Research on extrasolar planets is one of the most rapidly advancing fields of astrophysics. In just over a decade since the discovery of the first extra-solar planet orbiting around 51 Pegasi, 289 extrasolar planets have been discovered. This breakthrough is the result of the development of a wide range of new observational techniques and facilities for the detection and characterisation of extrasolar planets. In Utrecht we are building the Extreme Polarimeter (ExPo) to image extra-solar planets and circumstellar environments using polarimetry at contrast ratio of 10$^-9$. To test and calibrate ExPo, we have built a laboratory-based simulator that mimics a star with a Jupiter-like exoplanet as seen by the 4.2m William Herschel Telescope. The star and planet are simulated using two single- mode fibres in close proximity that are fed with a broadband arc lamp with a contrast ratio down to 10$^-9$. The planet is partially linearly polarized. The telescope is simulated with two lenses, and seeing can be included with a rotating glass plate covered with hairspray. In this paper we present the scientific requirements and the simulator design.