Retracing the Sun's path through the Galaxy to assess astronomical impacts on the solar system

The geological record shows there has been significant variation in both the Earth's climate and in the rates of biological species generation and extinction over the past 545 Myr (the Phanerozoic eon). Some of these variations can probably be explained in terms of internal cycle dynamics. However, it has been suggested by several authors that there may be a significant influence from astronomical sources. As the Sun orbits through the Galaxy it probably passes close to star forming regions, giant molecular clouds and the spiral arms. It also oscillates up and down through the Galactic disk. The Earth is therefore exposed to a varying environment and it has been proposed that the increased supernovae rate expected from increased proximity to massive star forming regions, and the resulting intense gamma ray and cosmic ray fluxes, could have a significant impact on the Earth's climate and so indirectly on life. Likewise, gravitational torques from giant molecular clouds could perturb the Oort cloud and send showers of comets into the inner solar system, with potentially devastating consequences for life on Earth.

Some researchers have even identified supposedly significant periodicities in the climate and/or extinction records leading them to suggest that periodic Galactic plane or spiral arm crossings on timescales of tens of Myr are responsible. While there is solid evidence for massive impacts on the Earth, and strong evidence linking at least one of these to a mass extinction, evidence for periodicity is far weaker. These are controversial suggestions, but the ideas are not implausible. Yet one of the weakest links in these arguments is knowledge of the changing environment the solar system has experienced over the past half billion years (approximately two orbits about the Galaxy).

Given accurate knowledge of the current position and velocity of the solar system, plus a time-dependent model of the mass distribution in the Galaxy, it is in principle possible to reconstruct the path of the Sun. (The Galaxy can be approximated as a collisionless system over some timescale, so this "unwinding" does not violate the second law of thermodynamics.) Some authors have done this, but generally with very simple models, and some have drawn rather selective conclusions from comparison with the data.

The goal of this project is to construct plausible models of the Galaxy, which include star forming regions and the spiral arms, and to use numerical integration to trace back the path of the Sun. There is obviously a vast number of variations on the Galaxy model according to the shapes of the arms, the number and location of clouds, the exact density variations etc. and any one model could easily be made to fit the relatively sparse geological and climate data. The objective, therefore, is to look at the problem statistically in order to examine how probable it is that the Sun could have been exposed to certain environments and with what frequency/quasi-periodicity. The ultimate goal is to assess and quantify the plausibility of one or more astronomical mechanisms of climate change and biological variation.

This project will involve not only modelling the Galaxy and the Sun's motion, but also the mechanisms by which supernovae, Galactic torques etc. could affect the solar system. We will also investigate the sensitivity of the conclusions to the accuracy of the phase space data on the Sun. The project will build the models and lay the essential ground work for a later, more comprehensive investigation possible with the results from the upcoming Gaia mission. (By measuring accurate 3D positions and 3D velocities for hundreds of millions of stars, Gaia will much better constrain the mass distribution in the Galaxy as well as the current coordinates and velocity of the Sun.)

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