2003 EL61

the strangest known object in the Kuiper belt

2003 EL61 is one of the strangest known objects in the solar system. It is a big across as Pluto, but shaped like a cigar. Or perhaps like a football [American-style]. Or, most accurately, a football that has too little air in it and has been stepped on. It spins end over end every 4 hours like a football that has been kicked. It appears to be made almost entirely of rock, but with a glaze of ice over the surface. It is surrounded by two tiny satellites and is followed in its orbit by a swarm of other small icy bodies. Everything that we know about this body appears to tell us that in its past another object slammed into it at high speed and cracked it into pieces which flew all around the outer solar system and left what we see today. Follow the story below....

2003 EL61 looks and spins approximately like this:

What is 2003 EL61?

2003 EL61 is the third-largest known dwarf planet in the Kuiper belt, the region of space beyond Neptune that contains the larger dwarf planets Eris and Pluto as well as thousands of smaller objects.

How big is 2003 EL61?

Many times when objects like this are discovered we don't actually know how big we are, just how bright they are. How bright they are tells us how much sunlight they reflect. But they could be bright and reflect a lot of sunlight because they are large or they could be bright because they are highly reflective, like a ball of snow. In the case of 2003 EL61, however, we have gotten lucky, because soon after the discovery of the object we discovered a moon orbiting it. By following the orbit of the moon over the course of 6 months we are able to precisely determine the mass of 2003 EL61 and its moon. The mass is about 32% that of Pluto. Unfortunately, the mass does not directly tell us it's size. An object that weighs a lot can either be small and dense, like something made out of rock, or large and less dense, like something made out of ice.

The fast spin of 2003 EL61, however, gives away its size in a somewhat complicated way. When an object spins quickly, it stretches out, much like a pizza crust tossed into the air. A denser rockier object stretches out less than a less dense lighter object. By seeing just how much 2003 EL61 stretches out due to its spin we can tell how dense it is. We find that 2003 EL61 must be made almost entirely of rock or else its very fast 4 hour spin would stretch it out even more than it is already stretched!

Once we know how much 2003 EL61 weighs and we know what 2003 EL61 is made of we can figure out  how big it is. The answer is that it is as big as Pluto -- along its longest dimension.  Nothing else so large and so elongated or so quickly rotating is known anywhere in the solar system.

What is 2003 EL61 made out of?

From the spin we see that the density of 2003 EL61 is almost that of rock. By looking at sunlight reflected from the surface of 2003 EL61 and carefully analyzing the spectrum of the light we have found, however, that the surface appears to be pure ice  (again, a technical paper). It seems that 2003 EL61 is a large rocky body with a thin film of ice. A cross-section would look something like this:

This is not dissimilar to the cross-section of an M&M candy, though they have chocolate, rather than rock, on the inside. No one ever expected to find something quite like this in the outer solar system.......

The moons of 2003 EL61

The first moon to 2003 EL61 was discovered on January 28th 2005 by observations at the Keck Observatory. The observations were obtained with a brand new high powered system at Keck which used a laser projecting out of the telescope to correct for the turbulence of the atmosphere. This system, called laser guide star adaptive optics, allows astronomers on the ground to take images with the sharpness of images obtained from space by the Hubble Space Telescope. When we observed 2003 EL61 with this new facility we immediately saw that there was a faint moon in the vicinity. From 5 observations over the span of 6 months we have precisely determined the orbit of the satellite to 2003 EL61.

A (technical) scientific paper describing the discovery of this satellite can be read in the October issue of the Astrophysical Journal Letters.

Here is an image of the satellite from the night of 30 June 2005. 2003 EL 61 is the bright object in the center and the satellite appears directly below about 0.5 arcseconds. To the south of 2003 EL61 you can also see a faint object which turns out to be a second satellite.

The second satellite is fainter than the first, but still detected in three of the five images of the first satellite. You can read the (again, technical) scientific paper describing the second satellite (and the satellite to Eris), too.

About 10% of Kuiper belt objects have satellites, but until recently no other object in the Kuiper belt was known to have more than one satellite. Recently, however, 2 small satellites around Pluto were also discovered. We suspect that other Kuiper belt objects (at least large Kuiper belt objects) might also have multiple satellite systems, but very sensitive searches around all of the largest Kuiper belt objects using the Hubble Space Telescope have failed to turn up any new multiple systems.

From five observations like the one above we can mathematically solve for the precise orbit of the brighter satellite. You can see the orbit of the satellite along with the five observations below. Three observations of the fainter satellite are not sufficient to rigorously solve for the orbit of the satellite, but, if we assume it is on a circular orbit, we come up with a solution like the one seen below.

The orbits of the two satellites are seen almost edge on, which is why they look so elongated. The orbits are tilted to each other by about 40 degrees. We're pretty sure that the orbit that we derived for the second satellite it incorrect. The two satellites appear large enough that they effect each other's orbits sufficiently that we will not be able to determine both of the orbits accurately without much much more data. We're hard at work on that right now!

From the two satellites, we have learned several important things about the 2003 EL61 system. First, both satellites appear  to be quite small. The larger one is perhaps 1% of the mass of the primary while the smaller is only 0.2% of the mass of the primary.

The brighter satellite is on a long period orbit, circling 2003 EL61 every 49 days, in contrast to Pluto's large satellite Charon which orbits in just 6 days. The longer orbital period of  the satellite is a consequence both of the more distant orbit of the satellite from 2003 EL61 and the lower mass of 2003 EL61. The orbit is almost circular but not quite (because of the affect of the second moon!).

Seen from the earth the satellite reaches a maximum distance from 2003 EL61 of about 1.5 arcseconds. This distance is just enough to enable to the satellite to be seen with telescopes even without high power laser adaptive optics systems. This fact  has enabled us to study the larger moon in detail to determine that it is, essentially, a large chunk of ice (yet another technical paper).

The biggest scientific benefit of the large satellite is that it allows us to calculate the mass of the system. Once we know the distance od the satellite from the primary and we know the time it take to orbit we can simply calculate the mass. A higher mass object would pull the satellite more tightly and cause it to orbit faster. Knowing the mass allows us to figure out the size, as discussed above.

Why is 2003 EL61 spinning fast, shaped like a football, made out of ice-covered rock, and surrounded by tiny satellites?

Good question. We have constructed a hypothesis which attempts to explain everything going on on 2003 EL61. The hypothesis goes like this: 4.5 billion years ago, when the solar system was in the process of forming, the object that is now 2003 EL61 was a ball made half of ice and half of rock and about the size of Pluto (much like what we think Pluto is today). Sometime early in its history, another large Kuiper belt object collided with 2003 EL61 obliquely. This collision knocked away most of the ice (which would have been on the outside) of 2003 EL61, leaving just a rock covered in a thin film of ice. The oblique impact caused 2003 EL61 to spin rapidly. The rapid spin elongated 2003 EL61 into the football shape we see today. Some of the debris from the impact coalesced into the satellites, which would have been initially much closer but which have been evolving outward with time.

This story contains some elements of how we think the Earth and Moon formed: a Mars-sized body hit the proto-earth and the moon coalesced from the debris following the impact. There is also an analogy to the formation of Charon around Pluto which was thought to have been from a Kuiper belt impact at a similar time. Finally there is an analogy to the planet Mercury, which was once thought to be much larger but was smashed by a huge object early in its history which removed much of the rocky outer layer and left mostly the iron core that we see today.

One of the interesting predictions of this hypothesis is that the debris from the collision, from which the satellites formed, should have been composed predominantly of ice, rather than rock.  The satellites themselves, then, should be almost purely ice. The best way to determine the composition of the satellites would be if we could determine their density. Unfortunately, the satellites are too small to measure the densities any time soon. We can, however, measure the composition of the surface of the brighter satellite. We find that it, like 2003 EL61 itself, is composed of water ice. On the satellite the water ice appears perhaps 100% pure. While we don't know for certain that there is not rock under the surface, we think that such pure water ice is a likely indication that there is water ice all the way down, which is consistent with our hypothesis.

The family of 2003 EL61

While the odd characteristics of 2003 EL61 and its moons initially led us to the hypothesis of a giant impact, the smoking gun came when we found the other icy debris left over from the collision in orbits similar to 2003 EL61 itself. In a paper just published in Nature we show the existence of five small objects in the Kuiper belt which look just like the moons of 2003 EL61 and appear to be additional chunks of the icy outer layer of 2003 EL61 that got blasted off the surface. These chunks appear to be about 10% of the total amount of material blasted off the surface. Some of the rest probably evaporated into space, but some smaller ones are probably still in the vicinity waiting to be found. We recognized these chunks as debris from 2003 EL61 because we first noticed that a small number of Kuiper belt objects appeared to have unique surfaces composed of almost pure ice. Most of the other Kuiper belt objects have much more complex surfaces that are difficult to understand, but 2003 EL61, its satellites, and these five new objects all appeared unique. While trying to understand why these we different it suddenly became glaringly obvious that all of these objects were on very similar orbits, and that if you traced those orbits back in time you would be able to connect them to a single location where they were once part of a larger body (this is an oversimplification, but gets the main point).

This is pretty exciting stuff! We had never before had evidence for such catastrophic disruptions in the outer solar system, though they appear moderately common within the asteroid belt in the inner solar system. What is even more interesting is that the collision occurred near a region of space where Kuiper belt objects don't live for long without having their orbits become unstable (for the curious: the orbits become unstable because they go around the sun precisely 7 times for  every 12 times that Neptune goes around the sun, and, over time, the fact that this pattern repeats gives the Kuiper belt objects slight perturbations every orbit which eventually build up enough to cause the object to be unstable).  When the orbits become unstable, the objects can eventually work their way in towards the inner solar system where we would call them comments. It is clear that the giant impact that made the 2003 EL61 family must have created many many tiny fragments that have lit up the earth's skies in the past. Even more interesting, 2003 EL61 is on an unstable orbit and will possibly become a comet itself. When it does it will probably be 10,000 times brighter than the spectacular comet Hale-Bopp, making it something like the brightness of the full moon and easily visible in the daytime sky. The only catch is that all of this will happen in perhaps 1 billion years, so you have a little bit of waiting to do to see it!

When will 2003 EL61 get a real name like Eris and Sedna and Quaoar?

We don't know. We submitted a name to the International Astronomical Union in mid September 2006 for 2003 EL61 and both of its satellites. The name were based in Hawaiian mythology to pay homage to the place where the satellites were discovered. We have heard nothing from the IAU since.