Scott S. Sheppard
Small bodies in the solar system; star, planet, and solar system formation; brown dwarfs
B.A., Physics, Oberlin College, 1998 M.S., Astronomy, University of Hawaii, 2001 Ph.D., Astronomy, University of Hawaii, 2004
The major planets in our Solar System travel around the Sun in fairly circular orbits and on similar planes. However, since the discovery of wildly varying planetary systems around other stars, 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, Carnegie’s Earth and Planets Laboratory astronomer Scott Sheppard studies the dynamical and physical properties of small bodies, such as asteroids, comets, moons, trans-neptunian objects (bodies that orbit beyond Neptune), and free floating substellar objects. These small bodies in our Solar System have a fossilized imprint from the formation and migration of the major planets in our Solar System.
The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun; the gas giant planets, which are further out; and the frozen objects of the Kuiper belt, which lie just beyond Neptune's orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was known to exist for its entire orbit until Sheppard and colleagues discovered a second object, dwarf planet 2012 VP113. It has a very eccentric orbit that is even more distant than Sedna. Sheppard has determined that the total population of these so called inner Oort cloud objects is likely bigger than the Kuiper Belt and main asteroid belt. Some of these inner Oort cloud objects could rival the size of Mars or Earth.
There are several competing theories for how the inner Oort cloud might have formed, but all require the Solar System to have been in a state vastly different than now since the inner Oort cloud objects are currently decoupled from any known major planet, yet have disturbed inclined, eccentric orbits. Thus the inner Oort cloud is a window into our Solar System's past. Sheppard and colleague are currently obtaining the widest and deepest survey for Solar System objects ever obtained to discover more inner Oort cloud members.
Active asteroids have stable orbits between Mars and Jupiter like other asteroids. However, unlike other asteroids, they sometimes have the appearance of comets, when dust or gas is ejected from their surfaces. The reasons for this loss of material and subsequent tail in active asteroids are unknown, although there are several theories such as recent impacts or sublimation of exposed ices. Sheppard and colleagues discovered an unexpected tail on asteroid 62412, an object which had been known as a typical asteroid for over a decade. Using Magellan Telescopic observations, Sheppard found 62412 to have a very fast rotation. It thus appears the activity in this asteroid is created by rotational fissioning of material off the surface of 62412. Sheppard and colleagues estimate that there are likely about 100 active asteroids in the main asteroid belt, based on their discovery.
Sheppard is also the co-discoverer of the first trailing Neptune Trojan and first high inclination leading Neptune Trojan. Trojans are asteroids that are locked into the same orbital period as a planet but lead or follow the planet by about 60 degrees. At these spots, the gravitational pull of the planet and the Sun combine to lock the asteroids into synchronized orbits with the planet. The presence of high inclination Trojans 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 many similarities with their Jupiter counterparts.
In another 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 moons around 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 and formation scenarios.
Sheppard discovered the first contact binary Kuiper belt object. A contact binary contains two objects that are drawn together by tidal friction like the Earth and the Moon to orbit about one another. The large amount of angular momentum in the Kuiper Belt suggests it was much denser in the distant past. 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.