  Trans-Neptunian objects (TNOs) contain the most primitive and thermally
least-processed materials from the early accretional phase of the solar system.
They allow us to study interrelations between various classes of small bodies,
their origin and evolution.
  Using FORS1 of the ESO VLT, we have obtained linear-polarization measurements
in the Bessell R filter for five TNOs at different values of their phase angle
(i.e., the angle between the Sun, the object, and the Earth). Due to the large
distance of the targets (> 30 AU), the observed range of phase angles is
limited to about 0-2 deg. We have analyzed our new observations of five TNOs,
and those of another four TNOs obtained in previous works, and discovered that
there exist two classes of objects that exhibit different polarimetric
behaviour. Objects with a diameter > 1000 km, such as, e.g., Pluto and Eris,
show a small polarization in the scattering plane (~0.5%) which slowly changes
in the observed phase angle range. In smaller objects such as, e.g., Ixion and
Varuna, linear polarization changes rapidly with the phase angle, and reaches ~
1% (in the scattering plane) at phase angle 1 deg. The larger objects have a
higher albedo than the smaller ones, and have the capability of retaining
volatiles such as CO, N2 and CH4. Both of these facts can be linked to their
different polarimetric behaviour compared to smaller objects.
  In spite of the very limited range of observable phase angles, ground-based
polarimetric observations are a powerful tool to identify different properties
of the surfaces of TNOs. We suggest that a single polarimetric observation at
phase angle ~1 deg allows one to determine whether the target albedo is low or
high.
