Spitzer Space Telescope - General Observer Proposal #3283


Characterization of the Icy Planetoids of the Outer Solar System

Principal Investigator: Michael Brown
           Institution: Caltech

Co-Investigators: 
Kristina Barkume, Caltech                                             
John Stansberry, University of Arizona                                
Chad Trujillo, Gemini Observatory                                     
Bill Reach, Spitzer Science Center                                    

Science Category: Kuiper belt objects 
 Observing Modes: IrsStare MipsPhot MipsSed 
  Hours Approved: 21.3 

Abstract:
The last two years have seen an explosion in the discoveries of what
is essentially a previously unknown class of solar system objects:
the icy planetoids of the outer solar system.  These objects -- Kuiper
belt objects with diameters greater than 500  km -- bear more
resemblance to Pluto and to the icy satellites of the giant planets
than they do to cometary nuclei.  GTO surveys have concentrated on the
modest-sized Kuiper belt objects and the icy satellites.  The icy
planetoids -- generally unknown at the time the GTO lists were frozen
--remain largely unexamined.  These poorly understood bodies provide
an ideal laboratory for the study of the chemical, geophysical, and
accretional evolution of the outer solar system.  We propose a
comprehensive program designed to take advantage of the unique
capabilities of Spitzer to study the size-distribution,
differentiation, and chemical evolution of this unique class of icy
planetoids.  All 14 currently known icy planetoids have been detected
in our ongoing large-scale survey of the outer solar system;  we
continue to discover new objects this size at the rate of
approximately one every other month.  Our total target list thus
includes all 20  planetoids that will be known during this cycle.
Critically, this sample of icy planetoids is known to be complete to
45 AU:  our survey has detected all objects with an absolute magnitude
less than 4.8 (9% albedo size of 500  km) out to a heliocentric
distance of 45 AU within the 4000  square degrees covered (which
includes most of the sky within 7 degrees of the ecliptic).  Such a
carefully selected sample is critical for accurate interpretation of
many of the results of a survey such as this and is only possible with
a sample chosen from a carefully controlled survey.  With these
observations and the ground-based auxiliary observations we are
obtaining, we will have a comprehensive picture of these poorly
studied icy planetoids which dominate the outer solar system.