Scott Sheppard's Current research

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My current research interests focus on the origins of the Solar System through observations of small, primordial bodies in our Solar System and as well as other Solar Systems forming around young stars. Some of my main research focus is below and here . See a list of my publications .

The Kuiper Belt and small bodies of the solar system

I have been observing light curves and colors of outer solar system objects. Through observing Kuiper Belt objects (KBOs) short and long term variablility we have determined several properties of the Kuiper Belt:

I have observed most of the brightest known KBOs light curves, giving the first large sample of light variations of KBOs. These bright objects should be large (D > 200 km) and thus in the absence of rotation near spherical due to self compression. I found that the KBOs have significantly faster rotation periods and larger amplitudes than their main belt counterparts. Dave Jewitt and I proposed that (20000) Varuna with its large amplitude and short period is a rotationally distorted, low density rubble pile. Statistically, the trans-Neptunian objects are less spherical than their main belt asteroid counterparts, indicating a higher specific angular momentum perhaps resulting from the formation epoch. If these objects are equilibrium rubble piles distorted by centripetal forces due to their own rotation, the implied densities must be comparable to or less than that of water. Such low densities may be naturally explained if the KBOs are internally porous.

I further measured phase darkening for many of the KBOs from 0 to 2 degrees. This was the first time phase functions of KBOs had been measured. The similarity of the slopes of the phase functions of all KBOs in our sample suggests comparative uniformity of the surface compositions, physical states and albedos. The steep slopes are consistent with low albedo porous surfaces. This is quite unlike the shallow phase curve of the high albedo Pluto.

I have also observed optical colors of KBOs showing some have distinct color distributions depending on their locations in the outer solar sysetm.

I'm also working to survey the outer Solar System for new objects .

I have been active in recovering Kupier Belt Objects (KBOs) in order to better determine their orbits. Check out the current list of KBOs which seems to be growing exponentially since the first one was discovered in 1992. Also check out the current list of Centaurs .

For a timeline and history of the most important events in the solar system check out my history of small bodies in the solar system

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Irregular Satellites of the Planets

I'm leading a survey along of the space around Venus, Mars, Jupiter, Saturn, Uranus and Neptune to find faint satellites of these planets. To see the latest on this project visit the Satellite page . To date we have found over 60 new satellites.

Irregular satellites have eccentric orbits that can be highly inclined or even retrograde relative to the equatorial planes of their planets. These objects cannot have formed by circumplanetary accretion as did the regular satellites which follow uninclined, nearly circular, prograde orbits. Instead, they are likely products of early capture from heliocentric orbit. At present it is practically impossible for planets to permanently capture satellites since no efficient dissipation mechanism exists. Satellite capture could only have occurred efficiently towards the end of a planet's formation from the dissipation mechanisms of gas drag from an extended atmosphere and/or the enlargement of a planet's gravitational sphere caused by the planet's mass growth (Heppenheimer & Porco 1977; Pollack et al. 1979). If so, satellite capture occurred on the same timescale as the planet's growth.

The new discoveries show that the giant planets possess about the same number of irregular satellites measured to a given size (and also comparable numbers of satellite groupings) even though the planet masses vary by more than an order of magnitude. This is especially remarkable given that the ice giants Uranus and Neptune may have had formation histories quite different from the gas giants Jupiter and Saturn.

Distinct dynamical groups are found, many dominated by a relatively large satellite. Fragmentation of the parent satellites could be caused by impact with interplanetary projectiles (principally comets) or by collision with other satellites, assuming both were once much more numerous than now. Gas drag acting on the fragments would produce size-dependent sorting of the orbits within each group that is not observed. For these reasons we believe that the disruptions occurred after capture and after the dissipation of the gas left over from planet's formation.

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