The First Planets

Both observations and theory indicate that there is a lower heavy element abundance threshold for planet formation at around 0.1 of solar ([Fe/H] of ~-1.0).   The known planet-host stars in this low metallicity range are overwhelmingly members of the old and dynamically excited galactic thick-disk and halo. These old galactic stellar populations are characterized by an enhanced ratio of alpha-capture elements (O, Ne, Mg Si, S, Ar and Ca) to Fe compared to the equivalent solar ratios.  The terrestrial planets formed from this alpha-element rich refractory material should have much larger mantle (rock) mass fractions and smaller Ni-Fe core mass fractions than the inner planets in our solar system.  The galactic production of the dominant radiogenic nuclei in planetary interiors (40K, 232Th, 235U and 238U) scales approximately with the alpha-capture elements.  When coupled with the lithophile nature of these radiogenic nuclei, this should give these first terrestrial planets enhanced interior heating compared with our solar system and with planets now being formed around thin-disk stars.   We explore the long-term thermal evolution of these first planets using a planetary interior thermal evolution code that follows the heat production and heat flow over the life of the galaxy.  The program goal is to determine whether these planets were able to develop and maintain conditions for habitability during their long lifetimes.