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.

be possible to distinguish between different stopping mechanisms by comparing the orbital characteristics of the exoplanet with those of the primary. For example, Kuchner & Lecar predict that planets around early A-type main sequence stars will collect at a radius much further from the star (0.3 au) than the radius where solartype stars collect. Further data is required in order to robustly test this hypothesis, e.g., Fig. 6.9. The rise in the number of exoplanets towards longer periods is becoming more apparent as more planets are discovered and is very well reproduced by exoplanet migration scenarios which envisage planets migrating inwards (e.g., Armitage et al. 2002; Trilling et al. 2003) as well as outwards (Masset & Papaloizou 2003).

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