Planets With Rings: Can You Name All Four?
Rings and Ring-Shadows
Smoke Rings and Ice Rings
Ever since early astronomer Galileo Galilei used his newly-invented telescope and gazed at the planet Saturn in 1610, we've known that some planets have rings.
In the last thirty years, astronomers have used telescopes and space probes to discover rings around three more planets. Can you name them? In fact, there may be five ringed planets in our solar system, but one of them is not like the others.
Let's take a photo-tour of our ringed neighbors, starting with the most famous and the most beautiful: Saturn, the sixth planet from our Sun.
17th-Century Astronomers Solve the Mystery of Saturn's Rings
Saturn Seen from Earth
Saturn, "Lord of the Rings"
Saturn is often nicknamed "The Lord of the Rings," after J.R.R. Tolkien's famous book.
In small telescopes or high-powered binoculars, it's hard to pick up any details. Saturn's rings confused Galileo: he kept tweaking his telescope trying to make out the "ears" or "handles." At first, he thought they were moons like the ones he had discovered around Jupiter, but they were too large, and did not move in the same way. He was even more mystified when they vanished a few years later, then returned.
Astronomers devised various explanations for the changing shape of Saturn's "handles." Finally, Christiaan Huygens, who had also discovered Saturn's moon Titan, proposed that the "handles" were a disc that did not touch the planet. Advances in mathematics and telescopes over the next two centuries determined that the disc was not solid, but a belt of particles.
Thanks to the Voyager and Cassini space probes, we now know that Saturn's rings are marvelously textured with gaps, lanes, ripples, even spokes caused by small moons tugging the particles out of their disc-shape. In a way, Saturn's ring and moon system is a miniature analog for the dust and gas disc out of which our solar system form.
My favorite ring is Saturn's "F Ring." When Voyager 2 sent back photos of it in 1981, scientists were very puzzled by its kinked, uneven path. The Cassini spacecraft solved this problem in the mid-2000s, as this video from NASA's website demonstrates:
Saturn's F-Ring: Why Is It So Kinky?
Jupiter's Rings: Infrared
Jupiter Blows Smoke Rings
Jupiter, the fifth planet from the Sun, is by far and away the largest planet, but its rings are the most delicate. They were discovered by the Voyager 1 spacecraft in 1979.
While Saturn's rings are composed of a flurry of ice chunks from pebble to trailer-sized, rushing around the planet in a 30-foot-thick disc that in many places is practically opaque, Jupiter's incredibly thin, dark rings are made of tiny dust particles the size of molecules in cigarette smoke.
This means they are only visible when the sun is directly behind Jupiter, backlighting them. Even then, its two "gossamer rings," external to the main ring, are usually invisible. (See diagram on NASA site).
Jupiter's wispy rings only exist in the vicinity of some of its small, rocky inner moons: Adrastea, Metis, Amalthea and Thebe. Scientists have recently confirmed they are formed by dust blasted off these moons by microscopic meteorite impacts (small, because heavier debris would fall to the moon's surface). In fact, the rings around all the outer planets except Saturn appear to have formed like Jupiter's, from moon-dust.
Jupiter's Main Ring Backlit By the Sun
Uranus' Rings Seen By Voyager
Bangles of Uranus
The "Ice Giant" Uranus took a beating: its small, ice-rock core was apparently smacked so hard by a planetary collison (or two) early in its history that it now spins on its side. Around the core is a frozen ocean, then an outer atmosphere of gas (see diagram of the cores of the gas giants).
Uranus is about four times the size of Earth.
In 1977, two astronomy teams flew telescopes on airplanes to get above most of the Earth's atmosphere, with the goal of trying to measure Uranus' atmosphere as a star passed behind it. To their surprise, they saw the star flicker several times before passing behind the planet, then flicker in the same pattern after emerging on the other side. The teams correctly realized that Uranus, like Saturn, must have rings. (Source)
Uranus' rings are dust rings like Jupiter's, probably blown off the planet's many moons. 13 have been discovered so far. The outermost one, recently discovered by Hubble, is actually blue! (Here's a faint image of it from the Keck Telescope).
Once Uranus' rings revealed that Saturn was not the only ringed planet, the race was on to check the other gas giants. Rings were later found around Jupiter (by the Voyager spacecraft) and Neptune (likewise).
Rings of Uranus: The Planet on Its Side
The Arcs of Neptune
Using the same technique that Earth-based astronomers had used to discover the rings of Uranus, they turned telescopes towards Neptune.
Sure enough, when stars passed behind the planet, they flickered going in or going out. There was just one problem: the pattern of starlight-dimming was never a symmetrical pattern, as if Neptune's rings were in completely different positions on opposite sides of the planet. Baffled astronomers began to wonder if the rings only went partway around the planet!
Voyager 2 finally solved this enigma when it reached Neptune in 1989. Straining to see in the sun's dim light and Neptune's glare, Voyager found that the rings had thicker segments in short discontinuous arcs, shaped by nearby moons. The rest of the rings were so thin that they were practically transparent, allowing starlight to pass through without a flicker.
A Photo Gallery of Ringed PlanetsClick thumbnail to view full-size
The Universe is a good science/astronomy TV series from the History Channel from 2007 through 2011. The best episodes are seasons 1 to 3, but "The Hunt for Ringed Planets" in season 4 is a fascinating episode with beautiful CGI imagery of the ring systems of every ringed planet based on data from all our space probes. Prominent astronomers tell their story in easy-to-understand language with lots of fun examples.
Are There Rings Around Any Other Planets?
Our best telescopes are now discovering many hundreds of exoplanets, planets around other stars. (As of June 2012, that number is up to 786, illustrated by this xkcd cartoon). Most of the planets discovered are gas giants, since they are so large that they are easier to detect from afar.
Can we see rings around any of them? Not yet. Detecting rings tests the limits of our best exoplanet-hunter, the incredible telescope Kepler. If most rings are dust rings like Jupiter's, they may be too faint to cause their star's light to flicker enough for us to detect them. But sooner or later, I think we will see the telltale flicker of another Saturn-like world.
Closer to home, we have three more possible candidates for ringed planets, or at least dwarf planets.
Someday, Mars may tear apart its moon Phobos, which is currently spiraling in towards the planet. In fact, astronomers have been searching the area around Mars for faint dust rings blown off by its two moons, but so far nothing has turned up.
Earth actually has an artificial ring created by our geostationary communications satellites and a messy halo of other satellites and space junk: tons and tons of it! That ring will be the first visible sign to any visitors that our planet is inhabited.
Finally, some astronomers are guessing that the moons of Pluto may create faint rings. We'll know in 2015 when the New Horizons spacecraft reaches it.
Chariklo, the Amazing Ringed Asteroid
UPDATE: There Is a Ring Around Your Asteroid!
Crassness aside, I am over the moon about this news: astronomers have discovered an asteroid with rings.
Asteroids are small, right? They can be the size of California or England. A few hundred miles across, or even smaller. Nobody expected them to have rings.
However, in March 2014, the European Southern Observatory (ESO) announced that they had found one, purely by accident.
They were observing the asteroid Chariklo as it passed in front of a star. To their surprise, the star flickered: once, twice just before disappearing behind the asteroid, and once, twice as it emerged from the far side. That can only mean one thing: the star's light was dimmed as it passed behind a pair of rings!
We can't see Chariklo's rings. In fact, even in powerful telescopes, this 250 km (160 mi) asteroid looks like a dim, fuzzy dot.
Scientists are now studying the data to try to understand how Chariklo can have rings. I find the idea of Chariko fascinating, especially because one theory suggests it may have tiny shepherd moons keeping the rings in place the way Saturn does.
If this news weren't fascinating enough, Chariklo is not a normal asteroid. It's a"Centaur," a mini-world that's halfway between a comet and an asteroid, orbiting out between Neptune and Pluto.
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