A paper by IU Astronomy graduate student Armaan Goyal, titled “Statistical Reevaluation of the Ultra-Short-Period Planet Classification Boundary”, has recently been accepted for publication in the Astronomical Journal!
Ultra-short-period planets (USPs), defined as having orbital periods shorter than 1 day, represent a highly unusual class of objects that tend to be small and widely separated from their neighboring worlds. As such, these planets, and the systems they occupy, can help us understand some of the most rare and extreme processes that may occur during the evolution of multi-planet systems!
However, the conventional 1 day boundary used to define the USP population was chosen solely for its convenience nearly two decades ago, and has stood as an entirely arbitrary cutoff ever since. As such, it has long remained to be seen whether this 1 day boundary is truly an appropriate definition for the USP population, or if these exists any alternative boundaries that are perhaps better suited for studying the bizarre properties of these planets.
To this end, Armaan presents in his paper a blind population analysis of 376 multi-planet systems from Kepler, K2, and TESS to search for a physically motivated cutoff for the USP population. Leveraging a variety of statistical techniques, Armaan identifies that planets in multi-planet systems are systematically smaller up to an orbital period of 1 day, but remain widely separated from their neighbors until a period of 2 days!
Armaan's findings not only provide long-awaited confirmation that the 1 day cutoff indeed separates USPs as a physically distinct population, but also uncover evidence that an additional 2 day cutoff as an important marker for understanding the history of planets that lie exceptionally close to their stars. Armaan's results broadly support USP formation theories where planets first experience chaotic inward migration, which rapidly separates them from their neighboring planets, and then lose some of their solid mass due to the extreme temperatures and gravitational pull they experience at their new orbits, which shrinks them to the characteristic small sizes we observe.
This work was conducted in collaboration with IU’s own Professor Songhu Wang. The full paper can be found at: https://iopscience.iop.org/article/10.3847/1538-3881/adb487
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