Astrophysical impacts on habitable planetary systems

Sammanfattning: Finding life on a planet other than the Earth would easily be one of the greatest discoveries of all time, particularly so if it is found outside our Solar System. To do so, we must understand the conditions required for life to emerge and develop. Whilst an enormous amount of research has gone into understanding the requirements for life on Earth to have spontaneously arisen and then evolved, much less focus has been put on how factors outside the Earth and even outside the Solar System may affect the so-called habitability of the Earth. This thesis summarizes three papers in which I have studied different astrophysical effects that can impact the habitability of planets. In Paper I, I investigate how the habitability of planets is affected by the Galactic orbit of their host star by considering how often they encounter giant molecular clouds. Paper I shows that for the Sun these encounters and potential extinction events due to core-collapse supernovae happen roughly once per billion years. In Paper II, I consider how planet-planet scattering between gas giants following an orbital instability can wipe out habitable planets in a system. Paper II shows that in any exoplanet system where there is an observed close-in eccentric giant, it will be very unlikely to find any habitable planet. In Paper III, I examine how a system can be made uninhabitable if the host star picks up an additional planet during a close encounter with another star in its birth cluster. Paper III shows that this can happen either by the new planet ending up on an orbit that destabilizes the existing planets in the system, or more interestingly, by significantly increasing the impact rate of large asteroids for billions of years.