Double-blind test program for astrometric planet detection with Gaia

Aims. The scope of this paper is twofold. First, it describes the simulation scenarios and the results of a large-scale, double-blind test campaign carried out to estimate the potential of Gaia for detecting and measuring planetary systems. The identified capabilities are then put in context by highlighting the unique contribution that the Gaia exoplanet discoveries will be able to bring to the science of extrasolar planets in the next decade. Methods. We use detailed simulations of the Gaia observations of synthetic planetary systems and develop and utilize independent software codes in double-blind mode to analyze the data, including statistical tools for planet detection and different algorithms for single and multiple Keplerian orbit fitting that use no a priori knowledge of the true orbital parameters of the systems. Results. 1) Planets with astrometric signatures alpha similar or equal to 3 times the assumed single-measurement error sigma(psi) and period P <= 5 yr can be detected reliably and consistently, with a very small number of false positives. 2) At twice the detection limit, uncertainties in orbital parameters and masses are typically 15-20%. 3) Over 70% of two-planet systems with well-separated periods in the range 0.2 <= P <= 3 9 yr, astrometric signal-to-noise ratio 2 <= alpha/sigma(psi) <= 50, and eccentricity e <= 0.6 are correctly identified. 4) Favorable orbital configurations (both planets with P <= 4 yr and alpha/sigma(psi) = 10, redundancy over a factor of 2 in the number of observations) have orbital elements measured to better than 10% accuracy > 90% of the time, and the value of the mutual inclination angle i(rel) determined with uncertainties <= 10 degrees.5) Finally, nominal uncertainties obtained from the fitting procedures are a good estimate of the actual errors in the orbit reconstruction. Extrapolating from the present-day statistical properties of the exoplanet sample, the results imply that a Gaia with sigma(psi) = 8 mu as, in its unbiased and complete magnitude-limited census of planetary systems, will discover and measure several thousands of giant planets out to 3-4 AUs from stars within 200 pc, and will characterize hundreds of multiple-planet systems, including meaningful coplanarity tests. Finally, we put Gaia's planet discovery potential into context, identifying several areas of planetary-system science (statistical properties and correlations, comparisons with predictions from theoretical models of formation and evolution, interpretation of direct detections) in which Gaia can be expected, on the basis of our results, to have a relevant impact, when combined with data coming from other ongoing and future planet search programs.