The flipped orbit of KELT-19Ab inferred from the symmetric TESS transit light curves

Dozens of planets are now discovered with large orbital obliquity, and have become the proof for the dynamical evolution of planetary orbits. In the current samples, there is an apparent clustering of planets around 90 degrees, and also an absence of planets around 180 degrees although the latter is expected by some theories. Statistical extrapolation using Hierarchical Bayesian Analysis have recently refuted the significant clustering around 90 degrees and suggested that the distribution may actually be broader. In this work, the symmetric TESS transit light curve of KELT-19Ab is analysed using gravity darkening to measure its true obliquity. Its large sky projected obliquity lambda = -179.7 degrees(+3.7 degrees)(-3.8 degrees) makes KELT-19Ab the only currently known planet with obliquity potentially close to 180 degrees. We apply spectroscopic constraints on vsini and lambda as well as theoretical constraints on the limb-darkening coefficients to find that the KELT-19Ab's obliquity is psi = 155 degrees(+17 degrees)(-21 degrees), in favour of a flipped orbit. The result is consistent with the statistically inferred uniformity of obliquity distribution, and also highlights the applicability of the gravity darkening technique to symmetric light curves.