Predicting Planets in Known Extra-Solar Planetary Systems II: Testing for Saturn-mass Planets

Recent results have shown that many of the known extrasolar planetary systems contain regions which are stable for massless test particles. We examine the possibility that Saturn-mass planets exist in these systems, just below the detection threshold, and attempt to predict likely orbital parameters for such unseen planets. To do this, we insert a Saturn-mass planet into the stable regions of these systems and integrate its orbit for 100 million years. We conduct 200-600 of these experiments to test parameter space in HD37124, HD38529, 55Cnc, and HD74156. In HD37124 the global maximum of the survival rate of Saturns in parameter space is at semimajor axis a = 1.03 AU, eccentricity e=0.1. In HD38529, only 5% of Saturns are unstable, and the region in which a Saturn could survive is very broad, centered on 0.5<a<0.6, e<0.2. In 55Cnc we find three maxima at (a,e) = (1.0 AU, 0.02), (2.0 AU, 0.08), and (3.0 AU, 0.17). In HD74156 we find a broad maximum with $a$ = 0.9-1.2 AU, e<=0.15. Several of these maxima are located in the habitable zones of their parent stars and are therefore of astrobiological interest. We suggest the possibility that companions may lie in these locations of parameter space, and encourage further observational investigation of these systems.Comment: submitted to ApJ 9 pages, 8 figures, 3 table

Vega's hot dust from icy planetesimals scattered inward by an outward-migrating planetary system

Vega has been shown to host multiple dust populations, including both hot exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100 AU. We use dynamical simulations to show how Vega's hot dust can be created by long-range gravitational scattering of planetesimals from its cold outer regions. Planetesimals are scattered progressively inward by a system of 5-7 planets from 30-60 AU to very close-in. In successful simulations the outermost planets are typically Neptune-mass. The back-reaction of planetesimal scattering causes these planets to migrate outward and continually interact with fresh planetesimals, replenishing the source of scattered bodies. The most favorable cases for producing Vega's exo-zodi have negative radial mass gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized planet exists beyond ~15 AU because the planet preferentially ejects planetesimals before they can reach the inner system. Direct-imaging planet searches can therefore directly test this mechanism.Comment: Updated references. Accepted to MNRAS Letters. 5 pages, 4 figures. Blog post about the paper at http://planetplanet.net/2014/03/31/vega-a-planetary-poem

Orbital Dynamics of Multi-Planet Systems with Eccentricity Diversity

Since exoplanets were detected using the radial velocity method, they have revealed a diverse distribution of orbital configurations. Amongst these are planets in highly eccentric orbits (e > 0.5). Most of these systems consist of a single planet but several have been found to also contain a longer period planet in a near-circular orbit. Here we use the latest Keplerian orbital solutions to investigate four known systems which exhibit this extreme eccentricity diversity; HD 37605, HD 74156, HD 163607, and HD 168443. We place limits on the presence of additional planets in these systems based on the radial velocity residuals. We show that the two known planets in each system exchange angular momentum through secular oscillations of their eccentricities. We calculate the amplitude and timescale for these eccentricity oscillations and associated periastron precession. We further demonstrate the effect of mutual orbital inclinations on the amplitude of high-frequency eccentricity oscillations. Finally, we discuss the implications of these oscillations in the context of possible origin scenarios for unequal eccentricities.Comment: 12 pages, 9 figures, accepted for publication in the Astrophysical Journa

High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability

The water content and habitability of terrestrial planets are determined during their final assembly, from perhaps a hundred 1000-km "planetary embryos" and a swarm of billions of 1-10 km "planetesimals." During this process, we assume that water-rich material is accreted by terrestrial planets via impacts of water-rich bodies that originate in the outer asteroid region. We present analysis of water delivery and planetary habitability in five high-resolution simulations containing about ten times more particles than in previous simulations (Raymond et al 2006a, Icarus, 183, 265-282). These simulations formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five planets in the habitable zone. Every planet from each simulation accreted at least the Earth's current water budget; most accreted several times that amount (assuming no impact depletion). Each planet accreted at least five water-rich embryos and planetesimals from past 2.5 AU; most accreted 10-20 water-rich bodies. We present a new model for water delivery to terrestrial planets in dynamically calm systems, with low-eccentricity or low-mass giant planets -- such systems may be very common in the Galaxy. We suggest that water is accreted in comparable amounts from a few planetary embryos in a "hit or miss" way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects such as planetary mass and location, and giant planet properties.Comment: Astrobiology, in pres

Small firms, global economies: The economic sociology of the northwest Atlantic sea urchin industry

This dissertation examines the organizational dynamics of firms operating in the East Coast sea urchin industry. First, I examine the confluence of economic and political conditions under which the industry evolved. As a part of the export driven growth of the past decade in the US the East Coast sea urchin industry benefited from political conditions, which encouraged development of global markets for US products. Along with this, the Japanese have displayed a seemingly insatiable demand for sea urchin roe through the 1990s. In 1971 the governance of the international monetary system changed from a fixed to floating exchange rate. The yen\u27s value began to increase compared to other currencies making export of products to Japan financially viable. These economic and political conditions set the stage for the East Coast sea urchin industry. To explain how the industry developed I draw on theory and research from Institutional Economics and Economic Sociology to address entrepreneurial processes, labor market processes, and exchange processes. A supply of sea urchins existed on the East Coast as the Japanese demand grew, but the link between supply and demand remained unfilled. Entrepreneurs in the industry established firms to link supply and demand. With a link between supply and demand established, firms were faced with mobilizing a labor force for the new industry. Labor market processes include the recruitment of workers by entrepreneurs and the development of skills and technologies for completing essential activities within the productive system. With the recruitment of labor it becomes necessary to coordinate the essential activities of the productive system. Participants coordinate exchange within a productive system through hierarchies, networks or markets. As the East Coast sea urchin industry was established and evolved these three processes (entrepreneurial, labor market, and exchange) combined to facilitate establishment and growth of the new industry. The entrepreneurs that moved into the new industry attempted to increase the size of their firms and the amount of production. Existing production arrangements on the working waterfront provided access to the natural resource, flexible technologies, an existing knowledge base, and unique social arrangements that allowed the easy movement of a labor force into the new industry. Finally, a market form of organizing exchange developed with an institutionalized distrust between participants. The case of the East Coast sea urchin industry provides an opportunity to examine the sociology of economic life. Its position within a global economy, and its economic boom characteristics provide a valuable empirical case study in the growing field of economic sociology. (Abstract shortened by UMI.)

Terrestrial Planet Formation Constrained by Mars and the Structure of the Asteroid Belt

Reproducing the large Earth/Mars mass ratio requires a strong mass depletion in solids within the protoplanetary disk between 1 and 3 AU. The Grand Tack model invokes a specific migration history of the giant planets to remove most of the mass initially beyond 1 AU and to dynamically excite the asteroid belt. However, one could also invoke a steep density gradient created by inward drift and pile-up of small particles induced by gas-drag, as has been proposed to explain the formation of close-in super Earths. Here we show that the asteroid belt's orbital excitation provides a crucial constraint against this scenario for the Solar System. We performed a series of simulations of terrestrial planet formation and asteroid belt evolution starting from disks of planetesimals and planetary embryos with various radial density gradients and including Jupiter and Saturn on nearly circular and coplanar orbits. Disks with shallow density gradients reproduce the dynamical excitation of the asteroid belt by gravitational self-stirring but form Mars analogs significantly more massive than the real planet. In contrast, a disk with a surface density gradient proportional to $r^{-5.5}$ reproduces the Earth/Mars mass ratio but leaves the asteroid belt in a dynamical state that is far colder than the real belt. We conclude that no disk profile can simultaneously explain the structure of the terrestrial planets and asteroid belt. The asteroid belt must have been depleted and dynamically excited by a different mechanism such as, for instance, in the Grand Tack scenario.Comment: Accepted for publication in MNRA