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It is evident that life arose from cosmic processes just by examining the chemicals in our body. The iron in our blood and the calcium in our bones were made inside stars. All the heavy chemical elements were forged by star that exploded long ago. Terrestrial life is embedded in a cosmic web, and it seems reasonable to speculate that life is cosmically commonplace. There are three ingredients upon which life depends: water, energy, and organic molecules (or carbon). Recent discoveries inform us that these pre-requisites may exist well beyond the planets closely orbiting the sun. This area - where conditions might potentially support life - is called The Habitable Zone. Figure 09-18a shows such zone in the Milky Way and in particular a zone in the Solar System between Mars and Earth. The galactic habitable zone is envisioned as a ring around the center of our Milky Way galaxy and in between spiral arms. It may only contain about 20 percent of the galaxy's stars -- including our own sun. Near the core of the Milky Way, life may be unlikely -- comet impacts may be more frequent, and radiation levels are high. Meanwhile, the outer fringe of the galaxy is a difficult place to build life-supporting planets because there are fewer heavy elements. |
Figure 09-18a Habitable Zone [view large image] |
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The habitable zone in the Solar System is restricted by the Sun's radiation. If it is too close, the heat from the Sun would boil off waters and break down organic molecules. If it is too far, then water would freeze to ice. The habitable zone around a star depends on its mass. Stars with higher mass will provide more heat to its surrounding. Figure 09-18b shows the relationship between the habitable zone and the stellar mass. The inhabitant in the habitable zone is rather broadly defined to include perhaps just a strand of RNA (a primitive version of DNA). |
Figure 09-18b Habitable Zone & Stellar Mass [view large image] |
Figure 09-18c Habitable Zone, 2016 |
Figure 09-18c is a 2016 update of the habitable zone for the various types of stars. It is suggested that rocky planets associated with K type stars are the most favorable for the |
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whether it retains an atmosphere or not. The planet would be subjected to harmful radiation if it does not possess a magnetic field. Tidal locking would produce another un-favourable factor for life with temperature extremes on both sides. Thus far, it is known that all three Alpha Centauri stars have at least one planet. This is in consistent with the 2010 update for the fraction of stars with planets Fp in the Drake equation. It generates N ~ 400 civilizations within the Milky Way |
Figure 09-18d Alpha Centauri and Proxima b [view large image] |
that are currently broadcasting detectable signals. Figure 09-18d shows the appearance of the Alpha Centauri stars in the Southern sky, and an artist's rendition of Promima b including the life giving water. |
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Figure 09-18e Types of Terrestrial Planets [view large image] |
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Scientists are using the European Space Agency's (ESA) Venus Express to search for life on Earth. It may sound obvious and silly (and merit for an Ig Nobel Prize), but they are actually looking for the kind of signatures that might be present on other habitable planets. The signals would not be some artificial structures such as the Great Wall of China, which is indiscernible by the naked eye from about 100 km up. It is the form of molecular spectrum (Figure 09-18f) at both the visible and near-infrared regions that is unique from the life-support planet. The presence of water and molecular oxygen is not a good enough evidence. More subtle signals, such as the so-called red edge caused by photosynthetic life (in the near infrared) are more reliable. The analysis to see whether this red edge is visible is just at the beginning in mid 2008. Meanwhile, a team commissioned by NASA is using computer models to quantify the astrophysical, atmospheric and geological factors that influence whether a planet can harbor life. The findings are categorized into biosignature, antibiosignature, and habitability signature as shown in Figure 09-18f. |
Figure 09-18f Signatures of Life [view large image] |
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According to the stellar evolution theory, the young Sun radiated much less energy in the past billion years (Figure 09-18g). It was only about one billion years ago that it warmed the Earth to above the freezing point of water. The Cambrian explosion followed 1/2 billion years later to initiate the diversification of multicellular life. However geological evidence has shown that unicellular Organisms existed between 4.5 - 3.7 billion years ago even when there was not enough solar energy to liquefy the water. This is known as the "Faint Young Sun Paradox". Various suggestions have been proposed to address this puzzle including : greenhouse effect, lower albedo (reflectivity), and higher initial mass for the Sun. Another possibility is hydrothermal mounds, which could harbor simply life form under a sheet of ice (similar to that in Europa). |
Figure 09-18g Cold Earth |
Despite the progress scientists have made trying to resolve this issue during the past 40 years, it seems that the paradox will remain un-resolved for the foreseeable future. |