A collimated jet of molecular gas from a star on the asymptotic giant branch

Evolved stars of about one solar mass are in general spherically symmetric, yet the planetary nebulae that they produce in the next phase of their evolution tend not to exhibit such symmetry. Collimated 'jets' and outflows of material have been observed(1-5) up to similar to0.3 parsec from the central stars of planetary nebulae, and precession of those jets has been proposed(1) to explain the observed asymmetries. Moreover, it has recently been shown 6 theoretically that magnetic fields could launch and collimate such jets. Here we report the detection of a collimated and precessing jet of molecular gas that is traced by water-vapour maser spots similar to500 astronomical units (AU) from the star W43A in Aquila. We conclude that the jet is formed in the immediate vicinity of the star, and infer that elongated planetary nebulae are formed by jets during the short period, of less than 1,000 years, when the star makes its transition through the proto-planetary nebula phase to become a planetary nebula.