Abstract
Abstract The eccentricity of a planet’s orbit and the inclination of its orbital plane encode important information about its formation and history. However, exoplanets detected via direct imaging are often only observed over a very small fraction of their period, making it challenging to perform reliable physical inferences given wide, unconstrained posteriors. The aim of this project is to investigate biases (deviation of the median and mode of the posterior from the true values of orbital parameters, and the width and coverage of their credible intervals) in the estimation of orbital parameters of directly imaged exoplanets, particularly their eccentricities, and to define general guidelines to perform better estimations of uncertainty. For this, we constructed various orbits and generated mock data for each spanning ∼0.5% of the orbital period. We used the Orbits For The Impatient algorithm to compute orbit posteriors and compared those to the true values of the orbital parameters. We found that the inclination of the orbital plane is the parameter that most affects our estimations of eccentricity, with orbits that appear near edge on producing eccentricity distributions skewed away from the true values and often bimodal. We also identified a degeneracy between eccentricity and inclination that makes it difficult to distinguish posteriors of face-on, eccentric orbits and edge-on, circular orbits. For the exoplanet-imaging community, we propose practical recommendations, guidelines, and warnings relevant to orbit fitting.
Highlights
The orbit of a planet can reveal much about its history and properties
We did not perform tests with the rest of the orbital parameters because they do not alter the shape of the orbit; for example, varying the semimajor axis only changes the size of the orbit, and varying the longitude of the ascending node only rotates it around our line of sight
The metrics considered were the width of the 68% credible interval and the absolute difference of the median and mode of the posterior and the true value
Summary
The orbit of a planet can reveal much about its history and properties. Distinct population-level eccentricity distributions have been used to suggest different formation mechanisms between giant planets and brown dwarfs (Bowler et al 2020). Planets detected via the radial velocity (RV) technique exhibit different population-level eccentricities between long-period and short-period planets (Kipping 2013). Direct imaging has allowed us to detect planets at wider separations than indirect detection methods (Bowler 2016). These wide separations often imply very long orbital periods of hundreds to thousands of years. Even after years of observations, oftentimes we only see a given planet moving along a very small arc of its orbit, making orbit fitting challenging and potentially susceptible to systematic or statistical biases
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