Friday, 24 February 2017

Late Mixing and Migration in the Solar System: Completing the Inventory of the Outer Solar System

Background: The Kuiper Belt is a relic from the original protoplanetary disk, albeit one that has been dynamically disturbed and collisionally processed. The accretion process did not complete for objects beyond Neptune, and the evolution of the Solar System has been imprinted within their current orbital distribution. The strong dynamical connection of planetesimals to planets make Kuiper Belt Object (KBO) population and orbital properties valuable for studying Solar System formation and planet evolution. As an example, the recent discovery by Co-I Sheppard and colleagues of a high inclination Neptune Trojan indicates that Neptune was once on a much more eccentric orbit than now; our Solar System probably had a very chaotic evolution (Sheppard and Trujillo 2006).

The Kuiper Belt has several dynamical constituents created during various stages of planet evolution (Figure 3). There are over 1200 KBOs known with perihelia between about 30-50 AU, but only a few objects are known with perihelia significantly beyond 50 AU even though surveys should have found many such objects beyond this “edge” (Allen et al. 2001). The highly eccentric Sedna, with a perihelion at 76 AU, and the few objects like it, are an intermediate population between the Kuiper Belt and more distant unobserved Oort cloud. The discovery of these eccentric “detached” objects reveals our Solar System's chaotic history (Gladman et al. 2002; Morbidelli and Levison 2004). If these Sedna-like objects formed in their current locations, they must have initially been on circular orbits otherwise the large relative velocities would have made their accretion impossible (Stern 2005). If these objects obtained their large eccentricities through interactions with the giant planets, somehow their perihelia must have been raised away from the giant planets.

Research to Date: The strong size and heliocentric distance dependence of the flux density of sunlight scattered from an object requires an extremely deep survey in order to access the population of objects of size <100 km lying beyond 50 AU. Kuiper Belt surveys to date have mostly either covered large areas but been to shallow depths (wide-field imaging on a small telescope) or have gone deep but covered a very small area of sky (a large telescope but very Figure 3 Trans-Neptunian semi-major axis versus eccentricity where distinct dynamical Kuiper Belt small field-of-view, << 1 square degree). These surveys are not efficient in detecting objects well beyond 50 AU.

Proposed Work: Co-I Sheppard will perform the deepest wide-field survey for distant Sedna like objects ever obtained. He will use the wide-field imaging camera on the Magellan 6.5 meter telescope operated by Carnegie. This survey will show if objects beyond 50 AU are fainter than expected, if there is truly a dearth of objects, or if the Kuiper Belt continues again after some sizable gap possibly caused by a planet sized object.

The survey will probe the origin of Sedna and determine if this eccentric, possible organic rich body (Section 5.2.1) is unique (as once believed for Pluto) or just the tip of the iceberg of a new class of object. If so, what is the orbital and size distribution of the population? Several theories of Sedna's history have been put forth, and most are observationally testable by discovering more Sedna-type objects (see Morbidelli and Levison 2004; Gomes 2003). If additional Sedna type objects in the distant solar system have similar perihelia, this would indicate a massive planet near this location, similar to how Neptune has created the scattered disk of the Kuiper Belt. A close passing star may leave its signature on the Sedna-type objects (Brown et al. 2004). Simulations have shown that different distributions of Sedna-type objects are expected depending on the rogue star’s mass and approach distance (Morbidelli and Levison 2004), which in turn indicate how dense a stellar environment our Sun formed in. The Sednatype objects could have had their perihelions raised through several distant, slower encounters with other stars in the cluster in which our Sun formed in. In this scenario an isotropic distribution of perihelions is expected. If massive planetary embryos were the cause of the raised perihelion of Sedna-type objects, there will be a distinct signature on the population’s eccentricities and inclinations (Gladman et al. 2006).