Date: August 7, 2018
Time: 7:35 PM
From the cosmic observance of what has been recorded thus far in the history of this world called Earth a great foundation has delivered the corpse of examination and brought letter to tack in society. To found the basis of only one find would suffice. Our sun is a meteor. The burning mass at movement from its source would have been a better term of research as the planets in the surround of our sun are the fragmented bodies of the corpse itself.
Trajectory to our open galaxy has made this available sun a long and much timed event in this atmosphere. Our basic understanding thus far has clarified that finding. To know that our sun will turn to stone would have allowed us to not have wasted our time looking for a planet in the quote unquote Goldilocks Zone when we should have been looking for a sun. The study of that would have been perfected here and delivered a chart to know the extent of a sun's potential livelihood. This would have been to fact and in addition been an easier find as per the scientific community suns are at noted as plentiful and easily spotted with a telescope. Along with past research of measurement and along with the brightness of those specific stars our research team could have placed variables aside and invited real track to structure of value and station.
The 'Sun' is to be imagined as a hot burning "Meteor" and the flight in expectation would have bits and pieces (Planets, Moons, Rings with particle) that are caught as rocks in its pull (example here the ordinary comet and or meteor storm to place actual picture and stature). The expectation is no less and no more than the following, as the meteor spins without impact residue to subdue it's mass, than the obvious would be a cooling rock and thus stone would be shown, and the stone would inevitably be solid and afloat.
Chord: This would have had to have been increased should a larger galaxy afford "quote unquote" a point of impact. Thereby the burning suns are in action, and, the best residence (planets caught in circular pull of gravity and thrown) creating a live dynamic place that new life would be 'Creation Itself'.
Odd, yet so cool, our sun in this, The Milky Way will be solid stone, wow!! Where would such a ball of chip come from? Possibly a larger (Meteor) in dispel.
*Study meteor showers for the every form of displaced bodies to possibly origin our galaxy's pattern.
So, we are looking for suns not planets to terraform (catch in the swing and advantage the mass to plunder more day and less shade per planet per foot to motion and or circular sway). I wonder, would NASA now be able to see a stone sun knowing this tidbit of information? Case to clarify, would NASA be able to locate in the environment of outside the Milky Way a stone sun with or without particle (Planets) in rotation?
add'.1
Our Moon: The mass in exposure of obvious is an old sun!! To study the moon would be of great advantage to discuss in conversation the age of our universe and ultimately the cosmos. To this grand canyon of great finds the most mathematics could invite to answer the older books in the physics of why a figure would be a sum. Now on that first engagement we, The United States of America, my country, would be fact. Our circumspect towards our successful landing on the Moon would now be of the best reward for our findings now would be priceless!!
July 20, 2017: https://www.nasa.gov/mission_pages/apollo/apollo11.html
July 20, 1969: One Giant Leap For Mankind
July 1969. It's a little over eight years since the flights of Gagarin and Shepard, followed quickly by President Kennedy's challenge to put a man on the moon before the decade is out.
Apollo 11 Commander Neil Armstrong working at an
equipment storage area on the lunar module. This is one of the few
photos that show Armstrong during the moonwalk. Click image to enlarge.
Credits: NASA
Smoke and flames signal the opening of a historic journey as the Saturn V clears the launch pad. Click image to enlarge.
Credits: NASA
Buzz Aldrin climbs down the Eagle's ladder to the surface. Click image to enlarge.
Credits: NASA
Crater 308 stands out in sharp relief in this photo from lunar orbit. Click image to enlarge.
Credits: NASA
Now, on the morning of July 16, Apollo 11 astronauts Neil Armstrong, Buzz Aldrin and Michael Collins sit atop another Saturn V at Launch Complex 39A at the Kennedy Space Center. The three-stage 363-foot rocket will use its 7.5 million pounds of thrust to propel them into space and into history.
At 9:32 a.m. EDT, the engines fire and Apollo 11 clears the tower. About 12 minutes later, the crew is in Earth orbit. (› Play Audio)
After one and a half orbits, Apollo 11 gets a "go" for what mission controllers call "Translunar Injection" - in other words, it's time to head for the moon. Three days later the crew is in lunar orbit. A day after that, Armstrong and Aldrin climb into the lunar module Eagle and begin the descent, while Collins orbits in the command module Columbia. (› View Flash Feature)
Collins later writes that Eagle is "the weirdest looking contraption I have ever seen in the sky," but it will prove its worth.
When it comes time to set Eagle down in the Sea of Tranquility, Armstrong improvises, manually piloting the ship past an area littered with boulders. During the final seconds of descent, Eagle's computer is sounding alarms.
It turns out to be a simple case of the computer trying to do too many things at once, but as Aldrin will later point out, "unfortunately it came up when we did not want to be trying to solve these particular problems."
When the lunar module lands at 4:18 p.m EDT, only 30 seconds of fuel remain. Armstrong radios "Houston, Tranquility Base here. The Eagle has landed." Mission control erupts in celebration as the tension breaks, and a controller tells the crew "You got a bunch of guys about to turn blue, we're breathing again." (› Play Audio)
Armstrong will later confirm that landing was his biggest concern, saying "the unknowns were rampant," and "there were just a thousand things to worry about."
At 10:56 p.m. EDT Armstrong is ready to plant the first human foot on another world. With more than half a billion people watching on television, he climbs down the ladder and proclaims: "That's one small step for a man, one giant leap for mankind." (› Play Audio)
Aldrin joins him shortly, and offers a simple but powerful description of the lunar surface: "magnificent desolation." They explore the surface for two and a half hours, collecting samples and taking photographs.
They leave behind an American flag, a patch honoring the fallen Apollo 1 crew, and a plaque on one of Eagle's legs. It reads, "Here men from the planet Earth first set foot upon the moon. July 1969 A.D. We came in peace for all mankind."
Armstrong and Aldrin blast off and dock with Collins in Columbia. Collins later says that "for the first time," he "really felt that we were going to carry this thing off."
The crew splashes down off Hawaii on July 24. Kennedy's challenge has been met. Men from Earth have walked on the moon and returned safely home.
In an interview years later, Armstrong praises the "hundreds of thousands" of people behind the project. "Every guy that's setting up the tests, cranking the torque wrench, and so on, is saying, man or woman, 'If anything goes wrong here, it's not going to be my fault.'" (› Read 2001 Interview, 172 Kb PDF)
In a post-flight press conference, Armstrong calls the flight "a beginning of a new age," while Collins talks about future journeys to Mars.
Over the next three and a half years, 10 astronauts will follow in their footsteps. Gene Cernan, commander of the last Apollo mission leaves the lunar surface with these words: "We leave as we came and, God willing, as we shall return, with peace, and hope for all mankind."
Last Updated: Aug. 6, 2017
Editor: NASA Content Administrator
Tags: Apollo, Apollo 11, NASA History
Apollo
11 was the spaceflight that landed the first two people on the Moon.
Mission commander Neil Armstrong and pilot Buzz Aldrin, both American,
landed the lunar module Eagle on July 20, 1969, at 20:17 UTC. Wikipedia
Aug. 23, 2018
5:17 AM
5:17 AM
K.A.P.
Circumstellar habitable zone
A diagram depicting the Habitable Zone (HZ) boundaries, and how the boundaries are affected by star type. This new plot includes solar system planets (Venus, Earth, and Mars) as well as especially significant exoplanets such as TRAPPIST-1d, Kepler-186f, and our nearest neighbor Proxima Centauri b.
The habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale 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".
Since the concept was first presented in 1953,[6] many stars have been confirmed to possess a CHZ planet, including some systems that consist of multiple CHZ planets.[7] Most such planets, being super-Earths or gas giants, are more massive than Earth, because such planets are easier to detect. On November 4, 2013, astronomers reported, based on Kepler data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.[8][9] 11 billion of these may be orbiting Sun-like stars.[10] Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) from Earth in the constellation of Centaurus, is the nearest known exoplanet, and is orbiting in the habitable zone of its star.[11] The CHZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the CHZ might outnumber planets.[12]
In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.[13] Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of deep biospheres, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist within in lithospheres and asthenospheres of the Solar System.[14] Sustained by other energy sources, such as tidal heating[15][16] or radioactive decay[17] or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons.[18] Liquid water can also exist at a wider range of temperatures and pressures as a solution, for example with sodium chlorides in seawater on Earth, chlorides and sulphates on equatorial Mars,[19] or ammoniates,[20] due to its different colligative properties. In addition, other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.[21]
Contents
History
An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4).[22] The concept of a circumstellar habitable zone was first introduced in 1953 by Hubertus Strughold, who in his treatise The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars, coined the term "ecosphere" and referred to various "zones" in which life could emerge.[6][23] In the same year, Harlow Shapley wrote "Liquid Water Belt", which described the same theory in further scientific detail. Both works stressed the importance of liquid water to life.[24] Su-Shu Huang, an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.[25][26] A major early contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.[27]The theory of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of circumstellar habitable zone as well as various other determinants of planetary habitability, eventually guestimating the number of habitable planets in the Milky Way to be about 600 million.[2] At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization.[28] The term "Goldilocks zone" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.[29] In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone.[25] Kasting was the first to present a detailed model for the habitable zone for exoplanets.[3][30]
An update to habitable zone theory came in 2000, when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.[31][32] The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a galactic center that stars there are enriched with heavier elements, but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.[31]
Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an alternative biochemistry.[21] In 2013, further developments in habitable zone theory were made with the proposal of a circumplanetary habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.[33]
Determination
Thermodynamic
properties of water depicting the conditions at the surface of the
terrestrial planets: Mars is near the triple point, Earth in the liquid;
and Venus near the critical point.
The inner edge of the HZ is the distance where a runaway greenhouse effect vaporizes the whole water reservoir and,[13] as a second effect, induces the photodissociation of water vapor and the loss of hydrogen to space. The outer edge of the HZ is the distance from the star where adding more carbon dioxide to the atmosphere fails to keep the surface of the planet above the freezing point.[13]
Solar System estimates
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units,[36][37][38][39] though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to 462 °C (864 °F) at which water can only exist as vapour.[40] The entire orbits of the Moon,[41] Mars,[42] and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods.[43] At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year.[43] Despite indirect evidence in the form of seasonal flows on warm Martian slopes,[44][45][46][47] no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, Ceres[48] is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface Venus,[49] Mars,[50][51][52] Vesta[53] and Ceres,[54][55] suggesting a more common phenomena than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.According to extended habitable zone theory, planetary mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources cannot be determined by their position in the habitable zone.
For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone are: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU.
| Inner edge (AU) | Outer edge (AU) | Year | Notes |
|---|---|---|---|
| 0.725 | 1.24 | Dole 1964[2] | Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone. |
| 1.385–1.398 | Budyko 1969[56] | Based on studies of ice albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969[57] and North 1975.[58] | |
| 0.88–0.912 | Rasool and De Bergh 1970[59] | Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans. | |
| 0.95 | 1.01 | Hart et al. 1979[60] | Based on computer modelling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications. |
| 3.0 | Fogg 1992[34] | Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone. | |
| 0.95 | 1.37 | Kasting et al. 1993[25] | Founded the most common working definition of the habitable zone used today. Assumes that CO2 and H2O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the carbonate-silicate cycle. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84 - 1.67 AU. |
| 2.0 | Spiegel et al. 2010[61] | Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity. | |
| 0.75 | Abe et al. 2011[62] | Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth. | |
| 10 | Pierrehumbert and Gaidos 2011[37] | Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in our solar system. | |
| 0.77—0.87 | 1.02—1.18 | Vladilo et al. 2013[63] | Inner edge of circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 millibar. |
| 0.99 | 1.68 | Kopparapu et al. 2013[4] | Revised estimates of the Kasting et al. (1993) formulation using updated runaway greenhouse and water loss algorithms. According to this measure Earth is at the inner edge of the HZ and close to, but just outside, the runaway greenhouse limit. This applies to a planet with Earth-like atmospheric composition and pressure. |
| 0.38 | Zsom et al. 2013 [36] |
Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere. | |
| 0.95 | Leconte et al. 2013[64] | Using 3-D models, these authors computed an inner edge of 0.95 AU for our solar system. | |
| 0.95 | 2.4 | Ramirez and Kaltenegger 2017 [38] |
An expansion of the classical carbon dioxide-water vapor habitable zone [25] assuming a volcanic hydrogen atmospheric concentration of 50%. |
Extrasolar extrapolation
Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun,[4] a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at
,
or 0.5, the distance from the star, corresponding to a distance of 0.67
AU. Various complicating factors, though, including the individual
characteristics of stars themselves, mean that extrasolar extrapolation
of the CHZ concept is more complex.
Spectral types and star-system characteristics
A video explaining the significance of the 2011 discovery of a planet in the circumbinary habitable zone of Kepler-47.
With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions.[68] Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.[69] Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of spectral class K0 or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a tidal locking radius for red dwarfs. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment to life.[70]
Others maintain that circumstellar habitable zones are more common, and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking.[71] Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.[72] At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.[70] Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales.[73]
Stellar evolution
Natural
shielding against space weather, such as the magnetosphere depicted in
this artistic rendition, may be required for planets to sustain surface
water for prolonged periods.
In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes[79] and huge starspots which can cover 20% of the star's surface area,[80] have the potential to strip an otherwise habitable planet of its atmosphere and water.[81] As with more massive stars, though, stellar evolution changes their nature and energy flux,[82] so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.[81][83]
Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.[84] For example, the Sun is expected to engulf the previously-habitable Earth as a red giant.[85][86] However, once a red giant star reaches the horizontal branch, it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.[87] At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense.[88] Given that this new equilibrium lasts for about 1 Gyr, and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants.[87] However, around such a helium-burning star, important life processes like photosynthesis could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide.[89] Moreover, as Ramirez and Kaltenegger (2016)[86] showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant.[86] Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage.[86]
Desert planets
A planet's atmospheric conditions influence its ability to retain heat, so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.[90][91]Other considerations
Earth's hydrosphere. Water covers 71% of Earth's surface, with the global ocean accounting for 97.3% of the water distribution on Earth.
Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorised to outgassing, impact degassing and ingassing.[98] Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.[99] In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure.[63] Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained[63] because even a small shift in pressure or temperature could render water unable to form as a liquid.[100]
Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth),[25] a study[38] led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in our solar system would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos [37] had eliminated the CO2-H2O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism, and is lost within millions to tens-of-millions of years.
In the case of planets orbiting in the CHZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013, showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between the two sides.[71]
Planetary-mass natural satellites have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets.[33] More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like Io,[33] but must still remain within the Hill radius of the planet so that they are not pulled out of orbit of their host planet.[101] Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.[33]
Artist's concept of a planet on an eccentric orbit that passes through the CHZ for only part of its orbit
Extrasolar discoveries
Among exoplanets, a review in 2015 came to the conclusion that Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable.[105] These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e is 11.9 light-years away. It is in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of 68 °C (154 °F).[106]Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put ηe, the fraction of stars with planets in the CHZ, at 0.48,[4] meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.[107] However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.[108]
Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.[109] NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their CHZs.[110][111]
Early findings
The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. However these early detections were all gas giant sized, and many in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.[112] One of the first discoveries was 70 Virginis b, a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold." Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water.[113] 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the CHZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.[114]Gliese 876 b, discovered in 1998, and Gliese 876 c, discovered in 2001, are both gas giants discovered in the habitable zone around Gliese 876 that may also have large moons.[115] Another gas giant, Upsilon Andromedae d was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.
Announced on April 4, 2001, HD 28185 b is a gas giant found to orbit entirely within its star's circumstellar habitable zone[116] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.[117] Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,[118] though it is unclear whether such satellites could form in the first place.[119]
HD 69830 d, a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of HD 69830, 41 light years away from Earth.[120] The following year, 55 Cancri f was discovered within the CHZ of its host star 55 Cancri A.[121][122] Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.[123]
Though in theory such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been detected. Planets within the zone with the potential for solid surfaces were therefore of much greater interest.
Habitable super-Earths
The habitable zone of Gliese 581 compared with our Solar System's habitable zone.
A
diagram comparing size (artist's impression) and orbital position of
planet Kepler-22b within Sun-like star Kepler 22's habitable zone and
that of Earth in the Solar System
Kepler-22 b, discovered in December 2011 by the Kepler space probe,[127] is the first transiting exoplanet discovered around a Sun-like star. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet.[128] Gliese 667 Cc, discovered in 2011 but announced in 2012,[129] is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C.
Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163[130] is located 49 light years from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about 60° C.[131][132][133] HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.[134] In December 2012, Tau Ceti e and Tau Ceti f were found in the circumstellar habitable zone of Tau Ceti, a Sun-like star 12 light years away.[135] Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;[136] however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study.[137]
Near Earth-sized planets and Solar analogs
Comparison of the CHZ position of Earth-radius planet Kepler-186f and the Solar System (17 April 2014)
While larger than Kepler 186f, Kepler-452b's orbit and star are more similar to Earth's.
A solar analog (or "solar twin") is a star that resembles the Sun. To date no solar twin with an exact match as that of the Sun has been found, however, there are some stars that are nearly identical to the Sun, and are such considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation.[139] Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also very important to low luminosity variation.[140][141][142]
Using data collected by NASA's Kepler Space observatory and the W. M. Keck Observatory, scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone.[143]
On 7 January 2013, astronomers from the Kepler team announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-size exoplanet candidate (1.7 times the radius of Earth) orbiting Kepler-69, a star similar to our Sun, in the CHZ and expected to offer habitable conditions.[144][145][146][147] The discovery of two planets orbiting in the habitable zone of Kepler-62, by the Kepler team was announced on April 19, 2013. The planets, named Kepler-62e and Kepler-62f, are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.[146][147][148]
With a radius estimated at 1.1 Earth, Kepler-186f, discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method[149][150][151] though its mass remains unknown and its parent star is not a Solar analog.
Kapteyn b, discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 earth radii was found orbiting the habitable zone of the red subdwarf Kapteyn's Star, 12.8 light-years away.[152]
On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.[153] Announced 16 January, K2-3d a planet of 1.5 Earth radii was found orbiting within the habitable zone of K2-3, receiving 1.4 times the intensity of visible light as Earth.[154]
Kepler-452b, announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its G-class (solar analog) star Kepler-452.[155][156]
The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, TRAPPIST-1, was announced in May 2016.[157] The discovery is considered significant because it greatly increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.
Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf K2-72 around 227 light year from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.[158]
Announced on the 20 April 2017, LHS 1140b is a super-dense super-Earth 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs.[159] The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.
Discovered by radial velocity in June 2017, with approximately 3 times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light years away.[160]
At 11 light-years away, a second closest planet, Ross 128 b, was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 Earth's mass is it roughly Earth sized and likely rocky in composition.[161]
Discovered in March 2018, K2-155d is about 1.64 time the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.[162][163][164]
| Notable exoplanets – Kepler Space Telescope |
|---|
(Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).[153] |
Habitability outside the CHZ
The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the carbon chauvinism that underpins CHZ theory.
Outside the CHZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.[15][16] Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.[18]
With some theorising that life on Earth may have actually originated in stable, subsurface habitats,[166][167] it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.[168] Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface.[169]
Another possibility is that outside the CHZ organisms may use alternative biochemistries that do not require water at all. Astrobiologist Christopher McKay, has suggested that methane (CH
4) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on 1,610,000,000 km (1.0×109 mi; 11 AU) from the star.[21] This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife.[21] In addition, testing of a number of organisms has found some are capable of surviving in extra-CHZ conditions.[170]
Significance for complex and intelligent life
The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the CHZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a CHZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics)[31] and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation).[171] But others, such as Ian Stewart and Jack Cohen in their 2002 book Evolving the Alien argue that complex intelligent life may arise outside the CHZ.[172] Intelligent life outside the CHZ may have evolved in subsurface environments, from alternative biochemistries[172] or even from nuclear reactions.[173]On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvental ecosystems, supporting large complex life forms such as Riftia pachyptila.[174] Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone.[175] Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space.[176] In addition, the plants Rhizocarpon geographicum and Xanthoria elegans have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize.[177][178][179] The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.[179]
Species, including humans, known to possess animal cognition require large amounts of energy,[180] and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earth analogs in the CHZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits or space habitat analogues on the surface may be eliminated and complex Earth life can thrive.[2]
Planets in the CHZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.[181] The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter ne, which is the average number of planetary-mass objects orbiting within the CHZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the Copernican mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe.[31] A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence[182][183] that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, CHZ candidates are the priority targets to narrow waterhole searches[184][185] and the Allen Telescope Array now extends Project Phoenix to such candidates.[186]
Because the CHZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and the 2003 Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the CHZ.[187][188][189][190] The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its CHZ.[121] A Message from Earth in 2008,[191] and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.
Terraforming
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An artist's conception shows a terraformed Mars in four stages of development.
The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction short story ("Collision Orbit") published during 1942 in Astounding Science Fiction,[1] but the concept may pre-date this work.
Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods of altering the climate of Mars may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to it. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world.
Contents
History of scholarly study
The renowned astronomer Carl Sagan proposed the planetary engineering of Venus in an article published in the journal Science in 1961.[2] Sagan imagined seeding the atmosphere of Venus with algae, which would convert water, nitrogen and carbon dioxide into organic compounds. As this process removed carbon dioxide from the atmosphere, the greenhouse effect would be reduced until surface temperatures dropped to "comfortable" levels. The resulting carbon, Sagan supposed, would be incinerated by the high surface temperatures of Venus, and thus be sequestered in the form of "graphite or some involatile form of carbon" on the planet's surface.[3] However, later discoveries about the conditions on Venus made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated sulfuric acid solution. Even if atmospheric algae could thrive in the hostile environment of Venus's upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thick—the high atmospheric pressure would result in an "atmosphere of nearly pure molecular oxygen" and cause the planet's surface to be thickly covered in fine graphite powder.[3] This volatile combination could not be sustained through time. Any carbon that was fixed in organic form would be liberated as carbon dioxide again through combustion, "short-circuiting" the terraforming process.[3]Sagan also visualized making Mars habitable for human life in "Planetary Engineering on Mars" (1973), an article published in the journal Icarus.[4] Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term "planetary ecosynthesis" instead.[5] The study concluded that it was possible for Mars to support life and be made into a habitable planet. The first conference session on terraforming, then referred to as "Planetary Modeling", was organized that same year.
In March 1979, NASA engineer and author James Oberg organized the First Terraforming Colloquium, a special session at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his book New Earths (1981).[6] Not until 1982 was the word terraforming used in the title of a published journal article. Planetologist Christopher McKay wrote "Terraforming Mars", a paper for the Journal of the British Interplanetary Society.[7] The paper discussed the prospects of a self-regulating Martian biosphere, and McKay's use of the word has since become the preferred term. In 1984, James Lovelock and Michael Allaby published The Greening of Mars.[8] Lovelock's book was one of the first to describe a novel method of warming Mars, where chlorofluorocarbons (CFCs) are added to the atmosphere.
Motivated by Lovelock's book, biophysicist Robert Haynes worked behind the scenes[citation needed] to promote terraforming, and contributed the neologism Ecopoiesis,[9] forming the word from the Greek οἶκος, oikos, "house",[10] and ποίησις, poiesis, "production".[11] Ecopoiesis refers to the origin of an ecosystem. In the context of space exploration, Haynes describes ecopoiesis as the "fabrication of a sustainable ecosystem on a currently lifeless, sterile planet". Fogg defines ecopoiesis as a type of planetary engineering and is one of the first stages of terraformation. This primary stage of ecosystem creation is usually restricted to the initial seeding of microbial life.[12] As conditions approach that of Earth, plant life could be brought in, and this will accelerate the production of oxygen, theoretically making the planet eventually able to support animal life.
Aspects and definitions
Beginning in 1985, Martyn J. Fogg began publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1992. In his book Terraforming: Engineering Planetary Environments (1995), Fogg proposed the following definitions for different aspects related to terraforming:[12]- Planetary engineering: the application of technology for the purpose of influencing the global properties of a planet.
- Geoengineering: planetary engineering applied specifically to Earth. It includes only those macroengineering concepts that deal with the alteration of some global parameter, such as the greenhouse effect, atmospheric composition, insolation or impact flux.
- Terraforming: a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life as we know it. The ultimate achievement in terraforming would be to create an open planetary ecosystem emulating all the functions of the biosphere of Earth, one that would be fully habitable for human beings.
- Habitable Planet (HP): A world with an environment sufficiently similar to Earth as to allow comfortable and free human habitation.
- Biocompatible Planet (BP): A planet possessing the necessary physical parameters for life to flourish on its surface. If initially lifeless, then such a world could host a biosphere of considerable complexity without the need for terraforming.
- Easily Terraformable Planet (ETP): A planet that might be rendered biocompatible, or possibly habitable, and maintained so by modest planetary engineering techniques and with the limited resources of a starship or robot precursor mission.
Habitability requirements
An absolute requirement for life is an energy source, but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before the surface of an astronomical body is able to support life. Of particular interest is the set of factors that has sustained complex, multicellular animals in addition to simpler organisms on Earth. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."[15]
Preliminary stages
Once conditions become more suitable for life of the introduced species, the importation of microbial life could begin.[12] As conditions approach that of Earth, plant life could also be brought in. This would accelerate the production of oxygen, which theoretically would make the planet eventually able to support animal life.Prospective targets
Artist's conception of a terraformed Mars
Mars
In many respects, Mars is the most Earth-like planet in the Solar System.[16] It is thought that Mars once had a more Earth-like environment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years.[17]The exact mechanism of this loss is still unclear, though three mechanisms in particular seem likely: First, whenever surface water is present, carbon dioxide (CO
2) reacts with rocks to form carbonates, thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when plate tectonics works to cause volcanic eruptions that vent carbon dioxide back to the atmosphere. On Mars, the lack of such tectonic activity worked to prevent the recycling of gases locked up in sediments.[18]
Second, the lack of a magnetosphere around Mars may have allowed the solar wind to gradually erode the atmosphere.[18] Convection within the core of Mars, which is made mostly of iron,[19] originally generated a magnetic field. However the dynamo ceased to function long ago,[20] and the magnetic field of Mars has largely disappeared, probably due to "... loss of core heat, solidification of most of the core, and/or changes in the mantle convection regime."[21] Results from the NASA MAVEN mission show that the atmosphere is removed primarily due to Coronal Mass Ejection events, where outbursts of high-velocity protons from the sun impact the atmosphere. Mars does still retain a limited magnetosphere that covers approximately 40% of its surface. Rather than uniformly covering and protecting the atmosphere from solar wind, however, the magnetic field takes the form of a collection of smaller, umbrella-shaped fields, mainly clustered together around the planet's southern hemisphere.[22]
Finally, between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment of objects in the Solar System. The low gravity of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space.[23]
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it.[24] A thicker atmosphere of greenhouse gases such as carbon dioxide would trap incoming solar radiation. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other.[25] Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured.[26]
Artist's conception of a terraformed Venus
Venus
Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 450 °C (723.15 K) surface temperature.[27][28] These goals are closely interrelated, because Venus's extreme temperature is thought to be due to the greenhouse effect caused by its dense atmosphere. Sequestering the atmospheric carbon would likely solve the temperature problem as well.
Artist's conception of what the Moon might look like terraformed
The Moon
Although the gravity on Earth's moon is too low to hold an atmosphere for geological spans of time, if given an atmosphere, it would retain the atmosphere for spans of time that are long compared to human lifespans.[29] Landis[29] and others[30][31] have thus proposed that it could be feasible to terraform the moon, although not all agree with that proposal.[32] Landis estimates that a 1 PSI atmosphere of pure oxygen on the moon would require on the order of two hundred trillion tons of oxygen, and suggests it could be produced by reducing the oxygen from an amount of lunar rock equivalent to a cube about fifty kilometers on an edge. Alternatively, he suggests that the water content of "fifty to a hundred comets" the size of Halley's comet would do the job, "assuming that the water doesn't splash away when the comets hit the moon."[29] Likewise, Benford calculates that terraforming the moon would require "about 100 comets the size of Halley's."[30]Other bodies in the Solar System
Other possible candidates for terraforming (possibly only partial or paraterraforming) include Titan, Callisto, Ganymede, Europa, and even Mercury, Saturn's moon Enceladus, and the dwarf planet Ceres.Other possibilities
Biological Terraforming
Many proposals for planetary engineering involve the use of genetically engineered bacteria.[33][34]As synthetic biology matures over the coming decades it may become possible to build designer organisms from scratch that directly manufacture desired products efficiently.[35] Lisa Nip, Ph.D. candidate at the MIT Media Lab's Molecular Machines group, said that by synthetic biology, scientists could genetically engineer humans, plants and bacteria to create Earth-like conditions on another planet.[36][37]
Gary King, microbiologist at Louisiana State University studying the most extreme organisms on Earth, notes that "synthetic biology has given us a remarkable toolkit that can be used to manufacture new kinds of organisms specially suited for the systems we want to plan for" and outlines the prospects for terraforming, saying "we'll want to investigate our chosen microbes, find the genes that code for the survival and terraforming properties that we want (like radiation and drought resistance), and then use that knowledge to genetically engineer specifically Martian-designed microbes". He sees the project's biggest bottleneck in the ability to genetically tweak and tailor the right microbes, estimating that this hurdle could take "a decade or more" to be solved. He also notes that the it would be best to develop "not a single kind microbe but a suite of several that work together".[38]
DARPA is researching using photosynthesizing plants, bacteria, and algae grown directly on the Mars surface that could warm up and thicken its atmosphere. In 2015 the agency and some of its research partners have created a software called DTA GView − a 'Google Maps of genomes', in which genomes of several organisms can be pulled up on the program to immediately show a list of known genes and where they are located in the genome. According to Alicia Jackson, deputy director of DARPA's Biological Technologies Office by this they have developed a "technological toolkit to transform not just hostile places here on Earth, but to go into space not just to visit, but to stay".[39][40][41][42]
Paraterraforming
Also known as the "worldhouse" concept, paraterraforming involves the construction of a habitable enclosure on a planet which encompasses most of the planet's usable area.[43] The enclosure would consist of a transparent roof held one or more kilometers above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. The worldhouse concept is similar to the concept of a domed habitat, but one which covers all (or most) of the planet.Adapting humans
It has also been suggested that instead of or in addition to terraforming a hostile environment humans might adapt to these places by the use of genetic engineering, biotechnology and cybernetic enhancements.[44][45][46][47][48]Issues
Ethical issues
There is a philosophical debate within biology and ecology as to whether terraforming other worlds is an ethical endeavor. From the point of view of a cosmocentric ethic, this involves balancing the need for the preservation of human life against the intrinsic value of existing planetary ecologies.[49]On the pro-terraforming side of the argument, there are those like Robert Zubrin, Martyn J. Fogg, Richard L. S. Taylor and the late Carl Sagan who believe that it is humanity's moral obligation to make other worlds suitable for life, as a continuation of the history of life transforming the environments around it on Earth.[50][51] They also point out that Earth would eventually be destroyed if nature takes its course, so that humanity faces a very long-term choice between terraforming other worlds or allowing all terrestrial life to become extinct. Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life.
The opposing argument posits that terraforming would be an unethical interference in nature, and that given humanity's past treatment of Earth, other planets may be better off without human interference. Still others strike a middle ground, such as Christopher McKay, who argues that terraforming is ethically sound only once we have completely assured that an alien planet does not harbor life of its own; but that if it does, we should not try to reshape it to our own use, but we should engineer its environment to artificially nurture the alien life and help it thrive and co-evolve, or even co-exist with humans.[52] Even this would be seen as a type of terraforming to the strictest of ecocentrists, who would say that all life has the right, in its home biosphere, to evolve without outside interference.
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