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Habitable Zone

Habitable Zone 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]

Habitable Zone and Stellar Mass Habitable Zone, 2016 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 [view large image]

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 development of life. Thus, the K type Alpha Centauri B is the subject of many science fictions and films. Its proximity to the Sun allows all kinds of interstellar travels possible. The fiction becomes a bit closer to reality when giant planet HD 131399Ab is discovered in July 2016 orbiting Alpha Centauri A (instead of B). BTW, Alpha Centauri is a triplet star system with Alpha Centauri A, B and Proxima Centauri, which is a smaller type M star further away from the A-B pair (see Figure 09-18c).

In the first half of 2016, telescopes around the world granted observation time to check out presence of planet around Proxima Centauri. The collective efforts culminated in an August 24 Letter, which confirms the existence of a small rocky planet (~ 1.25 MEarth) orbiting Proxima every 11.1 days. It means that the planet is very close to the star at 0.05 AU (much closer than Mercury to the Sun at 0.4 AU). However, the parent star is M type, which emits much less radiation producing a surface temperature from ~ 35oC to -35oC depending on whether it retains an
Proxima b 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 that are currently broadcasting detectable signals.

Figure 09-18d Alpha Centauri and Proxima b [view large image]

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.

Types of Terrestrial Planets
    The criterions for a good parent star include:
  1. It remains at approximately the same luminosity for billions of years to offer a chance for the development of life.
  2. It forms out of an interstellar cloud with enough metal content to build the terrestrial planets.
  3. It stays in between galactic spiral arms for as long as possible to minimize the chance of collision with other stars.
  4. It does not emit gigantic flares that singe the surfaces of nearby planets.
  5. It does not form multiple system with companion stars that swoop in and out of the habitable zone.
  6. A giant gas planet at the outer zone to control the cometary traffic is also important.
Figure 09-18e shows the four types of terrestrial planets at different location of the habitable zone. The corresponding temperatures could be too scorched (with no water), too humid, too cold, or just right. The best location for an earth-like world is in the habitable zone's center.

Figure 09-18e Types of Terrestrial Planets [view large image]

It is now realized that the size of the habitable zone around a star could double because many of the moons around the giant planets receive a lot of tidal heating, which keeps the water to remain in a liquid state. If the exomoons around gas giants are as large as Earth, their sheer size would boost the prospects for life. Only the gravitational pull of such a large body would be able to hold onto a thick, sheltering atmosphere. What's more, a large moon is more likely to have a magnetic field, which protects life from damaging radiation.

Signature of Life 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]

Cold Earth 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
[view large image]

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.

See "Stars and Habitable Planets" for more detail on the subject.

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