117,462 research outputs found
Thing-makers, Tool Freaks and Prototypers: How the Whole Earth Catalog’s Optimistic Message Reinvented the Environmental Movement in 1968
In the fall of 1968 a Stanford-trained biologist, organizer of the legendary Trips Festival and Merry Prankster named Stewart Brand published the first Whole Earth Catalog. Between 1968 and 1972, the Catalog reached millions of readers and won the National Book Award. The title and iconic cover image of this counterculture classic celebrated the first publicly released NASA photographs showing the whole planet Earth from space. These images profoundly changed the way humans thought about the environment. And the Catalog played an important role in that change
Gas Hydrates as a New Type of Fuel
For millions of years the nature has accumulated a huge amount of mineral
deposits that are successfully used by the humanity as energy sources during many
centuries. But recently, an acute question concerning the length of the period of time
during which such resources as coal, oil and natural gas will be enough for further
development has risen. Many countries, having insignificant reserves of classic kinds
of energy-carriers, are totally dependent on the conjuncture of the energy raw
materials on the world market. Nowadays, according to the opinion supported by a
great majority of the scientists and experts, gas hydrates are considered to be as the
most perspective alternative source of energy on the planet
Reassessing the Source of Long-Period Comets
We present numerical simulations to model the production of observable
long-period comets (LPCs) from the Oort Cloud, a vast reservoir of icy bodies
surrounding the Sun. We show that inner Oort Cloud objects can penetrate
Jupiter's orbit via a largely unexplored dynamical pathway, and they are an
important, if not the dominant, source of known LPCs. We use this LPC
production to place observationally motivated constraints on the population and
mass of the inner Oort Cloud, which are consistent with giant planet formation
theory. These constraints indicate that only one comet shower producing late
Eocene bombardment levels has likely occurred since the Cambrian Explosion,
making these phenomena an improbable cause of additional extinction events.Comment: Originally published in Science (9/4/09), 30 pages, 9 figures, main
article and online material combine
Habitable Zone Lifetime of Exoplanets around Main Sequence Stars
Funding: Dean's Scholarship at the University of East Anglia.The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star’s main sequence lifetime. We describe the time that a planet spends within the HZ as its ‘‘habitable zone lifetime.’’ The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the ‘‘classic’’ HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a ‘‘hybrid’’ HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79 · 109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns.Publisher PDFPeer reviewe
The non-resonant, relativistic dynamics of circumbinary planets
We investigate the non-resonant, 3-D (spatial) model of the hierarchical
system composed of point-mass stellar (or sub-stellar) binary and a low-mass
companion (a circumbinary planet or a brown dwarf). We take into account the
leading relativistic corrections to the Newtonian gravity. The secular model of
the system relies on the expansion of the perturbing Hamiltonian in terms of
the ratio of semi-major axes , averaged over the mean anomalies. We
found that the low-mass object in a distant orbit may excite large eccentricity
of the inner binary when the mutual inclination of the orbits is larger than
about of 60 deg. This is related to strong instability caused by a phenomenon
which acts similarly to the Lidov-Kozai resonance (LKR). The secular system may
be strongly chaotic and its dynamics unpredictable over the long-term time
scale. Our study shows that in the Jupiter-- or brown dwarf-- mass regime of
the low-massive companion, the restricted model does not properly describe the
long-term dynamics in the vicinity of the LKR. The relativistic correction is
significant for the parametric structure of a few families of stationary
solutions in this problem, in particular, for the direct orbits configurations
(with the mutual inclination less than 90 degrees). We found that the dynamics
of hierarchical systems with small may be qualitatively
different in the realm of the Newtonian (classic) and relativistic models. This
holds true even for relatively large masses of the secondaries.Comment: 18 pages, 17 figures, accepted to Monthly Notices of the Royal
Astronomical Societ
Are the Kepler Near-Resonance Planet Pairs due to Tidal Dissipation?
The multiple-planet systems discovered by the Kepler mission show an excess
of planet pairs with period ratios just wide of exact commensurability for
first-order resonances like 2:1 and 3:2. In principle, these planet pairs could
have both resonance angles associated with the resonance librating if the
orbital eccentricities are sufficiently small, because the width of first-order
resonances diverges in the limit of vanishingly small eccentricity. We consider
a widely-held scenario in which pairs of planets were captured into first-order
resonances by migration due to planet-disk interactions, and subsequently
became detached from the resonances, due to tidal dissipation in the planets.
In the context of this scenario, we find a constraint on the ratio of the
planet's tidal dissipation function and Love number that implies that some of
the Kepler planets are likely solid. However, tides are not strong enough to
move many of the planet pairs to the observed separations, suggesting that
additional dissipative processes are at play.Comment: 20 pages, including 7 figures; accepted for publication in Ap
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