 The cold population also contain a concentration of objects, referred to as the kernel, with semi-major axes at 44–44.5 AU.The second, the "dynamically hot" population, has orbits much more inclined to the ecliptic, by up to 30°.The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up.