  Cyclic C5H (c-C5H), the radical formed by substituting an ethynyl group CCH
for the hydrogen atom in the c-C3H radical, has been detected for the first
time in the space. The c-C5H species is an isomer of the well-known linear
radical l-C5H and is 6 kcal/mol less stable. A total of 17 rotational
transitions were detected for the c-C5H species in TMC-1 within the 31.0-50.3
GHz range using the Yebes 40m radio telescope. We derive a column density of
(9.0 +/- 0.9)e10 cm-2 for c-C5H. Additionally, we observed 12 lines for l-C5H
and derive a column density for it of (1.3 +/- 0.3)e12 cm-2, which results in
an abundance ratio c-C5H/l-C5H of 0.069. This is in sharp contrast with the
value found for the analogue system c-C3H/l-C3H, whose ratio is 5.5 in TMC-1.
We discuss the formation of c-C5H and conclude that this radical is probably
formed in the reaction of atomic carbon with diacetylene.
