The distribution of exoplanets with semimajor axes in Figs. 6.4 and 6.5 suggest that exoplanets show key differences with period. The short-period peak has been an important motivation in the development of migration theories for exoplanets (Lin et al. 1996; Murray et al. 1998; Trilling et al. 1998; Ward 1997; Armitage et al. 2002; Trilling, Lunine & Benz 2003). This peak can be explained by a stopping mechanism such as Lindblad resonances between the planet and disk (Kuchner & Lecar 2002) and photoevaporation (Matsuyama, Johnstone & Murray 2003) limiting exoplanet migration to periods of less than 3 days. Once there exists a sample of short-period exoplanets derived from a wider range of primary masses it should
Fig. 6.8. Median spectroscopic [Fe/H] values for the primaries of exoplanets plotted as a function of semimajor axis. The metal-rich Nature of exoplanets is evident from this plot, with no binned data approaching solar metallicity. The dashed line represents all planets above 1 Mjup sin i. The error bars are from y/(number) statistics and are only indicative. No relationship between semi-major axis and metallicity is apparent and thus any metallicity dependence of migration can be expected to be relatively modest.
Fig. 6.9. The plot shows median MstaTQ values for exoplanet host stars against semimajor axis for their exoplanets. The upper plot indicates a relative lack of stars with masses between 0.4 and 0.7 M© around which planets have been found. This is not unexpected since surveys target a greater fraction of 'Solar mass' stars and more recently have also focussed telescope time on the lowest-mass stars observable with current experiments. The lower plot indicates median Mstar0 values, the dashed line represents the sub sample of all planet hosts with above 1 Mjup sin i. No strong relationship between host mass and semi-major axis, which might be a proxy for mass-dependent migration, is indicated. The error bars are from y/ (number) statistics and are only indicative.
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