  We present the first investigation of Th abundances in Solar twins and
analogues to understand the possible range of this radioactive element and its
effect on rocky planet interior dynamics and potential habitability. The
abundances of the radioactive elements Th and U are key components of a
planet's energy budget, making up 30% to 50% of the Earth's (Korenaga 2008;
All\`egre et al. 2001; Schubert et al. 1980; Lyubetskaya & Korenaga 2007; The
KamLAND Collaboration 2011; Huang et al. 2013). Radiogenic heat drives interior
mantle convection and surface plate tectonics, which sustains a deep carbon and
water cycle and thereby aides in creating Earth's habitable surface. Unlike
other heat sources that are dependent on the planet's specific formation
history, the radiogenic heat budget is directly related to the mantle
concentration of these nuclides. As a refractory element, the stellar abundance
of Th is faithfully reflected in the terrestrial planet's concentration. We
find that log eps Th varies from 59% to 251% that of Solar, suggesting
extrasolar planetary systems may possess a greater energy budget with which to
support surface to interior dynamics and thus increase their likelihood to be
habitable compared to our Solar System.
