One of the biggest modern discoveries about the solar system is that IT is chaotic (laskar 1989, 1994). On million year timescales, nearby trajectories exponentially diverge; on billion year…

This is a $333,226 grant awarded by the National Aeronautics and Space Administration (NASA) to the University of Chicago on January 1, 2017 for research on "Gigayear Instabilities in Planetary Systems". The research will focus on studying the long-term dynamical stability of exoplanetary systems, including investigating the potential for instability on billion-year timescales that could impact planetary formation and evolution. The project will use numerical simulations and analytical methods to characterize the chaotic behavior of planetary systems, with a focus on the most populous known category of exoplanets - close-in super-Earths and Neptunes. The work will also explore scenarios where planets may become stranded in distant clouds before destabilizing the inner system later on, as well as study the impact of stellar companions on planetary system stability, using the Alpha Centauri system as a case study. This grant does not have a set-aside designation.

One of the biggest modern discoveries about the solar system is that IT is chaotic (laskar 1989, 1994). On million year timescales, nearby trajectories exponentially diverge; on billion year timescales, planets can develop large eccentricities and even collide. This is possible because our planets interact with enough energy and with the right secular timescales. This has the potential to put the planet mercury on an unstable orbit in the future, before the sun exhausts its fuel. Currently, as a standard step in the analysis, exoplanet observing teams check whether the planetary systems they are discovering are stable. This usually involves a few megayear numerical integration, and the system usually passes that test. However, the signatures of continuing instability have not been looked for in the exoplanet population, nor has its implications for planetary formation and evolution been fully recognized. We will study several specific evolutionary scenarios in which instability may manifest only on gigayear timescales, i.e. midway through the lives of the host STARS. This is relevant to the solicitation in that IT characterizes the dynamics of exoplanetary systems. In the first project, we will compare n body, numerically calculated secular, and fourier expansion secular theories to determine what essential ingredients go into the conclusion that a general planetary system is chaotic. We will apply these tools to specific realizations of kepler discovered close in planetary systems consisting of three or more neptunes or super earths, which is the most populous known exoplanet population. We will thus find the common ailments afflicting middle age planetary systems. In the second project, we will consider how planets might get stranded in their kuiper and oort clouds during early system evolution, only to destabilize the inner system later on. Various investigators have wondered whether the solar system is accompanied by a massive planetary companion, including a prominent proposal of one earlier this year. We will use n body and analytic secular methods to study planet scattering in the context of stellar birth clusters. External torques may lift the periastra of scattering planets, placing them in the comet cloud, and we will calculate the probability for such planets to return on gigayear timescales. In a final project, we will set our gaze on the alpha centauri system. As is well known, IT is a triple stellar system with a very distant third star proxima centauri. Observers continue to debate whether IT hosts a planet and whether or not a space mission should be dedicated to this system. As theorists, we propose to calculate the probability that proxima has, or will, substantially alter the orbit of the binary, and what dynamical effect this has on a potential planetary system. If the binary was more circular in the past, then planet formation was easier around each star. If the binary is getting more eccentric now, IT can destabilize its planetary system, even at this late date. This project will also use n body and analytic secular methods.