Coupled radiative-convective/photochemical modeling was performed for Earth-like planets orbiting different types of stars (the Sun as a G2V, an F2V, and a K2V star). O2 concentrations between 1 and 10-5 times the present atmospheric level (PAL) were simulated. The results were used to calculate visible/near-IR and thermal-IR spectra, along with surface UV fluxes and relative dose rates for erythema and DNA damage. For the spectral resolution and sensitivity currently planned for the first generation of terrestrial planet detection and characterization missions, we find that O2 should be observable remotely in the visible for atmospheres containing at least 10-2 PAL of O2. O3 should be visible in the thermal-IR for atmospheres containing at least 10-3 PAL of O2. CH4 is not expected to be observable in 1 PAL O2 atmospheres like that of modern Earth, but it might be observable at thermal-IR wavelengths in “mid-Proterozoic-type” atmospheres containing ~ 10-1 PAL of O2. Thus, the simultaneous detection of both O3 and CH4 – considered to be a reliable indication of life – is within the realm of possibility. High-O2 planets orbiting K2V and F2V stars are both better protected from surface UV radiation than is modern Earth. For the F2V case the high intrinsic UV luminosity of the star is more than offset by the much thicker ozone layer. At O2 levels below ~ 10-2 PAL, planets around all three types of stars are subject to high surface UV fluxes, with the F2V planet exhibiting the most biologically dangerous radiation environment. Thus, while advanced life is theoretically possible on high-O2 planets around F stars, it is not obvious that it would evolve as it did on Earth.
We present results of numerical simulations that examine the dynamical stability of known planetary systems, a star with two or more planets. First we vary the initial conditions of each system on the basis of observational data. We then determine regions of phase space that produce stable planetary configurations. For each system we perform 1000 ~ 106 yr integrations. We examine υ And, HD 83443, GJ 876, HD 82943, 47 UMa, HD 168443, and the solar system. We find that the resonant systems, two planets in a first-order mean motion resonance (HD 82943 and GJ 876) have very narrow zones of stability. The interacting systems, not in first-order resonance, but able to perturb each other (υ And, 47 UMa, and the solar system), have broad stable regions. The separated systems, two planets beyond 10 : 1 resonance (we examine only HD 83443 and HD 168443) are fully stable. We find that the best fits to the interacting and resonant systems place them very close to unstable regions. The boundary in phase space between stability and instability depends strongly on the eccentricities and (if applicable) the proximity of the system to perfect resonance. Furthermore, we also find that the longitudes of periastron circulate in chaotic systems but librate in regular systems. In addition to 106 yr integrations, we also examined stability on ~108 yr timescales. For each system we ran ~10 long-term simulations, and find that the Keplerian fits to these systems all contain configurations that are regular on this timescale.
Here, we make use of 110 publicly available complete genome sequences to understand how the core components of nitrogenase, including NifH, NifD, NifK, NifE, and NifN proteins, have evolved. These genes are universal in nitrogen fixing organisms-typically found within highly conserved operons-and, overall, have remarkably congruent phylogenetic histories. Additional clues to the early origins of this system are available from two distinct clades of nitrogenase paralogs: a group composed of genes essential to photosynthetic pigment biosynthesis and a group of uncharacterized genes present in methanogens and in some photosynthetic bacteria. We explore the complex genetic history of the nitrogenase family, which is replete with gene duplication, recruitment, fusion, and horizontal gene transfer and discuss these events in light of the hypothesized presence of nitrogenase in the last common ancestor of modern organisms, as well as the additional possibility that nitrogen fixation might have evolved later, perhaps in methanogenic archaea, and was subsequently transferred into the bacterial domain.