You Are Viewing. Trending Price New. No ratings or reviews yet. Be the first to write a review. Best Selling in Nonfiction See all. Permanent Record by Edward Snowden , Hardcover 1. Open Borders Inc. Save on Nonfiction Trending price is based on prices over last 90 days. You may also like. Arthur C. Elizabeth Berg Hardcover Books. Further, where an atmosphere is less dense than 0. The temperature range at which water is liquid is smaller at low pressures generally. 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.
Mars, by contrast, is nearly or perhaps totally geologically dead and has lost much of its atmosphere. Earth may in fact lie on the lower boundary of habitability: if it were any smaller, plate tectonics would be impossible. Conversely, " super-Earths ", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.
Exceptional circumstances do offer exceptional cases: Jupiter 's moon Io which is smaller than any of the terrestrial planets is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant. Saturn 's Titan , meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface.
Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding. A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth.
The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets. Finally, a larger planet is likely to have a large iron core.
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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. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core  —but it is a significant component of the process. As with other criteria, stability is the critical consideration in evaluating 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 stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent e. 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.
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Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. 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, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now in the Quaternary than it has been in the past, coinciding with reduced polar ice , warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt see Snowball Earth.
The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature. The Earth's tilt varies between A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity. The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.
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In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to ocean tides , which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate. 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 very simple amino acids such as glycine , have been found in meteorites and in the interstellar medium. 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 , is rare.
Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's crust. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as silica a compound of silicon and oxygen, accounting for oxygen's relative abundance.
Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller—Urey experiment showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize amino acids. Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans.
Comets impacting with the Earth in the Solar System's early years would have deposited vast amounts of water, along with the other volatile compounds life requires onto the early Earth, providing a kick-start to the origin of life. 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.
One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life.
Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as mutation , selection , and genetic drift , operate in micro-organisms that act on and respond to changing micro-environments. 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.
The Lawn Hill crater has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of Mars. Earth environments that cannot support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the Atacama desert , generally considered the driest place on Earth, appears unable to support life, and it has been subject to study by NASA and ESA for that reason: it provides a Mars analog and the moisture gradients along its edges are ideal for studying the boundary between sterility and habitability.
The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation. In determining the feasibility of extraterrestrial life, astronomers had long focused their attention on stars like the Sun.
However, since planetary systems that resemble the Solar System are proving to be rare, they have begun to explore the possibility that life might form in systems very unlike our own. This may be partly sample bias, as massive and bright stars tend to be in binaries and these are most easily observed and catalogued; a more precise analysis has suggested that the more common fainter stars are usually singular, and that up to two thirds of all stellar systems are therefore solitary.
The separation between stars in a binary may range from less than one astronomical unit AU, the average Earth—Sun distance to several hundred.kirtihospital.com/dropship-express-how-to-start.php
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In latter instances, the gravitational effects will be negligible on a planet orbiting an otherwise suitable star and habitability potential will not be disrupted unless the orbit is highly eccentric see Nemesis , for example. However, where the separation is significantly less, a stable orbit may be impossible. If a planet's distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.
Theoretical work by Alan Boss at the Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars.
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One study of Alpha Centauri , the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. The HZ for Centauri A is conservatively estimated at 1. Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size from 0. Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0. At those distances, the star's gravity would cause tidal locking.
One side of the planet would eternally face the star, while the other would always face away from it. The only ways in which potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side, or if there was a gas giant in the habitable zone, with a habitable moon , which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the planet.
It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis. This pessimism has been tempered by research. Martin Heath of Greenwich Community College , has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap.
Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants. Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever.
Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the infrared , and on the Earth the process depends on visible light. There are potential positives to this scenario. Numerous terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, for instance, which would be possible in a red dwarf system. Because of the lack of a day-night cycle, including the weak light of morning and evening, far more energy would be available at a given radiation level.
Red dwarfs are far more variable and violent than their more stable, larger cousins. For a planet around a red dwarf star to support life, it would require a rapidly rotating magnetic field to protect it from the flares. A tidally locked planet rotates only very slowly, and so cannot produce a geodynamo at its core. The violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1. If a planet forms far away from a red dwarf so as to avoid tidal locking, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.
It was long assumed to be quiescent in terms of stellar activity. Yet, in , astronomers observed an intense stellar flare , surprisingly showing that Barnard's Star is, despite its age, a flare star. Red dwarfs have one advantage over other stars as abodes for life: far greater longevity. It took 4. Red dwarfs, by contrast, could live for trillions of years because their nuclear reactions are far slower than those of larger stars, meaning that life would have longer to evolve and survive.
While the likelihood of finding a planet in the habitable zone around any specific red dwarf is slight, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity. Massive stars are thus eliminated as possible abodes for life. However, a massive-star system could be a progenitor of life in another way — the supernova explosion of the massive star in the central part of the system. This supernova will disperse heavier elements throughout its vicinity, created during the phase when the massive star has moved off of the main sequence, and the systems of the potential low-mass stars which are still on the main sequence within the former massive-star system may be enriched with the relatively large supply of the heavy elements so close to a supernova explosion.
However, this states nothing about what types of planets would form as a result of the supernova material, or what their habitability potential would be. In a review of the factors which are important for the evolution of habitable Earth-sized planets, Lammer et al. Class I habitats are planetary bodies on which stellar and geophysical conditions allow liquid water to be available at the surface, along with sunlight, so that complex multicellular organisms may originate. Class II habitats include bodies which initially enjoy Earth-like conditions, but do not keep their ability to sustain liquid water on their surface due to stellar or geophysical conditions.
Mars, and possibly Venus are examples of this class where complex life forms may not develop. Class III habitats are planetary bodies where liquid water oceans exist below the surface, where they can interact directly with a silicate-rich core. 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 galactic habitable zones are better suited to life than others; the Solar System in which we live, in the Orion Spur , on the Milky Way galaxy's edge is considered to be in a life-favorable spot: .
Thus, relative isolation is ultimately what a life-bearing system needs. If the Sun were crowded amongst other systems, the chance of being fatally close to dangerous radiation sources would increase significantly. Further, close neighbors might disrupt the stability of various orbiting bodies such as Oort cloud and Kuiper belt objects, which can bring catastrophe if knocked into the inner Solar System.
While stellar crowding proves disadvantageous to habitability, so too does extreme isolation. A star as metal-rich as the Sun would probably not have formed in the very outermost regions of the Milky Way given a decline in the relative abundance of metals and a general lack of star formation. Thus, a "suburban" location, such as the Solar System enjoys, is preferable to a Galaxy's center or farthest reaches. 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 and hydrocarbons are sometimes suggested as alternative solvents to water. The astrobiologist Dirk Schulze-Makuch and other scientists have proposed a Planet Habitability Index whose criteria include "potential for holding a liquid solvent" that is not necessarily restricted to water.
More speculative ideas have focused on bodies altogether different from Earth-like planets. Astronomer Frank Drake , a well-known proponent of the search for extraterrestrial life , imagined life on a neutron star : submicroscopic "nuclear molecules" combining to form creatures with a life cycle millions of times quicker than Earth life. First, they help to stabilize the orbits, and thereby the climates of the inner planets. Related tags 2 2 25 2 2 s 3 AR 2 2 AR 2. What is MDS? LibraryThing's MDS system is based on the classification work of libraries around the world, whose assignments are not copyrightable.
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