This thesis sheds new light on two main subjects. First, the discovery of new extended planetary nebulae (PNe) in the Galactic Plane, thanks to the recent Isaac Newton Telescope Photometric Hα Survey (IPHAS). The new objects, located in a right ascension range between 18h and 20h, were identified using different diagnostic diagrams in the optical, infrared and radio regimes. We investigated their nature and distribution on the Plane compared to known planetary nebulae and found some ``hidden populations'' of PNe. These new findings also confirm the efficiency of the survey to detect faint planetary nebulae at low latitudes. The analysis of the radio and infrared properties of the newly found nebulae, allowed us to constrain and correct the diagnostic diagrams used so far and which are mostly dedicated to bright PNe. Combined with an investigation of the recombination status and the age of the nebulae, our analysis suggests that we are dealing with large, old and generally dusty objects. A totally new aspect of PNe has therefore been revealed.
The issue of distance estimation has also been approached with the use of a new extinction-distance method developed within the framework of IPHAS.
The last investigation carried out by IPHAS concerns the interaction with the interstellar medium (ISM). We observed several examples of interaction which were compared with hydrodynamical models. The interactions are classified into four stages according to the degree of disruption of the nebula, and we were able to identify objects at each stage i.e. from the undisturbed to the totally disrupted morphology.
The second part of the thesis is dedicated to the discovery and the study of magnetic fields in planetary nebulae and the role they can play in their shaping. This point was already mentioned while studying the ISM interaction for the IPHAS nebulae and how magnetic fields could disrupt the PNe. The observation of 4 bipolar PNe and Proto-PNe (NGC 7027, NGC 6537, NGC 6302 and CRL 2688) in the sub-mm regime with SCUBA, has indicated the occurrence of toroidal magnetic fields in all the objects via dust grain alignment.
The toroidal fields could account for the bipolar morphology observed but the lack of information concerning the strength of the field prevents us from jumping to such a conclusion. We also investigated the possible relation between magnetic fields and, first the chemistry of the selected targets: we showed two oxygen-rich and two carbon-rich nebulae (and we observed two different fields' organisation), and then their evolutionary stage: we showed nebulae from the Proto-PN to the PN stage (and we identified long-lived fields).
Finally we discussed the methods that would allow us to derive the strength, as well as all those helping to define how magnetic fields act in planetary nebulae.