We propose to create Galaxy, an advanced execution system (AES) built on top of the PlanetLab networking testbed. The Galaxy AES uses PlanetLab to aggregate and manage volunteer computing resources and is designed to facilitate research of existing P2P and novel volunteer-computing systems. The system software is composed of several pieces, including: an execution engine, a mapping service for assigning tasks to volunteer nodes, and networking layers to enable medium-grained parallel computation in the wide-area. In this paper, we describe the Galaxy architecture in detail and present a possible application workload that can be used to test the system. The major objectives of the Galaxy project are to deliver a prototype general-purpose wide-area computing testbed and to successfully deploy loosely-coupled scientific-computing applications using volunteer computing cycles.