California Institute of Technology
Planetary Science Department
Kuiper Belt Satellites (especially 2003 EL61 = Haumea)
Kuiper Belt Families
In March 2007, our group published a paper in Nature about the discovery of the first Kuiper belt family. A family is a group of objects that are thought to be pieces of a single progenitor that got partially blown apart by a huge impact. The largest remaining piece is EL61, now Haumea, and it sure looks like it got hit by something that sent it spiraling about 10 trillion 4-hour rotations ago. Pieces of the debris (or the "splash") eventually coalesced into the two moons. But some of the pieces flew off completely and left EL61 to become their own KBOs. When this happens, the pieces go into nearby orbits and that's what I study: how close are all the orbits of these family members?
In the asteroid belt, such families have been known for almost 100 years since they were discovered by Hirayama. They are identified by looking at orbital elements. Orbital elements are numbers that describe how each asteroid goes around the sun: the semi-major axis is roughly the average size of the orbit, the eccentricity measures the elliptical or non-circular nature of the asteroid's orbit, and the inclination measures the tilt of the orbit with respect to the plane of the planets. For example, EL61's semi-major axis is about 43 AU, with an eccentricity of 0.19, and an inclination of 28 degrees relative to Earth's orbital plane. (Go here, to visualize EL61's orbit.) A family will have similar sets of (proper or time-averaged) orbital elements, and they can be easily seen among asteroids.
Families in the Kuiper belt have a somewhat different character than asteroid families, so I undertook a detailed study of the orbits of KBOs in a scientific paper to appear in the Astronomical Journal. As part of this study, we found that the dynamical clustering of the EL61 family members is very tight, i.e. the family members identified in our Nature paper (1995 SM55, 1996 TO66, 2002 TX300, 2003 OP32, and 2005 RR43) are the very closest KBOs to the center of the collision. We also identified a few new family members (1999 OY3, 2003 UZ117, and 2005 CB79), though the latter two aren't fully confirmed yet. See this plot from the paper. There are also a couple of lists of dynamically close objects which need to be studied spectroscopically to see if they have the same strong signal of water ice as the known family members. All in all, the dynamics-side of the EL61 family hypothesis works very well, even when you dig into the details.
From studying the orbits of the family members, we were also able to show that the family occurred more than a billion years ago! How exactly this works is complicated, but basically the orbit of EL61 has slowly been getting more elliptical over time. This is because it is in a special place in the outer solar system called a "resonance." EL61 is specifically in the 12:7 mean motion resonance so that Neptune orbits the sun exactly 12 times when EL61 goes around exactly 7 times. This means that the relative position of Neptune and EL61 is not random, but biased. So, although the gravitational perturbation of Neptune is weak compared to the Sun, it tends to add up, which doesn't happen for non-resonant objects (even in nearby orbits like family members). The biased perturbations of the resonant configuration tend to keep objects in the resonance (i.e. resonances are generally stable and are hard to escape). In the case of a weak resonance, like the 12:7, the resonance has the additional effect of slowly chaotically changing the eccentricity of EL61's orbit. (This does not happen in strong resonances, like Pluto's 3:2 resonance; the "strength" of a resonance is roughly measured by the difference between the two numbers in the period ratio, e.g. 12-7=5 (weak), but 3-2=1 (strong).) You can see the diffusion of orbital eccentricity in an animation. Our studies show that it takes objects at least a billion years to get from where we think EL61 started (near the center of the family members) to where it is now. So, the family should be at least a billion years old. This is the expected result: the current Kuiper belt is too sparse to have a collision of the magnitude of the EL61 satellite-and-family-forming impact. However, early in the history of the solar system, the transneptunian region is thought to have had many more objects, making this probability more likely. The details of this analysis have been studied by Hal Levison and colleagues and a talk of his on this subject is available here. Our results appear to be fully consistent with theirs and vice versa with the expectation that the EL61 family was created near the beginning of the solar system. Unfortunately, this method of dating the collision can only give a rough estimate (3 +/- 2 billion years) of the age. However, someday with many more known family members, we might be able to increase the precision of our age estimates, which would be really nice for those of us who are trying to piece together the history of the outer solar system.
One big surprise with these family members is their surfaces. From the spectra which we've taken of these objects, every one has a surface that appears to be relatively fresh. For example, the objects are covered with crystalline water ice, which is thought to break down (into amorphous water ice) in only 10 million years. Based on our current understanding of the surfaces of these objects, they should not be a billion years old. This seems to be a contradiction, but I think it is really an indication that we don't fully understand how KBO surfaces work. (Not a surprise, since we really don't know a lot about them.) May I also note that some family members are thought to be among the largest members of the Kuiper belt. This is based on an assumption of their albedo, a measure of surface reflectivity. Since they are family members, it seems reasonable that they would share the same high albedos as EL61, its moons, Charon, and other objects with similar spectra. This implies that they are much smaller than typically expected.
In any case, the EL61 family will be really useful in future studies of the formation and evolution of the outer solar system. Feel free to contact me if you have any questions about it.
NEW! PAPER DESCRIBING THE ORBITS OF THE SATELLITES OF HAUMEA ACCEPTED. Here is an animation of the orbits of Hi'iaka and Namaka as seen from Earth over the last few years (2005=red, 2008=purple). Haumea is in the center, to scale, Hi'iaka is the circle, orbiting every 49 days and Namaka is the diamond, orbiting every 18 days. (Looks best in Firefox.)Not only is 2003 EL61 the progenitor of the only known Kuiper belt family, it also has two large satellites. The outer satellite is brighter and its orbit was described in a 2005 scientific paper. In a classic problem of celestial mechanics, Brown et al. started with 5 snapshots of the outer satellite taken at different times and calculated the orbit that would put the outer satellite in those positions at those times. To do this, you need to find the orbital elements that describe the orbit: semi-major axis (typical distance from EL61), period of orbital revolution, eccentricity, inclination, and a few angles that describe the orientation of the orbit ellipse as viewed from Earth. The semi-major axis, period, and mass of EL61 are related through Kepler's Third Law. Solving this for the 49-day, 49500 km orbit of the outer satellite around EL61 implies that it has a mass about 1/3 the mass of Pluto (4 x 1021 kg). In the image on the right, the black crosses show the observed location and the colored circles are the expected location from the orbit. You see that there is an excellent agreement. EL61 is drawn to scale in the center.
The inner satellite is fainter and orbits closer. This makes it harder to see not only because it's faint, but it also spends a lot of time so close (as seen from Earth) to EL61, that it gets hidden in the glare. So, we only saw it 3 times in 2005, not enough to get a good orbit. In an effort to get more information about EL61 and its satellites, we've continued to observe it in 2006 and 2007 and have seen the inner satellite continue to move around. So, why can't we just figure out its orbit like the outer satellite? When we try, nothing works: there is no single orbit that matches up with all the pictures we taken... because its orbit is changing.
This is actually expected: the gravitational interactions with the outer satellite push and tug on the inner orbit in such a way that, even over the course of a few months, the orbit has moved. (The inner satellite also slightly changes the outer satellite orbit, but much less because the outer satellite is more massive and harder to push around.) I've written a orbit-finding program to try to account for these motions and I'm currently investigating the inner orbit. It will not be easy to find the solution and we may need to wait for more data (we can observe EL61 again next spring). Once we do know the inner orbit, we can begin with more detailed studies of the interactions of these satellites including mass estimates, whether they are in resonance, their formation and evolution, etc. Stay tuned. (It wouldn't be research if we already knew the answer.)
I'm also interested in the satellites of other KBOs and how they formed and how their orbits have evolved. This is a pretty new subject so there is much to explore and understand.
Over the last 10 years, scientists have discovered nearly 300 planets orbiting around other stars. Like Kuiper belt objects, the first thing we learn about these planets is their orbits. Even that is enough to generate many questions; many of these remain mostly unsolved (such as the details of the origin of the wild eccentricity distribution, compared to our relatively circular solar system or the effects of tides on multi-planet systems). I have recently finished a large project on figuring out what is on the insides of these planets . Imagine a raw and a hard-boiled egg: they will have the same mass and size. However, if you spin them, the raw egg will wobble around and the hard-boiled egg will spin more rigidly. Using a similar technique, Aaron Wolf and I explained how to determine the "squishiness" (technically speaking, the planetary Love number) of very hot Jupiters, Jupiter-size planets that are extremely close to their parent stars.