in 1977 could have shown you rings around Uranus. The rings were discovered by astronomers observing a stellar occultation. This is an event that occurs when a star is hidden [occulted] by another object passing between it and the observer. If a planet covers the star as it passes, the timing can provide useful information about the planet.

A team of astronomers studying Uranus saw that the star had dimmed several times both before and after Uranus hid it. They had found rings, something they were definitely not expecting.

The first actual sight of the rings came via the Voyager 2 spacecraft in 1986. In addition to the nine known rings, it discovered two new ones. In 2003-2005, the Hubble Space Telescope found two more for a total of 13. Astronomers think the rings are made of debris left by collisions of Uranus's moons.

Despite finding that Uranus had rings, astronomers weren't expecting Jupiter to have any. After all, Jupiter is notably closer than Saturn, but no one had ever seen any sign of rings. Yet in 1979 that's what a Voyager 1 flyby showed.

It's nearly impossible to see Jupiter's rings from Earth. They're thin and dusty and hard to see even up close. Most of what we learned about the rings came from the Galileo orbiter in the 1990s. The data showed that they were probably created from the dust related by meteor impacts on Jovian moons.

Distant Neptune was the last of the giant planets found to have rings. The first hint of them occurred in 1984 at La Silla Observatory in Chile when astronomers found ring arcs around Neptune. This suggested rings, but it wasn't conclusive until Voyager 2 made its flyby in 1989. Neptune has six rings, and they're dark like the rings of Uranus. However they're fainter and thinner.

Mars doesn't have a ring that you haven't heard about. Yet it may well have had a ring in the distant past, and will have another in the distant future. Here's the story that's convinced some planetary scientists.

We know that in the distant past, Mars was hit by a substantial impactor, possibly the one that created the North Polar Basin. Some researchers think this impact initiated a cycle of events in which the material thrown into orbit by the impact formed a ring. Then, after a time, the material clumped together to make a moon. If the moon's orbit then decayed, as is happening now to Phobos, Martian gravity would break it up into a ring again.

What made the Moon? The current favored theory is that 4.5 billion years ago a Mars-sized body crashed into the young Earth. A massive amount of debris from both the Earth and the impactor was thrown into orbit. For a time Earth would have had a ring. The orbiting material then clumped together to make the Moon. Unlike that of little Phobos, the Moon's orbit isn't decaying. In fact, our Moon is very slowly moving into a higher orbit.

10199 Chariklo
Rings aren't just for planets. The asteroid Chariklo, which orbits between Saturn and Uranus, has two rings. They were a surprise finding during an occultation in 2014. They probably formed from a debris disc following a collision. The shape of the ring pair suggests that there may also be a small moon amid the debris.

Haumea, officially classified as a dwarf planet in 2008, is the first trans-Neptunian object known to have a ring. The ring is around the dwarf planet's equator. How did it form? How does it stay in a stable orbit around such a small body? Hard questions to answer. Haumea is out beyond Pluto, and the ring is so faint we can't actually see it. It was discovered during a stellar occultation.

You Should Also Read:
Cassini-Huygens - the Prime Mission
Voyager 2- the Grand Tour
Haumea - the Ringed Dwarf Planet

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