Against all odds? Forming the planet of the HD196885 binary

HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose orbit places it at the limit for orbital stability. The presence of a planet in such a highly perturbed region poses a clear challenge to planet-formation scenarios. We investigate this issue by focusing on the planet-formation stage that is arguably the most sensitive to binary perturbations: the mutual accretion of kilometre-sized planetesimals. To this effect we numerically estimate the impact velocities $dv$ amongst a population of circumprimary planetesimals. We find that most of the circumprimary disc is strongly hostile to planetesimal accretion, especially the region around 2.6AU (the planet's location) where binary perturbations induce planetesimal-shattering $dv$ of more than 1km/s. Possible solutions to the paradox of having a planet in such accretion-hostile regions are 1) that initial planetesimals were very big, at least 250km, 2) that the binary had an initial orbit at least twice the present one, and was later compacted due to early stellar encounters, 3) that planetesimals did not grow by mutual impacts but by sweeping of dust (the "snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab was formed not by core-accretion but by the concurent disc instability mechanism. All of these 4 scenarios remain however highly conjectural.Comment: accepted for publication by Celestial Mechanics and Dynamical Astronomy (Special issue on EXOPLANETS

Soil and Freshwater and Marine Sediment Food Webs: Their Structure and Function

The food webs of terrestrial soils and of freshwater and marine sediments depend on adjacent aboveground or pelagic ecosystems for organic matter input that provides nutrients and energy. There are important similarities in the flow of organic matter through these food webs and how this flow feeds back to primary production. In both soils and sediments, trophic interactions occur in a cycle in which consumers stimulate nutrient cycling such that mineralized resources are made available to the primary producers. However, aquatic sediments and terrestrial soils differ greatly in the connectivity between the production and the consumption of organic matter. Terrestrial soils and shallow aquatic sediments can receive organic matter within hours of photosynthesis when roots leak carbon, whereas deep oceanic sediments receive organic matter possibly months after carbon assimilation by phytoplankton. This comparison has implications for the capacity of soils and sediments to affect the global carbon balance.