We search 0.02 deg ^ { 2 } of the invariable plane for trans-Neptunian objects ( TNOs ) 25 AU or more distant using the Advanced Camera for Surveys ( ACS ) aboard the Hubble Space Telescope .
With 22 ksec per pointing , the search is > 50 \% complete for m _ { 606 W } \leq 29.2 .
Three new objects are discovered , the faintest with mean magnitude m = 28.3 
( diameter \approx 25 km ) , which is 3 mag fainter than any previously well-measured Solar System body .
Each new discovery is verified with a followup 18 ksec observation with the ACS , and the detection efficiency is verified with implanted objects .
The three detections are a factor \sim 25 
fewer than would be expected under extrapolation of the power-law differential sky density for brighter objects , \Sigma ( m ) \equiv dN / dmd \Omega \propto 10 ^ { \alpha m } , \alpha \approx 0.63 .
Analysis of the ACS data and recent TNO surveys from the literature reveals departures from this power law at both the bright and faint ends .
Division of the TNO sample by distance and inclination into “ classical Kuiper belt ” ( CKB ) and “ Excited ” samples reveals that \Sigma ( m ) differs for the two populations at 96 % confidence , and both samples show departures from power-law behavior .
A double power law \Sigma ( m ) adequately fits all data .
Implications of these departures include the following .
( 1 ) The total mass of the “ classical ” Kuiper belt is \approx 0.010 M _ { \oplus } , only a few times Pluto ’ s mass , and is predominately in the form of \sim 100 km bodies ( barring a secondary peak in the mass distribution at < 10 km sizes ) .
The mass of Excited objects is perhaps a few times larger .
( 2 ) The Excited class has a shallower bright-end magnitude ( and presumably size ) distribution ; the largest objects , including Pluto , comprise tens of percent of the total mass whereas the largest CKBOs are only \sim 2 \% of its mass .
( 3 ) The derived size distributions predict that the largest Excited body should be roughly the mass of Pluto , and the largest CKB body should have m _ { R } \approx 20 —hence Pluto is feasibly considered to have originated from the same physical process as the Excited TNOs .
( 4 ) The observed deficit of small TNOs occurs in the size regime where present-day collisions are expected to be disruptive , suggesting extensive depletion by collisions .
The Excited and CKB size distributions are qualitatively similar to some numerical models of growth and erosion , with both accretion and erosion appearing to have proceeded to more advanced stages in the Excited class than the CKB .
( 5 ) The lack of detections of distant TNOs implies that , if a mass of TNOs comparable to the CKB is present near the invariable plane beyond 50 AU , that distant population must be composed primarily of bodies smaller than \approx 40 km .
( 6 ) There are too few small CKBOs for this population to be the reservoir of Jupiter-family-comet precursors without a significant upturn in the population at diameters < 20 km .
With optimistic model parameters and extrapolations , the Excited population could be the source reservoir .
Implications of these discoveries for the formation and evolution of the outer Solar System are discussed .