All right, students, here's your quick quiz for the day. How many bodies in our solar system have rings?
If you answered “One!” you're undoubtedly referring to Saturn. Visitors to Rowan's public observatory open houses (such as the one coming up April 25) can sometimes glimpse the glorious rings of Saturn.
If you answered “Four!” you know that in the past 40 years we have discovered rings, much narrower and dimmer than Saturn's, circling the other three gas giants Jupiter, Uranus, and Neptune.
But now there is a fifth body. Amazingly, the new rings appear around an asteroid no more than 150 miles across!
Asteroid 10119 Chariklo is the largest of the asteroids called Centaurs, which are located beyond the orbit of Jupiter. Chariklo's path is in the space between Saturn and Uranus.
One of the things we like to know about an asteroid is its shape. Is it nearly spherical? Is it stretched out like a potato? (Many are.)
But the largest telescope in the world can't see a tiny body that far away as anything more than a faint dot. We must use indirect techniques to determine its characteristics.
Last summer, Chariklo was predicted to pass in front of a star called UCAC4 248-108672. By observing how much the star's light was dimmed, and for how long, astronomers could determine how wide the asteroid was. If that observation was carried out at several telescopes, we would see the star disappear behind different parts of the asteroid, and could get a rough idea of its shape.
Telescopes at seven different sites in South America measured the dip in the star's brightness. Fortunately, they started taking their measurements well before the event, and continued for some time afterwards, which is good observational technique. Later, upon inspecting each light curve (a plot of brightness against time,) they noticed two tiny unexpected dips before and after the main event.
The mini-dips were equally spaced before and after. Moreover, the time interval between the mini-dips varied depending on the location of each telescope, so that it was possible to figure out the shapes of the objects that produced them.
To the surprise of the astronomers, the best fit to the data strongly suggested the objects were narrow, dense, circular rings, like the rings of the much-larger gas giants, in orbit around the asteroid. No one had ever seen rings orbiting anything smaller than Neptune, which is four times the diameter of the Earth and 200 times that of Chariklo, but it's hard to devise a different explanation.
It was always thought that the gravitational force of such a small body would not be able to hold rings in place. Certainly none of the small planets, like our Earth, have rings.
But it leads to an interesting supposition. The rings of Saturn are kept in their gorgeous arrangements by nearby small moons, often called shepherd moons. Might there be one or more moons in orbit around Chariklo that do the same thing for it?
There is a lot to be learned about this system. What are the rings made of? How were they formed? Luckily, since phenomena in nature are seldom unique, it's likely we'll be seeing more rings, now that we know to look for them!
Information about the planetarium, including observatory open houses, planetarium shows, and the upcoming jazz concert “Fly Me to the Moons!” can be found at www.rowan.edu/planetarium/.