Posted : February 2011
Author : the admin
The habitable zone (HZ) is the distance from a star where an
Earth-like planet can maintain liquid water on its surface and Earth-like life.
The habitable zone is the intersection of two regions that must both be
favorable to life; one within a planetary system, and the other within a
galaxy. The habitable zone is not to be confused with the planetary
habitability. While planetary habitability deals solely with the planetary
conditions required to maintain carbon-based life, the habitable zone deals
with the stellar conditions required to maintain carbon-based life, and these
two factors are not meant to be interchanged. A ” Goldilocks planet ” is a
planet that falls within a star’s habitable zone, and the name is often
specifically used for planets close to the size of Earth. The name comes from
the story of Goldilocks and the Three Bears, in which a little girl chooses
from sets of three items, ignoring the ones that are too extreme (large or
small, hot or cold, etc.), and settling on the one in the middle, which is
“just right”. Likewise, a planet following this Goldilocks Principle is one
that is neither too close nor too far from a star to rule out liquid water on
its surface and thus life (as humans understand it) on the planet. However,
planets within a habitable zone that are unlikely to host life (e.g., gas
giants) may also be called Goldilocks planets. The best example of a Goldilocks
planet is the Earth itself. Along with the characteristics of planets and their
star systems, the wider galactic environment may also impact habitability.
Scientists considered the possibility that particular areas of galaxies are
better suited to life than others:
* It is not in a globular cluster where immense star densities
are inimical to life, given excessive radiation and gravitational disturbance.
* It is not near an active gamma ray source.
* It is not near the galactic center where once again star
densities increase the likelihood of ionizing radiation. A supermassive black
hole is also believed to lie at the middle of the galaxy which might prove a
danger to any nearby bodies.
9. Less Alterations in Luminosity of its Star
Changes in luminosity are common to all stars, but the
severity of such fluctuations covers a broad range. Most stars are relatively
stable, but a significant minority of variable stars often experience sudden
and intense increases in luminosity and consequently the amount of energy
radiated toward bodies in orbit. These are considered poor candidates for
hosting life-bearing planets as their unpredictability and energy output
changes would negatively impact organisms. Particularly, living things adapted
to a specific temperature range would probably be unable to survive too great a
temperature deviation. Further, upswings in luminosity are generally accompanied
by massive doses of gamma ray and X-ray radiation which might prove lethal.
Atmospheres do mitigate such effects, but atmosphere retention might not occur
on planets orbiting variables, because the high-frequency energy buffeting
these bodies would continually strip them of their protective covering.
8. High Metallicity of its Star
In astronomy and physical cosmology, the metallicity of an
object is the proportion of its matter made up of chemical elements other than
hydrogen and helium. Since stars, which comprise most of the visible matter in
the universe, are composed mostly of hydrogen and helium, astronomers use for
convenience the blanket term “metal” to describe all other elements
collectively. A low amount of metal significantly decreases the probability
that planets will have formed around that star. Thus, any planets that did form
around a metal-poor star would probably be low in mass, and thus unfavorable
for life.
7. Good Jupiters
“Good Jupiters” are gas giant planets, like the solar
system’s Jupiter, that orbit their stars in circular orbits far enough away
from the habitable zone to not disturb it but close enough to “protect”
terrestrial planets in closer orbit in two critical ways. First, they help to
stabilize the orbits, and thereby the climates, of the inner planets. Second,
they keep the inner solar system relatively free of comets and asteroids that
could cause devastating impacts. Jupiter orbits the Sun at about five times the
distance between the Earth and the Sun. This is the rough distance we should
expect to find good Jupiters elsewhere. Jupiter’s “caretaker” role was
dramatically illustrated in 1994 when Comet Shoemaker-Levy 9 impacted the
giant; had Jovian gravity not captured the comet, it may well have entered the
inner solar system.
6. More Mass
Low-mass planets are poor candidates for life for two
reasons. First, their lesser gravity makes atmosphere retention difficult.
Constituent molecules are more likely to reach escape velocity and be lost to
space when buffeted by solar wind or stirred by collision. Secondly, smaller
planets have smaller diameters and thus higher surface-to-volume ratios than
their larger cousins. Such bodies tend to lose the energy left over from their
formation quickly and end up geologically dead, lacking the volcanoes,
earthquakes and tectonic activity which supply the surface with life-sustaining
material and the atmosphere with temperature moderators like carbon dioxide.
Plate tectonics appear particularly crucial, at least on Earth: not only does
the process recycle important chemicals and minerals, it also fosters
bio-diversity through continent creation and increased environmental complexity
and helps create the convective cells necessary to generate Earth’s magnetic
field. “Low mass” is partly a relative label; the Earth is considered low mass
when compared to the Solar System’s gas giants, but it is the largest, by
diameter and mass, and densest of all terrestrial bodies. It is large enough to
retain an atmosphere through gravity alone and large enough that its molten
core remains a heat engine, driving the diverse geology of the surface.
Finally, a larger planet is likely to have a large iron core. This allows for a
magnetic field to protect the planet from stellar wind and cosmic radiation,
which otherwise would tend to strip away planetary atmosphere and to bombard
living things with ionized particles.
5. Less Eccentric Orbit
The orbital eccentricity of an astronomical body is the amount
by which its orbit deviates from a perfect circle. As with other criteria,
stability is the critical consideration in determining the effect of orbital
and rotational characteristics on planetary habitability. Orbital eccentricity
is the difference between a planet’s farthest and closest approach to its
parent star divided by the sum of said distances. It is a ratio describing the
shape of the elliptical orbit. The greater the eccentricity the greater the
temperature fluctuation on a planet’s surface. Although they are adaptive,
living organisms can only stand so much variation, particularly if the
fluctuations overlap both the freezing point and boiling point of the planet’s
main biotic solvent (e.g., water on Earth). If, for example, Earth’s oceans were
alternately boiling and freezing solid, it is difficult to imagine life as we
know it having evolved. The more complex the organism, the greater the
temperature sensitivity. The Earth’s orbit is almost wholly circular, with an
eccentricity of less than 0.02; other planets in our solar system (with the
exception of Mercury) have eccentricities that are similarly benign.
4. Axial Tilt
A planet’s movement around its rotational axis must also
meet certain criteria if life is to have the opportunity to evolve. A first
assumption is that the planet should have moderate seasons. If there is little
or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic,
seasons will not occur and a main stimulant to biospheric dynamism will
disappear. The planet would also be colder than it would be with a significant
tilt: when the greatest intensity of radiation is always within a few degrees
of the equator, warm weather cannot move poleward and a planet’s climate
becomes dominated by colder polar weather systems. If a planet is radically
tilted, meanwhile, seasons will be extreme and make it more difficult for a
biosphere to achieve homeostasis.
3. Biomass
It is generally assumed that any extraterrestrial life that
might exist will be based on the same fundamental biochemistry as found on
Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and
nitrogen, are also the most common chemically reactive elements in the
universe. Indeed, simple biogenic compounds, such as amino acids, have been
found in meteorites and in the interstellar medium. These four elements
together comprise over 96% of Earth’s collective biomass. Carbon has an
unparalleled ability to bond with itself and to form a massive array of
intricate and varied structures, making it an ideal material for the complex
mechanisms that form living cells. Hydrogen and oxygen, in the form of water,
compose the solvent in which biological processes take place and in which the
first reactions occurred that led to life’s emergence. The energy released in
the formation of powerful covalent bonds between carbon and oxygen, available
by oxidizing organic compounds, is the fuel of all complex life-forms. These
four elements together make up amino acids, which in turn are the building blocks
of proteins, the substance of living tissue. In addition, neither sulfur,
required for the building of proteins, nor phosphorus, needed for the formation
of DNA, RNA, and the adenosine phosphates essential to metabolism, are rare.
Thus, while there is reason to suspect that the four “life elements” ought to
be readily available elsewhere, a habitable system probably also requires a
supply of long-term orbiting bodies to seed inner planets. Without comets there
is a possibility that life as we know it would not exist on Earth.
2. Microenvironment
One important qualification to habitability criteria is that
only a tiny portion of a planet is required to support life. The discovery of
life in extreme conditions has complicated definitions of habitability, but
also generated much excitement amongst researchers in greatly broadening the
known range of conditions under which life can persist. For example, a planet
that might otherwise be unable to support an atmosphere given the solar
conditions in its vicinity, might be able to do so within a deep shadowed rift
or volcanic cave. Similarly, craterous terrain might offer a refuge for
primitive life.
1. Different Metabolism Mechanism
While most investigations of extraterrestrial life start
with the assumption that advanced life-forms must have similar requirements for
life as on Earth, the hypothesis of other types of biochemistry suggests the
possibility of lifeforms evolving around a different metabolic mechanism. In Evolving
the Alien, biologist Jack Cohen and mathematician Ian Stewart argue
astrobiology, based on the Rare Earth hypothesis, is restrictive and
unimaginative. They suggest that Earth-like planets may be very rare, but
non-carbon-based complex life could possibly emerge in other environments. The
most frequently mentioned alternative to carbon is silicon-based life, while
ammonia is sometimes suggested as an alternative solvent to water.
~Blog Admin~
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