Kuiper belt objects (KBOs) are a class of small bodies located beyond 30 AU from the Sun, and Centaur objects are another class of those orbiting mainly between Jupiter and Saturn. With the rapid discovery of growing number each year, over 1,000 KBOs and 60 Centaurs were known to date. Two out of three dwarf planets, which are new class by IAU resolution voted in Prague are the member of KBOs. It is widely believed that they are remnant planetesimals from the early accretion phases of the Solar System and constitute an important reservoir of primitive materials. Centaurs are often speculated to be objects in transition from the Kuiper belt, as their orbits dynamically evolve into Jupiter-family comet orbits or they get ejected from the Solar System.<br />
Photometric observations constrain the size, surface albedo, and rotational properties from its lightcurve. The lightcurve is useful to estimate the shape and lower limit of bulk density of the body. The results of several observations of KBOs and Centaurs with 1-m telescope at Wise Observatory and 200-inch telescope at Palomar Observatory will be presented. In particular, detailed results of new Centaur comet 174P/Echeclus will be given. <br />
In parallel, I will introduce the thermal model, which solves the energy equation and the mass balance equation numerically. The energy sources of young KBOs taken to be the external heat source from the early Sun and the internal heat source from radioactive species (in particular 26Al) are comparable at the typical distance of KBOs from the Sun (~ 40 AU). The models suggest that KBOs are likely to lose the ices of very volatile species during early evolution and the internal structure of KBOs is most probably not uniform but stratified with the outer layers being less altered by evolution. Additionally, cometary activity was not rare for the orbits of Centaurs and some of scattered KBOs even without radiogenic heating.<br />