Probing the physical properties of Kuiper Belt Objects

Emmanuel Lellouch
Observatoire de Paris, France

Once thought to be restricted to Pluto, the outskirts of the Solar
System beyond Neptune are now known to harbor a collection of small
bodies the Kuiper Belt objects which represent the remnants of
planetesimals that formed during the early phases of planetary
accretion. With over 1300 known objects, the orbital characterization
of this Trans-Neptunian population (TNOs) is now relatively well
established, showing several population families (classical, resonant,
scattered/detached, Centaurs) of various dynamical origins. The
physical characterization of TNOs has also progressed significantly in
the last 20 years, with numerous results obtained on their colors
(from visible and IR photometry), surface composition (spectroscopy),
rotation state and shape (optical light-curves) and binarity (direct
imaging).  In recent years, new techniques, including thermal
radiometry, stellar occultations and high-resolution spectroscopy,
have provided access to other fundamental parameters, such as size,
mass density, albedo, and thermo-physical properties (i.e. thermal
inertia and emissivity), as well as constraints on their
atmospheres. Knowing these quantities is necessary not only for a
complete characterization ("portrait") of the individual objects, but
also, if they can be measured on a sufficiently large sample, to
assess possible correlations between physical and orbital
characteristics, possibly testifying of physical processes at work
within the population (e.g. collisions, surface irradiation,
maintenance of volatile ices...). We will discuss recent findings in
the field, presenting general results on the population as a whole, as
well as on several prominent objects, particularly the dwarf planets
(Pluto, Eris, Haumea, and Makemake).