* Astronomy

Title: ExELS: an exoplanet legacy science proposal for the ESA Euclid mission I. Cold exoplanets
Authors: M.T. Penny, E. Kerins, N. Rattenbury, J.-P. Beaulieu, A.C. Robin, S. Mao, V. Batista, S. Calchi Novati, A. Cassan, P. Fouque, I. McDonald, J.B. Marquette, P. Tisserand, M.R. Zapatero Osorio (Euclid Exoplanets Working Group)

The Euclid mission is the second M-class mission of the ESA Cosmic Vision programme, with the principal science goal of studying dark energy through observations of weak lensing and baryon acoustic oscillations. Euclid is also expected to undertake additional Legacy Science programmes. One such proposal is the Exoplanet Euclid Legacy Survey (ExELS) which will be the first survey able to measure the abundance of exoplanets down to Earth mass for host separations from ~1 AU out to the free-floating (unbound) regime. The cold and free-floating exoplanet regimes represent a crucial discovery space for testing planet formation theories. ExELS will use the gravitational microlensing technique and will detect over 400 microlensing events per month over 1.6 deg² of the Galactic bulge. We assess how many of these events will have detectable planetary signatures using a detailed multi-wavelength microlensing simulator --- the Manchester-Besancon microLensing Simulator (MaBuLS) --- which incorporates the Besancon Galactic model with 3D extinction. MaBuLS is the first theoretical simulation of microlensing to treat the effects of point spread function (PSF) blending self-consistently with the underlying Galactic model. We use MaBuLS, together with current numerical models for the Euclid PSFs, to explore a number of designs and de-scope options for ExELS, including the two current spacecraft designs, the exoplanet yield as a function of filter choice, and the effect of systematic photometry errors. Using conservative extrapolations of current empirical exoplanet mass functions determined from ground-based microlensing and radial velocity surveys, ExELS can expect to detect a few hundred cold exoplanets around mainly G, K and M-type stellar hosts, including ~19 Earth-mass planets and ~3 Mars-mass planets for an observing programme totalling 10 months.

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