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Origin of compact exoplanetary systems during disk infall

Presentation #222.06 in the session Exoplanets Formation & Evolution (Poster)

Published onOct 23, 2023
Origin of compact exoplanetary systems during disk infall

A surprising discovery in exoplanet science is the existence of compact systems of Earth to super-Earth sized planets orbiting within 0.01-0.1 AU from their star, a region lacking planets in our Solar System. While compact systems are common, their origin is debated. A prevalent assumption is that compact systems formed after the infall of gas and solids to the circumstellar disk ended. However, observational, theoretical, and meteoritical evidence suggests accretion may commence earlier. We propose that compact systems are surviving remnants of planet accretion during the end stages of infall. The mass of a planet forming during infall is regulated by a balance between growth due to accretion of infalling solids, and inward gas-driven orbital migration that becomes faster as it grows. When planets reach a critical mass, further accretion accelerates inward migration, leading to planetary loss. This balance selects for similarly sized planets (“peas-in-a-pod” type architecture). We show that the planetary system is regulated to contain at any given time a few critical planetary masses, implying a maximum ratio of the total planetary system mass compared to the stellar mass.

This formation scenario is reminiscent of some models for the formation of satellites around giant planets. However, here a key difference is that the circumstellar gas disk lifetime may be substantially longer than the infall timescale, lengthening the time during which planet migration and loss may occur. In some planetary systems, including perhaps our Solar System, planets formed during infall may have ultimately been lost because the gas disk lifetime was long compared to the infall phase. However, for compact disks, we find that the disk lifetimes tend to be relatively short and planets that accrete during infall may survive the subsequent gas disk phase. Using N-body accretion simulations that include the effects of mass infall, planet migration, and gas disk dispersal, we show that the remnant planetary systems have a common mass of a few ×10−5 to 10−4 times the host star’s mass, comparable to the observed compact exoplanetary systems.

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