  Nearly all young stars are initially surrounded by `protoplanetary' discs of
gas and dust, and in the case of single stars at least 30\% of these discs go
on to form planets. The process of protoplanetary disc formation can result in
initial misalignments, where the disc orbital plane is different to the stellar
equator in single star systems, or to the binary orbital plane in systems with
two stars. A quirk of the dynamics means that initially misaligned
`circumbinary' discs -- those that surround two stars -- are predicted to
evolve to one of two possible stable configurations, one where the disc and
binary orbital planes are coplanar, and one where they are perpendicular (a
`polar' configuration). Prior work has found coplanar circumbinary discs, but
no polar examples were known until now. Here we report the first discovery of a
protoplanetary circumbinary disc in the polar configuration, supporting the
predictions that such discs should exist. The disc shows some characteristics
that are similar to discs around single stars, and that are attributed to dust
growth. Thus, the first stages of planet formation appear able to proceed in
polar circumbinary discs.
