Constraining isocurvature fluctuations with the Planck Surveyor

We consider the detection possibilities of isocurvature fluctuations in the future CMB satellite experiments MAP and Planck for different cosmological reference models. We present a simultaneous 10 parameter fit (8 for the case of open model) to determine the correlations between the cosmological parameters, including the isocurvature cold dark matter contribution to the anisotropy. Assuming that polarization information can be fully exploited, we find that an isocurvature perturbation can be detected by the Planck Surveyor if it contributes more than 4% to the angular power spectrum at large scales. In the absence of polarization data, the signal from isocurvature perturbations can be confused with tensor perturbations or early re-ionization effects, and the limit is larger by almost an order of magnitude.