SETI Institute Weekly Colloquium - Upcoming Speakers
The habitable zone (HZ) is the region around a star in which liquid water could exist on a planetary surface. Although most HZ studies have focused on the main-sequence period, here we argue that the pre-main-sequence HZ likely provides additional targets for observers. The spatial distribution of liquid water and its change during the pre-main-sequence phase of protoplanetary systems is important in understanding how planets become habitable.
Such worlds are interesting targets for future missions because the coolest stars could provide habitable conditions for up to 2.5 billion years post-accretion. Moreover, for a given star type, planetary systems are more easily resolved because of higher pre-main-sequence stellar luminosities, resulting in larger planet-star separation for cool stars than is the case for the traditional main-sequence (MS) habitable zone (HZ). Using 1-D radiative-convective climate and stellar evolutionary models, we calculate pre-main-sequence HZ distances for F1-M8 stellar types. We also show that accreting planets that are later located in the traditional MS HZ orbiting stars cooler than a K5 (including the full range of M-stars) receive stellar fluxes that exceed the runaway greenhouse threshold, and thus may lose substantial amounts of water initially delivered to them.
We predict that M-star planets need to initially accrete more water than Earth did or, alternatively, have additional water delivered afterwards to remain habitable. Our findings are also consistent with recent claims that Venus lost its water during accretion.
The Giant Impact theory is the leading explanation for the Moon's origin, but mysteries remain in the conditions leading up to the event. Collisions were common during the turbulent infancy of the Solar System and led to a small set of terrestrial planets. Dr. Quarles presents a numerical model that considers the penultimate orbits of the Solar System, when five terrestrial planets are present. From this model, he indicates which starting parameters for Theia (the proto-Moon) result in a late Giant Impact consistent with physical dating constraints. He also finds that the likely semimajor axis of Theia, at the epoch when the simulations begin, depends on the assumed mass ratio of the Earth-Moon progenitors (8/1, 4/1, or 1/1). The low eccentricities of the resulting terrestrial planets are most commonly produced when the progenitors have similar semimajor axes at the epoch when the model starts. Additionally, Dr. Quarles will show that perturbations from the giant planets can affect the dynamical evolution of the system leading to a late Moon Forming Giant Impact.
Hydrothermal fields on the prebiotic Earth are candidate environments for biogenesis. We propose a model in which molecular systems driven by cycles of hydration and dehydration in such sites undergo chemical evolution and selection in a dehydrated surface phase followed by encapsulation and combinatorial selection in a hydrated phase. This model is partly supported by recent science, and lies partly in the realm of speculation including a hypothesized pathway for the parallel evolution of the functional machinery of life. Complex models like this present challenges for science in the 21st century and we will describe a new technology to enable the simulation of such models.