Astrophysics Seminar: Alexander Thomas, University of Notre Dame

One of the key results of the Kepler mission is that many exoplanets appear to form in systems of similarly sized and similarly spaced planets. However, there is a class of systems that have similarly sized planets, yet show gaps in their spacing; there are also systems where planet sizes are diverse. In light of these cases of apparent diversity, one might wonder the extent to which “missing” (or failing to detect) a planet can skew our interpretation of the system architecture. We address this question with a simple experiment: starting from a large, homogeneous catalog, we remove planets and monitor how several well-defined metrics of the system architecture change. We first perform this test on a catalog of observed exoplanets. We then repeat our test on a catalog of synthetic planetary systems with underlying hyper-parameters that have been fit to reproduce the observed systems as faithfully as possible (though imperfectly). For both samples, we find that the failure to detect one or more planets tends to create more irregularly spaced planets, whereas the planet mass similarity and coplanarity are essentially unaffected. Our findings support the interpretation that planets in the same system have similar sizes and regular spacing and reinforce the need to develop an underlying model of planetary architectures that reproduces these observed patterns. Having established that failure to detect a planet most dramatically affects the apparent gap complexity of a system, we consider several explanations for the apparent gaps, including (1) the possibility of undetected planets in the gaps, (2) the possibility of truly empty gaps, and (3) the role of outer giant planets in structuring apparent gaps in inner systems, whether or not the gaps are truly empty.