Sunday, 30 March 2014

Two Rings Discovered Around the Centaur Chariklo

The tiny world of Chariklo and its two mysterious, icy rings.
Last Wednesday was an interesting day for astronomy, as the discovery of not one, but two strange new worlds right here in the Solar System were announced. A new dwarf planet, a tiny world orbiting the Sun far beyond Pluto, seems to have got most of the headlines. But I'm more interested in the other discovery, published here in Nature.

Most of the asteroids and other smaller worlds in the Solar System tend to be found in two areas: The Asteroid Belt  in between Mars and Jupiter, and the icy Kuiper Belt (of which Pluto is a member) out beyond the orbit of Neptune. The space in between is far from empty however.

In the late 1970s astronomers started noticing small objects orbiting in amongst the giant outer planets. Called Centaurs, these small rocks raised a lot of questions. Their orbits are inherently unstable, crossing over the orbits of  the huge gas giants. It's only a matter of time before they run smack into Jupiter, Saturn, Uranus or Neptune, colliding with them or being thrown out of the Solar System. This means they can't have been there long, so where did they come from?   

We also don't know what they really are. Are they comets, thrown in from the Oort Cloud in the depths of space? Maybe they're more like asteroids, scattered out of the Kuiper Belt by Neptune's gravity? Perhaps a mix of both? Whatever they are, they don't fit a neat explanation, and that makes them interesting.

In June 2013 several telescopes in South America were trained on the largest Centaur, a flattened sphere of rock 127 kilometers across called Chariklo. They were watching for a event called a stellar occultation, waiting for Chariklo to pass in between Earth and a background star. By looking at the way the Centaur blocked out the light coming from the star, the astronomers behind the telescope were hoping to find out things like what shape Chariklo is and how it rotates.

Right on time, the star disappeared as Chariklo moved across it, reappearing six seconds later as the Centaur moved on.

But just before it disappeared , the star dimmed very slightly for a split second, and then nearly vanished completely for a second more. As is came out from behind Chariklo it did it again, but this time in the opposite order: Flashing off, then dimming slightly.

The occultation of a background star by Chariklo, reveling the rings.
This image is one of my new favorite pictures. It shows the actual data from one of the telescopes. Time is along the bottom, going from left to right. Going up the graph shows how bright the background star was, going from 0 to 1, where zero means no light and one is the normal amount of light coming from the star. 

Going across you can clearly see the light dim a bit, dim a bit more, then go away completely as the star passes behind Chariklo. Then it comes back again and does the opposite.

This means that there must be something else blocking out the star for a moment as it gets close to Chariklo, causing two perfectly symmetrical dips in the star's light, either side of the Centaur. Only one thing would do that: Chariklo must have a pair of rings. 

Up until now we've only found four objects with rings: Jupiter, Uranus, Neptune and the stunning band of ice and dust around Saturn. These planets have one main thing in common: They're huge, many times the size of the Earth. That the fifth known ring system would belong to a rock smaller than Wales (to use a standard unit of measurement) was completely unexpected. 

The reason I like the plot so much is that is so clearly shows what's actually there: A big thing in the middle with two rings around it. Most plots need a long time and lots of extra information to work out what they show, but here it's nice and clear. 

The rings around such a small object raise a lot of questions. For a start, look at how much of the star's light the inner ring blocks. Over three-quarters doesn't get through. These rings aren't wispy clouds of dust, they're bits of ice and rock just as thick as Saturn's rings. How did rings like that form?

Chariklo's gravity is so small that is shouldn't be able to hold on to the rings for very long at all. As the Sun warms up the particles they radiate the heat away in all directions. But the Sun's heat is only coming from one direction, an imbalance that over time will give them a little kick, enough to escape Chariklo's tiny gravity. Either the rings are very young and won't be there much longer, or Chariklo has moons (that we haven't spotted) holding the rings in place. 

The discoverers suggest a few ideas about how the rings may have formed. Maybe something bumped into Chariklo, throwing up material off its surface but not so fast as to give it enough force to fly off entirely. Alternatively Chariklo may be slightly active, blowing stuff off itself in weak volcanic eruptions. Perhaps it could once have had two moons, which collided to form the ring.

We may never know the answers.  But even if the rest of Chariklo's secrets remain hidden, its intriguing and mysterious rings show us just how much we have to learn about space, even in our own back yard.

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A more traditional set of rings around Saturn. You're in this picture: Small dot, bottom right.


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