Why do some planets have rings and others don't?

Why do some planets have rings?

For centuries, the strange rings around Saturn have presented astronomers with a major puzzle. Galileo’s telescope was too weak to give him a clear view. He surmised first that Saturn had a bulge on each side, and then that he was looking at three planets in one. A few years later, in 1614, German astronomer Christoph Scheiner thought that the bulges were in fact two crescents: Saturn was a planet with handles! Forty years on, Christiaan Huygens, the Dutch mathematician and physicist who invented the pendulum clock, studied the planet with a more powerful telescope, and said that Saturn was surrounded by a flat ring, which encircled the planet at its equator. In 1665, French-Italian astronomer Gian Domenico Cassini observed that there were two rings, one inside another.

Saturn’s rings shine more brightly than the planet itself, and they are gigantic, stretching some 272 000 km (169 000 miles), more than twenty times the diameter of the Earth. Since Cassini’s discovery known still as the Cassini division other astronomers have puzzled over what makes up Saturn’s rings and those surrounding other giant and distant planets, Jupiter, Uranus and Neptune.

All these planets, because of their size, have tremendous gravitational force. In the 1850s, French astronomer Edouard Roche considered what would happen if the Moon were closer to Earth and therefore under a much stronger gravitational pull. He reasoned that what is known as the ‘tidal effect’ of the Earth on the Moon which tends almost to lift its surface, as the Moon’s gravity moves our oceans would impose such a strain that the Moon would break into pieces. Applying that reasoning to the rings around Saturn and other planets, Roche deduced that they could not be solid sheets of matter.

What causes rings around planets?

Since then, Voyager probes have sent back a mass of data about the rings surrounding the distant gas giants, but nobody has yet explained convincingly why the rings were formed. One favoured theory takes up Roche’s point that rings form when a moon, or perhaps an icy comet, comes too close to a large planet and gets torn apart by gravity. As pieces break away, they go into independent orbits around the planet, and through constant collisions eventually become clouds of fine dust and gas.

To support this moon crunching theory, astronomers assert that only the largest planets, with the strongest gravity, have rings. The rings differ in puzzling ways some are extremely thin and dark, and reflect no light but do not exist around the smaller inner planets, Mercury, Venus, Earth and Mars.

A counter argument is that the rings formed in the earliest days of the solar system from the same material that made up the planets. For a time, the large planets may have resembled a flattened disk of gas and dust. Gravity pulled the more solid material into a smaller space, condensing it eventually into a planet. The remaining dust and gas tightened into the ring systems that we see today.