The Smallest Ringbearers

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In the last decade, we have discovered three small bodies in our solar system that have rings:  Chariklo, 2060 Charon, and Haumea. Chariklo and 2060 Charon are smaller than the state of Maine. Haumea, pictured above, is about the size of Alaska. Before rings were discovered about these bodies, we thought rings could only form around the giant gas planets, like Saturn and Jupiter. Discovering the rings about these small bodies has forced us to revise what we believe it takes for a planet or to accumulate a ring. What can these discoveries tell us about how dark matter has accumulated around our terrestrial planet, Earth?

Although Haumea is the largest of the three bodies, its rings were not discovered first. Chariklo’s rings were discovered first, in 2013. Chariklo is a centaur, so named because it is half comet, half asteroid. It has two rings, with the proposed names Oiapoque and Chuí. The following is an artist’s conception of Chariklo and its rings.

These rings were discovered using the transit method. When Chariklo was known to pass in front of a star, it was observed by a dozen observatories, each measuring how the star dimmed.

In the plot above, you can see how the star dimmed as Chariklo passed in front of it. You can also see the dimmings that indicate a ring around Chariklo, in fact two rings! The dimming is show as video below.

The transit technique is used in other ways to detect and measure other bodies. When I was an undergrad at UT Austin, I had a friend in the astronomy department who was using a stellar transit to measure the width of a crater on the Moon.

Transit methods are also used to detect exoplanets. The dimming of a star is used to infer that a planet passed in front of it. In fact the level of dimming is used to infer the size of the planet. The more dim the star becomes, the larger the planet.

Transit methods are ironic to me: to detect one thing, another must disappear! It’s almost like the universe is intentionally playing with object permanence, that knowledge we develop as babies to know that if something leaves our view, it still exists. If our parents walk out of the room, they don’t disappear forever. But how our consciounsness grows past that.

The serendipitous discovery of Chariklo’s rings shows us that small bodies can host a ring. Moreover—that ring can be formed from captured material. The process of gravitational capture is basically being at the wrong place at the wrong time, also at the wrong speed. Capture is a passive process in the cosmic dance. When captured, a body, in this case a dust particle, comes near then becomes gravitationally bound to a new larger body, in this case, Chariklo. In orbital mechanics we use the process of capture actively to position spacecraft. Capture has notable natural examples as well outside of ring formation. Consider the moons of Mars. They show us that it’s not necessary for moons to form with their planet. I have another blog about capture in development.

Rings around such small bodies—asteroids, centaurs, moons—show us tthat rings form by capture. Moreover, we are now informed that when it comes to forming a ring, it’s not size so much as shape that is important.

Saturn, a gas giant, has rings so enormous that you can see them with a backyard telescope. The other gas giants have rings as well, but we did not know that until we sent probes to the outer solar system. With your backyard telescope, you can clearly see Jupiter and its moons, but not its rings. So, clearly, size is not everything when it comes to forming rings. So why does Saturn have such distinct rings, when Jupiter does not?

The key is shape. Saturn is more flattened that Jupiter. In fact, Saturn is the most flattened planet in our solar system. Its equatorial radius is 10% wider than its polar radius! When we describe this feature, we use the term oblate from geometry. The opposite is prolate. If you need a mnemonic to distinguish the two terms, just remember Bert and Ernie from Sesame Street. Bert has a prolate head, and Ernie’s is oblate.

We see that the shapes of these bodies are not pure spheres. One is oblate, and the other, Haumea, is prolate! But the latter spins about its minor (smaller) axis creating what I playfully would refer to as a virtually oblate body. From what we know about dynamic systems in the free-fall of space, this kind of spin in prolate bodies will always be the case. An elongated body in space will eventually rotate about its smallest axis (there are exceptions I won’t belabor). This has to do with the energy of the system. In this spin configuration, the energy of the system is minimized.

Another difference between the rings of these small bodies, compared to the rings of Saturn, is the composition. These rings are made of dust particles. Saturn created a ring of ice chunks, effectively. These small bodies formed rings with even smaller particles, dust.  This suggests there is a relationship betwen the shape and mass of the main body, and the size of the particles in the ring.

While the Earth may be far less oblate than Saturn or these asteroids, it is oblate. It is about 40 kilometers wider in diameter at the equator than the poles. And when we consider a ring of dark matter around the Earth, we have to recognize that dark matter is likely subatomic in size! So given the physical mechanism of ring formation requires two characteristics, oblateness of the main body, and relatively small size in the ring particles compared to the host, then it is admissible to consider Earth has a ring or disk of dark matter attached to it.

There is a third aspect of rings that keeps them stable: the gravitational interaction between the ring particles. We know the ice chunks and shepherd moons Saturns have an interplay due to mutual gravitational attraction. In the above image we see how a shepherd moon sheds waves into Saturn’s rings. The mutual gravitation among the particles causes waves but the waves dissipate into the ring. Would an Earthbound dark matter ring feature such stability?

Looking at the stability of Earth’s dark matter ring can provide a critical insight on the nature of dark matter: do dark matter particles attract one another? Or are they only attracted to normal, baryonic matter? We know from observation that the latter is true. If we can deduce the size and density of the Earth-bound dark matter disk or ring, then I assert we could infer whether dark matter interacts with itself. How? Well since we’ve conceptually only one disk or ring about the Earth, we need another structure, and there is a handy one already predicted by existing dark matter literature: the caustic.

In later posts, we will explore how our planet diverts the dark matter it encounters from the galactic dark matter halo. This diversion concentrates the dark matter into a small stream, called a caustic. Does the Earth’s galactic dark matter caustic interact with its ring? Knowing whether dark matter attracts itself tests a critical assumption about this mysterious form of matter. We will explore all of these concepts on this website, by simulation, and by observation. Both will be powered by software that I will post on this website, so please check in once in a while for cool stuff you can try on your own to help us explore the dark matter at our doorstep.