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Introduction
The first confirmed discoveries of directly imaged exoplanets were announced in late 2008 (Kalas et al. 2008; Marois et al. 2008). Of the two directly imaged systems announced at that time, the planets of HR8799 evoked the most excitement. Near-infrared imaging of the system revealed a family of three giant planets, each believed to be significantly more massive than Jupiter (see Fig. 1 from Marois et al. 2008, hereafter M08). Since their discovery, there have been extensive discussions of both the observed properties of the planets (Gozdziewski & Migaszewski 2009; Fabrycky & Murray-Clay 2010) and the accompanying dust disc (Reidemeister et al. 2009; Su et al. 2009), which was first discovered by the Infrared Astronomical Satellite (IRAS) in 1983 (Rhee et al. 2007).
Fig. 1.
Image of the three planets orbiting HR8799 (position marked by white cross) produced from Keck Ks band near infrared images (taken from M08).
A number of factors can influence planetary habitability, as discussed in the review by Horner & Jones (2010), elsewhere in these proceedings. One important prerequisite for the development of life is the long-term stability of the planetary system within which a habitable exoplanet resides. There are currently over 40 known multiple exoplanet systems1 (such as the five planets around the multiple star 55 Cancri, the three around Upsilon Andromedae and the three around Gliese 876), and the fine details of the dynamical evolution of such systems will undoubtedly play a pivotal role in determining the habitability of any as yet undetected telluric worlds within the classical 'habitable zone' of those systems (Jones et al. 2006). A stable climate and relatively quiescent impact regime are thought to be important prerequisites for the development of life, and both these factors depend upon the stability of the planetary system in question. To illustrate this, let us provide an example from the history of our own Solar System, which highlights the effect instabilities can have on the impact flux through the habitable zone.
Despite the current stability of the Solar System, evidence of much higher impact rates from the cratering record of planetary (and lunar) surfaces imply a more turbulent past. In particular, there is a significant weight of evidence that the...