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Help edit Lee Smith (baseball), Wikipedia's current collaboration of the week! Please help to bring it up to featured article standard.

Lee Arthur Smith (born December 4, 1957 in Shreveport, Louisiana) was a pitcher in Major Leagues. Smith played for eight teams in both the NL and AL in his 18-year career, beginning with the Cubs in 1980. Smith led the league in saves four times during his career and by the time of his retirement in 1997 (with the Expos), he was the all-time leader with 478 saves. Smith used his fastball and size (he stood 6'6") to intimidate batters during the late innings of the game and became one of the premier closers of the 1980's and early 1990's.

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My Featured Article

Understanding planetary habitability is partly an extropolation of Earth conditions

Planetary habitability is the measure of an astronomical body’s potential for developing and sustaining life. It may be applied both to planets and to the natural satellites of planets. Given that the existence of life beyond our own planet is at present unverified, delineating planetary habitability is largely an extrapolation of Earth conditions and the characteristics of our Sun and solar system which have proven particularly favourable to life’s flourishing here. Observation and exploration of the other planets and moons within the solar system provide critical information on defining habitability criteria.

The only absolute requirement for life is an energy source (usually but not necessarily sunlight) but the notion of planetary habitability implies that certain other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body is likely to give rise to life. This article is a discursive description of what conditions are presently considered essential in this regard and is not intended as a probability analysis of life emerging off of the planet Earth.

Suitable Stars

An understanding of planetary habitability must begin with stars, not planets themselves. While bodies with broadly Earth-like characteristics may be plentiful it is equally vital that their larger system be amendable to life. Under the auspices of SETI’s Project Phoenix, scientists Margaret Turnbull and Jill Tarter developed the “HabCat” or Catalogue of Habitable Stellar Systems in 2002. The catalogue was formed by winnowing the nearly 120 000 stars of the larger Hipparcos Catalogue into a core group of 17 000 “HabStars” and the selection criteria employed provide a good starting point for understanding which astrophysical factors are necessary to habitable planets [1].

Spectral class

The spectral class of a star indicates its photospheric temperature which (for main-sequence stars) correlates to overall mass. The appropriate spectral range for "hab-stars" is presently considered to be early-F, G, to mid-K. This corresponds to temperatures of a little more than 7 000 K down to a little more than 4 000 K; our Sun (not coincidentally) is directly in the middle of these bounds, classified as a G2 star. "Middle-class" stars of this sort have a number characteristics considered important to planetary habitability:

  • They live at least a few a billion years, allowing life a chance to evolve.
  • They emit enough high frequency ultraviolet radiation to initiate important atmospheric dynamics such as ozone formation.
  • They do not emit so much UV radiation that ionization destroys life as at it attempts to form.
  • Surficial liquid water may exist on planets orbiting them at a distance that does not induce tidal lock (see next section and 3.1).

Crudely, these stars are neither "too hot" nor "too cold" and they live just long enough that life should have a chance to begin. This spectral range likely accounts for between 5-10% of stars in our galaxy. Whether fainter late-K and M class ("red dwarf") stars are also suitable hosts for habitable planets is perhaps the most important open question in the entire field of planetary habitability given that the majority of stars fall within this range; this is discussed extensively below.

A stable habitable zone (HZ)

The HZ is a theoretical shell surrounding a star throughout which any planets present would have surficial liquid water. After an energy source, liquid water is considered the most crucial component for life given how integral it is to all life-systems on Earth. This may reflect the bias of a water-dependent species and if life is discovered in the absence of water (i.e., in a liquid-ammonia solution) the notion of an HZ may have to be greatly expanded or else discarded altogether as too restricting.

A “stable” HZ denotes two factors. First, the range of an HZ should not vary greatly over time. All stars increase in luminosity as they age and a given HZ naturally migrates outwards but if this happens too quickly (i.e., with a super-massive star) planets may only have a brief window inside the HZ and a correspondingly weaker chance to develop life. Secondly, no large mass body such as a gas giant should be present in or relatively close to the HZ thus disrupting the formation of Earth-like bodies. If, for example, Jupiter had appeared between the orbits of Venus and Earth the two smaller planets would almost certainly not have formed. It was once assumed that the inner-rock planets, outer-gas giants pattern observable in our solar system was likely to be the norm elsewhere but recent discoveries of extra-solar planets have over-turned this notion. Numerous Jupiter-sized bodies have been found in close orbit about their primary, disrupting potential HZs.

Low stellar variation

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 impact negatively on organisms. Most obviously, living things adapted to a particular temperature range, would likely be unable to stand a too great 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 (i.e., an absolute increase of 100% in our Sun's luminosity would not necessarily translate into a 100% temperature increase on Earth) but atmosphere retention might not occur in the first-place on planets orbiting variables, as being buffeted by high frequency energy would continually strip such bodies of their protective covering.

Our Sun, as in much else, is benign in terms of this danger: the variation between solar max and minimum is roughly 0.1% over its eleven-year solar cycle. There is strong (though not undisputed) evidence that even minor changes in our Sun's luminosity have had significant effects on the Earth's climate well within the historical ere; the Little Ice Age of the mid-second millenium, for instance, may have been caused by a relatively long-term decline in the sun's luminosity[2]. Thus, a star does not have to be a true variable for differences in luminosity to affect habitability.

The closest "solar twin" to our Sun is considered to be 18 Scorpii; interestingly (and perhaps unfortunately) the only significant difference between the two bodies is the amplitude of the solar cycle which appears to be much greater on 18 Scorpii[3].

High metallicity

While the bulk of material in any star is hydrogen and helium, there is a great variation in the amount of heavier elements (metals) stars contain. A high proportion of metals in a star co-relates to the amount of heavy material initially available in protoplanetary disks. A low amount of metal signifantly decreases the probability that planets will have formed around that star, under the solar nebula theory of planetary systems formation: "there must be a lower limit in metallicity below which there are not enough heavy elements to build an Earth-mass planet during the system’s formation [4]." Spectroscopic studies of systems where exoplanets have been found to date confirm the relationship between high-metal content and planet formation. High metallicity also places a youth requirement on hab-stars: stars formed early in the universe’s history have low metal content and a correspondingly lesser likelihood of having planetary companions.

Planetary characteristics

Mass Matters

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. Planets without a thick atmosphere lack the primal matter necessary for biochemistry, have little insulation and poor heat transfer across their surfaces (Mars with its thin atmosphere is colder than the Earth would be at similar distance) and lesser protection against high-frequency radiation and meteoroids. 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.

“Low-mass” is, of course, a partly relative label; the Earth is considered low-mass when compared to our Solar System’s gas giants but it is the largest, by diameter and mass, and densest of all terrestrial bodies. It is a large enough (along with Venus) 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. Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere. Thus, it would be fair to infer that the lower mass limit for habitability lies somewhere between Mars and Earth-Venus. Exceptional circumstances do breed exceptional cases, however: Jupiter's moon Io (smaller than the terrestrial planets) is volcanically dynamic due to the gravitational stresses induced by its orbit; neighbouring Europa may have a liquid ocean underneath a frozen shell due also to energy created in its orbiting a gas giant; Saturn's Titan, meanwhile has retained a thick atmosphere and has an outside chance of harbouring life. These satellites are exceptions but they prove that mass as a habitability criteria cannot be considered in isolation.

Interestingly, there is a “mass-gap” in our solar system between Earth and the two smallest gas giants, Uranus and Neptune, which are both roughly 14 Earth-masses. Assuming this is coincidence and that there is no geophysical barrier to the formation of intermediary bodies, we should except to find planets through out the galaxy between two and twelve Earth-masses. If the star system is otherwise favourable, such planets would be good candidates for life as they would be large enough to remain internally dynamic and atmosphere retentive over billions of years but not so large as to accrete the gaseous shell which limits the possibility of life formation.

Orbit and Rotation

As with other criteria, stability is the critical consideration in determing the impact of orbital and rotational characteristics on planetary habitability.

Orbital eccentricity is the difference between a planet's closest and farthest approach to its primary. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. While supremely adaptive, living organisms can only stand so much variation, particularly if the fluctuations over-lap both the freezing point and boiling point of the planet's main biotic solvent (i.e., water). If, for example, Earth's oceans were alternately boiling off into space and freezing solid it is difficult to imagine life as we know it having evolved. Fortunately, 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 Pluto and to a lesser extent Mercury) have eccentricities that are similarly benign. Data collected on the orbital eccentricities of extra-solar planets has been surprising and perhaps discouraging in terms of extraterrestrial possibilities: 90% have an orbital eccentricity greater than that found within our solar system, and the average is fully 0.25.[5]

A planet's movement around its rotational axis must also meet certain criteria if we are to expect life to evolve.

  • The day-night cycle should not be over-long. If a day takes years (or centuries as with Venus) the temperature differential between the day and night side will be pronounced and problems similar to those noted with extreme orbital eccentricity will come to the fore.
  • The planet should have moderate seasons. If the axial tilt is perpendicular to the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. If it is radically tilted, meanwhile, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis.
  • The rotational "wobble" should not be pronounced. Precession on Earth occurs over a 23 000 year cycle; if this period were radically shorter or if the wobble were more extreme drastic climatic changes would again impact on habitability.

The Earth's moon appears to play a crucial role in moderating our climate by stabalizing the axial tilt. It is been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e., a satellite the size of the moon is not only helpful but required to produce stability. This position remains controversial.

Chemistry

For life as we know it to exist on any world, an abundance of four elements must be present: carbon, hydrogen, oxygen, and nitrogen. By weight, these four elements make up over 96 percent of the bodies of all organisms on Earth. Carbon has an unparalelled 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, comprise the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy contained in the powerful hydrogen bond, released from the breakdown of carbohydrates, is life's fuel. These four elements together make up amino acids, which in turn are the building blocks of protiens, the substance of living tissue.

The four elements of life are also the commonest chemically reactive elements in the universe; indeed simple biogenic compounds, such as amino acids, have been found in meteorites and in interstellar space. However, of the four, only oxygen is present in any abundance in the Earth's crust.[[6]]

This relative scarcity is partly explained by the fact that three of these four elements, along with many of their most basic compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gaseous at warm temperatures. In the hot region close to the Sun, they could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed rocky crusts. Outgassing of these compounds through the first volcanoes would have contributed to the formation of the planets' atmospheres. The Miller experiments have shown that, with the application of energy, amino acids can form from the synthesis of the simple compounds within the primordial atmosphere. [[7]] Even so, volcanic outgassing could not have accounted for the sheer amount of water in earth's oceans. [[8]] The vast majority of the water, and arguably of the carbon, necessary for life must have come from the outer solar system, away from the Sun's heat, where it could remain solid. Comets impacting with the Earth in the Solar systems early years would have deposited vast amounts of the lighter, voilitile compounds life requires, including amino acids onto the early Earth, providing a kickstart to the evolution of life.

Other considerations

File:BarnardsStarPlanet.jpg
Artist's rendering of Barnard's star. The habitability of planets orbiting red dwarfs is perhaps the most important question in the field

The habitability of Red Dwarf planetary systems

Determining the habitability of red dwarf stars would help decide whether life in the universe is ubiquitous or vanishingly rare, since red dwarfs make up between seventy and ninety percent of all the stars in the galaxy (brown dwarfs are likely more numerous, but could never support life as we understand it, since what little heat they emit quickly disappears).

Astronomers for many years ruled out red dwarfs as potential abodes for life, because their small size (from 0.1-0.6 solar masses) means that their nuclear reactions proceed exceptionally slowly, and thus they emit very little light, from 3% that produced by the Sun to as little as 0.01%. Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earthlike surface temperatures; from 0.3 AU (just inside the orbit of Mercury) for a star like Lacaille 8760, to as little as 0.032 AU (such a world would have a year lasting a little over a day) for a star like Proxima Centauri [[9]]. At those distances, the star's gravity would cause tidal lock. The daylight side of the planet would eternally face the star, while the nightime side would eternally face away from it. The only way potential life could avoid either an inferno or an utter 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. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, and thus photosynthesis from taking place.

However, studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere need only be 15 percent thicker than Earth's for the star's heat to be effectively carried to the night side. This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models[10]. Martin Heath of Greenwich Community College, London, 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. So, a planet with deep enough sea basins and a thick enough atmosphere could, at least potentially, harbour life in a red dwarf system.

Mere 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, making photosynthesis difficult. Photosynthesis as we understand it would be further complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on our planet the process depends on visible light.

Red dwarfs are far more variable and violent than their stabler, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40 percent for months at a time, while at other times they emit gigantic flares that can double their brightness in a matter of minutes. Such variation would be very damaging for life, however it could also stimulate evolution, by increasing mutation rates and rapidly shifting climatic conditions.

There is, however, one major advantage that red dwarfs have over other stars as abodes for life: they live a long time. It took 4.5 billion years for humanity to emerge on Earth, and life as we know it will see suitable conditions for perhaps 2 billion years more. Red dwarfs, by contrast, can live for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both has longer to evolve and longer to survive.

"Good Jupiters"

"Good jupiters" are gas giant planets like our own Jupiter that orbit their stars in circular orbits far enough away from the HZ 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 of the inner planets and in turn their climates. Second, they keep the inner solar system relatively free of comets and asteroids that could cause devastating impacts[11]. Jupiter orbits our sun at about 5 times the Earth-sun distance and this is the rough distance we should expect to find good Jupiters elsewhere. Jupiter's caretaker role was dramatically illustrated in 1994 when the Comet Shoemaker-Levy 9 comet impacted the giant; had Jovian gravity not captured the comet it may well have entered the inner solar system.

The Galactic Neighbourhood

Scientists have also considered the possibility that particular areas of galaxies (Galactic habital zones) are better suited to life than others; our own system, in the Orion Spur, on our galaxy's edge is considered to be in a life-favourable spot. Well away from the galactic center it avoids various dangers:

Relative loneliness is ultimately what a life-bearing system needs. If Sol were crowded amongst other systems neighbours might disrupt the stability of various orbiting bodies (not least Oort cloud and Kuiper Belt objects, which can bring catastrophe if knocked into the inner solar system). Close neighbours also increase the likelihood of being fatally close to supernova explosions and pulsars.

See Also
06:30 UTC
Tuesday, 10 June

Profile

My name is Ryan Lee Ethan. I was born March 23, 1982 in Saugus, California. At the age of 6, I moved along with my family to Murrieta, California. At age 12, I moved to Las Vegas, Nevada where I currently reside. Here at Wikipedia I occupy the majority of time performing a variety of tasks. This may include RC Patrol (#en.wikipedia.vandalism, User:Cryptoderk's VandalFighter), helping out over at WP:CP, answering questions the best I can in #wikipedia, etc. I try to stay away from deletion issues (VFD, CSD, etc). Other than that I still spend a great deal of time learning, wikilink surfing, and helping to improve and create new and interesting articles.

My interests include:

Edit Counts

As of September 1st, 2005[12]

  • Articles: 332
  • Talk: 45
  • User: 169
  • User talk: 46
  • Wikipedia: 119
  • Image: 9
  • Template: 4

  • Total:724
  • Edited a total of 269 distinct pages.

My To-Do List

Quote of the Moment

"Ah, this is obviously some strange usage of the word 'safe' of which I wasn't previously aware."

-The Hitchhiker's Guide to the Galaxy, c. 5

IPs I May Use

68.104.69.70 (talk · contribs)
enThis user is a native speaker of the English language.
es-1Este usuario puede contribuir con un nivel básico de español.

Template:User 1337-2


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Multi-licensed with the Creative Commons Attribution Share-Alike License versions 1.0 and 2.0
I agree to multi-license my text contributions, unless otherwise stated, under Wikipedia's copyright terms and the Creative Commons Attribution Share-Alike license version 1.0 and version 2.0. Please be aware that other contributors might not do the same, so if you want to use my contributions under the Creative Commons terms, please check the CC dual-license and Multi-licensing guides.

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