Galactic Habitable Zones and Possibility of Carbon-Water Based Life
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Galactic habitable zones have much significance if we are searching a habitable planet to colonize or even we want to contact carbon water based intelligence.
The centerpiece of all life on Earth is carbon-based biochemistry. It has repeatedly been surmised that biochemistry based on carbon may also play a pivotal role in extraterrestrial life forms, if existent. This is due to the pronounced advantages of carbon, especially compared to its closest competitor (i.e., silicon)
, which include: its relatively high abundance, its bonding properties, and its ability to form very large molecules as it can combine with hydrogen and other molecules as, e.g., nitrogen and oxygen in a very large number of ways (Goldsmith & Owen 2002). In a paper by M. Cuntz and L. Gurdemir, they explored the relative damage to carbon-based macro-molecules in the environments of stars other than the Sun using DNA
as a proxy. They focused on the effects of photospheric radiation from main-sequence stars
, encompassing the range between F0 and M0. Their models consist of the following components:
1. The radiative effects on DNA are considered by applying a DNA action spectrum (Horneck 1995). It shows that the damage is strongly wavelength-dependent, increasing by more than seven orders of magnitude between 400 and 200 nm. The different regimes are commonly referred to as UV-A, UV-B, and UV-C. As wavelenth would decrease consequently its energy would increase. So it seems pretty obvious that it would cause more damage to DNA strands.
2.The planets are assumed to be located in the stellar habitable zone (HZ). Following the concepts by Kasting et al. (1993) and Underwood et al. (2003), they distinguished between the conservative and generalized HZ. The inner and outer edge of the conservative HZ are given by the onset of water loss and CO2 condensation, respectively, whereas the inner and outer edge of the generalized HZ are given by the runaway greenhouse effect and the breakdown of greenhouse heating, respectively, needed to permit the existence of fluid water on the planetary surface.
3. Stellar photospheric radiation is represented by using realistic spectra, which take into account millions or hundred of millions of lines for atoms and molecules (Castelli & Kurucz 2004, and related publications). Clearly, significant differences emerge between the different spectral types, both concerning the total amount of radiation and their spectral distribution.
4. They considered the effects of attenuation by a planetary atmosphere. The following cases are considered: Earth as today, Earth 3.5 Gyr ago, and no atmosphere at all.
[Image Details: It is clear from image that habitability is increasing towards left since biological damage is decreasing from right to left. They show the relative damage to DNA due to photospheric radiation from stars between spectral type F0 and M0. The results are normalized to today’s Earth, placed at 1 AU from a star of spectral-type G2V. We also considered planets at the inner and outer edge of either the conservative or generalized HZ as well as planets of different atmospheric attentuation.
Now the conclusions which are obvious from the plots are mainly:
1.All main-sequence stars of spectral type F to M have the potential of damaging DNA due to UV radiation. The amount of damage strongly depends on the stellar spectral type, the type of the planetary atmosphere and the position of the planet in the habitable zone (HZ).
2. The damage to DNA for a planet in the HZ around an F-star (Earth- equivalent distance) due to photospheric radiation is significantly higher (factor 5) compared to planet Earth around the Sun, which in turn is significantly higher than for an Earth equivalent planet around an M-star (factor 180). Small modifications of this picture occur for different planetary positions inside their respective HZs.
3. Regarding the cases studied, they found that the damage is most severe in the case of no atmosphere at all, somewhat less severe for an atmosphere corresponding to Earth 3.5 Gyr ago, and least severe for an atmosphere like Earth today.
However, the final conclusion are well known to us even you are not going to study that research paper. The planet to where we re going to establish our future colonies, must have atmosphere and should be located in the proximity of M type stars.
[Ref: Astrobiology in the Environments of Main-Sequence Stars: Effects of Photospheric Radiation By M. Cuntz and L. Gurdemir]