The origin and shaping of planetary nebulae: Putting the binary hypothesis to the test

Planetary nebulae (PNs) are circumstellar gas ejected during an intense mass-losing phase in the lives of asymptotic giant branch stars. PNs have a stunning variety of shapes, most of which are not spherically symmetric. The debate over what makes and shapes the circumstellar gas of these evolved, intermediate mass stars has raged for two decades. Today the community is reaching a consensus that single stars cannot trivially manufacture PNs and impart to them nonspherical shapes and that a binary companion, possibly even a substellar one, might be needed in a majority of cases. This theoretical conjecture has, however, not been tested observationally. In this review we discuss the problem from both the theoretical and observational standpoints, explaining the obstacles that stand in the way of a clean observational test and ways to ameliorate the situation. We also discuss indirect tests of this hypothesis and its implications for stellar and galactic astrophysics.