Earths life forms at the current oxygen concentration they could be

Earths life forms at the current oxygen concentration

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Earth’s life forms at the current oxygen concentration, they could be debilitating, even fatal, at higher pressures (e.g., [ 125 ]). Plus, wildfires on vegetated planets may limit O 2 abundance and act as a negative feedback when O 2 levels rise too high [ 126 , 127 ]. Nevertheless, aerotolerant anaerobes on Earth employ fermentation to produce adenosine triphosphate (ATP), which is a complex organic chemical, and are not negatively affected by the presence or absence of oxygen (e.g., [ 128 ]). Thus, the maximum oxygen pressure level sustainable by alien life is unknown.
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Geosciences 2018 , 8 , 280 13 of 48 6. Expanding the Spectral Range of the Classical HZ Given the various uncertainties in our current understanding (as explained above), recent work has instead adopted a more liberal view of the HZ (e.g., [ 2 , 51 , 129 132 ]), expanding it to include the detection of life beyond that suggested by the classical definition. As explained in the next section, that would also include atmospheres consisting of reduced gases like CH 4 or H 2 , ocean worlds, desert worlds, and even planets orbiting stars that are still in the pre-main-sequence (or post-main-sequence) phase of stellar evolution, among many more possibilities that will be discussed later. Previous calculations of the classical HZ had only computed boundaries for stars with main-sequence lifetimes >~1 or 2 Ga (F–M spectral classes) [ 1 , 26 ]. However, some argue (e.g., [ 2 , 130 ]) that such an HZ (e.g., [ 1 ]) may still be overly-pessimistic. For instance, we know that microbial life on our planet arose much more quickly than that (~700 Myr; [ 133 ]). Thus, main-sequence stars that last at least 700 Myr (which include A-stars with T EFF up to ~10,000 K) should be included in HZ definitions (e.g., [ 2 , 130 , 131 ]). After all, if a planet can sustain simple life, it is habitable (by definition), as originally defined by Kasting [ 1 ]. Again, it is possible that life emerges even more quickly than 700 Myr on other planets. For instance, evidence in zircons suggests that conditions on Earth may have been habitable by 4.3–4.4 Ga [ 74 ]. Overall, our own planet exhibits evidence that life can arise at least as quickly as 700 Myr, if not quicker. Classical HZ boundaries were originally computed for stars of T EFF = 2600–7200 (F–M stars) using the following fourth-order parameterization of the stellar effective flux ( S EFF ) [ 26 ]: S EFF = S sun + a · T * + b · T * 2 + c · T * 3 + d · T * 4 , (6) where T* = ( T EFF - 5780) and S sun is the S EFF value for a given HZ limit in our solar system. The quantities (a,b,c,d) are constants. However, the spectral range has recently been expanded to include A-stars (10,000 K) [ 2 ]. This extended classical CO 2 –H 2 O HZ, along with updated constants for Equation (6) (Table 1 ), is shown in Figure 5 . Geosciences 2018 , 8 , x FOR PEER REVIEW 13 of 48 worlds, and even planets orbiting stars that are still in the pre-main-sequence (or post-main-sequence) phase of stellar evolution, among many more possibilities that will be discussed later.
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