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The Detectability and Biasing Effects of the Shape of an Exoplanet in the Analysis of High SNR Light-curves

Presentation #102.364 in the session Poster Session.

Published onJun 20, 2022
The Detectability and Biasing Effects of the Shape of an Exoplanet in the Analysis of High SNR Light-curves

One of the many exciting things that JWST will enable is mapping exoplanets beyond anything that has previously been done, leveraging high SNR multi-wavelength observations to construct 3D models of a planet’s atmosphere. The caveat to this is that one must have a proper understanding of all potential biases and degeneracies, particularly from subtle effects which are typically ignored for lower SNR observations such as the shape of a planet. A typical assumption is that exoplanets are perfectly spherical; however, in our solar system we observe that Saturn is ~10% wider at its equator than at its poles, causing it to be oblate. Such a shape variation can alter the light-curve of a transiting planet primarily during ingress and egress. This is relevant for the method of eclipse mapping, which leverages the ‘scanning’ effect of a star passing in front of an exoplanet in order to measure flux across slices of its surface. Previous efforts have searched for oblateness among the best candidates which would show structure in their residuals from a model that incorrectly assumes a spherical planet. Our work has focused on studying the degree to which other orbital parameters such as inclination and semi-major axis may adjust themselves to ‘absorb’ the oblateness signal. This is done through a series of injection-retrieval tests, in which we simulate the lightcurves of oblate planets and fit them with a circular model, observing the difference between the true parameter values and those that are retrieved by the fit. For many cases we find no structure in the fit residuals, indicating that they may be an insufficient method of detecting oblateness and shape variations in general. This also leads to biased values for the parameters which are degenerate with oblateness, which we find may vary by several standard deviations from their true values. A large part of our work has been to quantify such an effect across the full sample of confirmed transiting exoplanets. We find that for flux errors below 100ppm, as many as 50 currently known planets could exhibit directly observable oblateness, indicating that many more than that would be susceptible to the “oblateness masking” mentioned above. We also consider the potential for oblateness to induce asymmetry in a lightcurve, mimicking a TTV signal typically associated with a perturbation from another body. Such effects have been considered in the past, but we now have a sample of over 4000 confirmed exoplanets against which to test such ideas and understand how they affect the planets we know to exist.

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