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The eccentricity of a planet's orbit and the inclination of its orbital planeencode important information about its formation and history. However,exoplanets detected via direct-imaging are often only observed over a verysmall fraction of their period, making it challenging to perform reliablephysical inferences given wide, unconstrained posteriors. The aim of thisproject is to investigate biases (deviation of the median and mode of theposterior from the true values of orbital parameters, and the width andcoverage of their credible intervals) in the estimation of orbital parametersof directly-imaged exoplanets, particularly their eccentricities, and to definegeneral guidelines to perform better estimations of uncertainty. For this, weconstructed various orbits and generated mock data for each spanning $\sim 0.5\%$ of the orbital period. We used the Orbits For The Impatient (OFTI)algorithm to compute orbit posteriors, and compared those to the true values ofthe orbital parameters. We found that the inclination of the orbital plane isthe parameter that most affects our estimations of eccentricity, with orbitsthat appear near edge-on producing eccentricity distributions skewed away fromthe true values, and often bi-modal. We also identified a degeneracy betweeneccentricity and inclination that makes it difficult to distinguish posteriorsof face-on, eccentric orbits and edge-on, circular orbits. For theexoplanet-imaging community, we propose practical recommendations, guidelinesand warnings relevant to orbit-fitting.

Biases in orbit fitting of directly-imaged exoplanets with small orbital coverage