pre main sequence HZ even if located in the HZ today should be ranked lower

Pre main sequence hz even if located in the hz today

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pre-main-sequence HZ, even if located in the HZ today, should be ranked lower than a planet that has always been inside the HZ. Moreover, if dense CO 2 –CH 4 atmospheres near the outer edge of hotter stars are suggestive of inhabitance [ 2 ], we should observe that a relatively large fraction of these planets are inhabited, as explained in Sections 7.2 and 7.3 . Supposing that an observable pCO 2 gradient from the inner and outer edge exists, we could infer that the outer edge distance is smaller in M-star systems that contain planets with dense CO 2 –CH 4 atmospheres [ 2 ]. Also, the H 2 in dense CO 2 atmospheres is likely to be of volcanic [ 132 ] rather than primordial [ 129 ] origin. Alternatively, if H 2 dominates the planetary atmosphere, then the source is probably primordial instead. Further, if we were to find big terrestrial planets with dense CO 2 –H 2 atmospheres, then we may conclude that they must have very high volcanic outgassing rates, low escape rates, or potent magnetic fields (ibid). Plus, higher scale heights for hydrogen-rich atmospheres can distinguish them from other types of rocky planets ([ 129 , 132 , 152 ]). Thus, the point here is that many planetary processes can be inferred simply by considering the implications of observations as predicted by HZ theory. Finally, ocean worlds are predicted to require fast rotation rates in order to sustain the equator-pole temperature gradients necessary to support life [ 216 ]. Thus, planets with high rotation rates that are located near the classical outer edge should be investigated for this type of planet (ibid). Plus, the resultant dense CO 2 atmospheres would have low scale heights that are easily distinguished from planets with fluffier H/He envelopes. If sufficiently water-rich, these worlds would also exhibit low bulk densities that can be used to distinguish them from other types of rocky planets (ibid). An example of how different HZ definitions can be used to complement the classic one is given in Figure 15 . Geosciences 2018 , 8 , x FOR PEER REVIEW 35 of 48 Figure 15. Sample flow chart using the classical HZ, along with CO 2 –H 2 , CO 2 –CH 4 , and pre-main- sequence HZ extensions to assess the potential habitability of planets. End states are in yellow. 16. Conclusions To conclude, the HZ is a tool for finding potentially habitable planets. Over the years, it has evolved as an aid in finding potentially habitable (albeit exotic) planets with CO 2− , CH 4− , and even H 2 -rich atmospheres; potentially life-bearing worlds around white dwarfs; ocean worlds; desert worlds; planets around red giant stars; worlds around pre-main-sequence stars; planets orbiting A-stars; and even worlds orbiting binary stars. The HZ can be utilized to rank potentially habitable planets in Figure 15. Sample flow chart using the classical HZ, along with CO 2 –H 2 , CO 2 –CH 4 , and pre- main-sequence HZ extensions to assess the potential habitability of planets. End states are in yellow.
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Geosciences 2018 , 8 , 280 36 of 48 16. Conclusions To conclude, the HZ is a tool for finding potentially habitable planets. Over the years, it has
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