These figures show several distinct dynamical Kuiper Belt populations. Figure A.) Vertical solid lines display resonances with Neptune as well as the Neptune Trojans in the 1:1 resonance at about 30 AU. Scattered disk objects have perihelia 30 < q < 40 AU that are shown by dashed lines. Scattered disk objects are likely to have high eccentricities from interactions with Neptune. Classical objects are in the lower center portion of the figure. An edge around 50 AU can be seen for low eccentricity objects.

Figure B.) Only three distant objects are known to fall significantly below the dashed perihelion line around 40 AU: Sedna, 2000 CR105 (upper left of Figure B) and 2004 XR190 (near 57 AU, Figure A). These three objects with high semi-major axes and perihelion distances do not fit easily within our current understanding of solar system formation.

Since Pluto was demoted, our solar system has been left with eight major planets. They travel around the Sun in fairly circular orbits and on similar planes. However, since the discovery of wildly varying planetary systems around other stars in the Milky Way, and given our increased understanding about small, primordial bodies in our celestial neighborhood, the notion that our solar system has always been so orderly is changing.

To understand solar system evolution in general and how ours came to be, DTM astronomer Scott Sheppard studies the dynamical and physical properties of small bodies, such as asteroids, comets, satellites, moons, trans-neptunian objects (bodies that orbit around Neptune and beyond), and young objects around other stars.

In one research area, Sheppard surveys our solar system for so-called irregular satellites. These bodies have been captured by their respective planets. Regular satellites, on the other hand, were created during disk accretion. Sheppard and colleagues have discovered over 70 of the irregular objects around Mars, Jupiter, Saturn, Uranus, and Neptune. During the survey, Sheppard determined that the giant planets all possess about the same number of irregular satellites, despite large differences in planetary mass.

Sheppard is also the codiscoverer of three of the five known Neptune “Trojans.” Trojans are asteroids that are locked into the same orbital period as a planet. The Neptune Trojans cluster in the elongated, curved region of a Lagrangian point, an area that is 60 degrees ahead in orbit. At these spots, the gravitational pull of the planet and the Sun combine to lock the asteroids into synchronized orbits with the planet. One of these Trojans is the first known high-inclination Trojan. The presence of this body implies that Neptune was on a much more eccentric orbit in the past. As Neptune went through the process of becoming more circular in orbit, it gained the ability to capture high-inclination objects. Sheppard has also learned that Neptune Trojans share similarities with their Jupiter counterparts.

Bodies called Kuiper Belt objects (KBOs) exist just beyond the orbit of Neptune. They are ancient, relatively small, icy rock bodies. Sheppard observed the brightness of the largest-ever sampling of these objects, discovering that a third of them have highly variable brightness over short periods. The finding implies that KBOs are piles of rubble, lacking internal strength, and points to the possibility that they originated from collisions during Kuiper Belt history.

Sheppard discovered the first contact binary KBO. A contact binary contains two objects that are drawn together by tidal friction—like the Earth and the Moon—to orbit about one another. Similar observations by Sheppard and his colleagues also yielded one of the first measurements of the bulk density of a KBO; the value is sufficiently low that a volatile-rich, porous structure is indicated. According to Sheppard, KBOs, as well as the asteroids, are fossils that reveal aspects of the evolution of the planets.