Habitable zone
In
The habitable zone is also called the Goldilocks zone, a metaphor, allusion and antonomasia 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 an HZ planet, including some systems that consist of multiple HZ planets.[7] Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect.[8] 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.[9][10] About 11 billion of these may be orbiting Sun-like stars.[11] 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.[12] The HZ is also of particular interest to the emerging field of habitability of natural satellites, because planetary-mass moons in the HZ might outnumber planets.[13]
In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.[14] 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 in lithospheres and asthenospheres of the Solar System.[15] Sustained by other energy sources, such as tidal heating[16][17] or radioactive decay[18] or pressurized by non-atmospheric means, liquid water may be found even on rogue planets, or their moons.[19] 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,[20] or ammoniates,[21] 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.[22]
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).[23]
The concept of a circumstellar habitable zone was first introduced[24] in 1913, by
The concept of habitable zones was further developed in 1964 by
An update to habitable zone concept came in 2000 when astronomers
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
It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment.[37][38] However, surface conditions are dependent on a host of different individual properties of that planet.[37][38] This misunderstanding is reflected in excited reports of 'habitable planets'.[39][40][41] Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.[38][40][42][43]
Determination
Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.[45]
Solar System estimates
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units,[46][47][48][49] 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 in most estimates 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 vapor.
Despite this, studies are strongly suggestive of past liquid water on the surface of Venus,[59] Mars,[60][61][62] Vesta[63] and Ceres,[64][65] suggesting a more common phenomenon 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 the extended habitable zone concept, 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 their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources.
For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.
Inner edge (AU) | Outer edge (AU) | Year | Notes |
---|---|---|---|
0.725 | 1.24 | 1964, Dole[2] | Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone. |
1.005–1.008 | 1969, Budyko[66] | 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[67] and North 1975.[68] | |
0.92–0.96 | 1970, Rasool and De Bergh[69] | 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. | |
0.958 | 1.004 | 1979, Hart[70] | Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications. |
3.0 | 1992, Fogg[44] | Used the carbon cycle to estimate the outer edge of the circumstellar habitable zone. | |
0.95 | 1.37 | 1993, Kasting et al.[28] | 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 | 2010, Spiegel et al.[71] | Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity. | |
0.75 | 2011, Abe et al.[72] | Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth. | |
10 | 2011, Pierrehumbert and Gaidos[47] | 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 the Solar System. | |
0.77–0.87 | 1.02–1.18 | 2013, Vladilo et al.[73] | Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar. |
0.99 | 1.67 | 2013, Kopparapu et al.[4][74] | Revised estimates of the Kasting et al. (1993) formulation using updated moist 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 moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapor. Once the stratosphere becomes wet, water vapor photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a CO2 dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in CO2 will not create enough warming to prevent CO2 catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.67 AU. This definition does not take into account possible radiative warming by CO2 clouds. |
0.38 | 2013, Zsom et al. [46] |
Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere. | |
0.95 | 2013, Leconte et al.[75] | Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System. | |
0.95 | 2.4 | 2017, Ramirez and Kaltenegger [48] |
An expansion of the classical carbon dioxide-water vapor habitable zone[28] assuming a volcanic hydrogen atmospheric concentration of 50%. |
0.93–0.91 | 2019, Gomez-Leal et al. [76] |
Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of the climate sensitivity in both cases. | |
0.99 | 1.004 | Tightest bounded estimate from above | |
0.38 | 10 | Most relaxed estimate from above |
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, according to Kopparapu's habitable zone estimate, 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 HZ concept is more complex.
Spectral types and star-system characteristics
Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain
With regard to spectral types,
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 modeling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking.[83] Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.[84] At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.[82] 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.[85]
Stellar evolution
Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the main sequence for fewer than 10 million years,[86] would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve.[87][88] Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now,[89] and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase.[90] In order to deal with this increase in luminosity, the concept of a continuously habitable zone has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.[90]
In red dwarf systems, gigantic
Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.
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.[102][103]
Other considerations
A planet cannot have a
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.[110] Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.[111] 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.[73] Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained[73] because even a small shift in pressure or temperature could render water unable to form as a liquid.[112]
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),[28] a study[48] 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 the 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[47] 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 HZs of red dwarf stars, the extremely close distances to the stars cause
Planetary mass
A planetary object that orbits a star with high
Extrasolar discoveries
A 2015 review concluded that the
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 HZ, at 0.48,[4] meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.[119] However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.[120]
Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.
Early findings
The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.[124] 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.[125] 16 Cygni Bb, also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the HZ, 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.[126]
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.[127] 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[128] and has a low orbital eccentricity, comparable to that of Mars in the Solar System.[129] Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,[130] though it is unclear whether such satellites could form in the first place.[131]
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.[132] The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A.[133][134] Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.[135]
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 discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.
Habitable super-Earths
The 2007 discovery of
Discovered in August 2011, HD 85512 b was initially speculated to be habitable,[138] but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.[120]
Near Earth-sized planets and Solar analogs
Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above 1.5 R🜨 the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core.[150] A genuinely Earth-like planet – an Earth analog or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology.
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, some stars are nearly identical to the Sun and are 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.[151] Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation.[152][153][154]
Using data collected by NASA's
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 the Sun, in the HZ and expected to offer habitable conditions.[156][157][158][159] 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.[158][159][160]
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[161][162][163] though its mass remains unknown and its parent star is not a Solar analog.
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.[165] However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 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.[166]
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.[167][168]
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.[169] The discovery is considered significant because it dramatically 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 years from the Sun: K2-72c and K2-72e are both of similar size to Earth and receive similar amounts of stellar radiation.[170]
Announced on the 20 April 2017,
Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away.[172]
At 11 light-years away, the 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 times Earth's mass, is it roughly Earth-sized and likely rocky in composition.[173]
Discovered in March 2018, K2-155d is about 1.64 times the radius of Earth, is likely rocky and orbits in the habitable zone of its red dwarf star 203 light years away.[174][175][176]
One of the earliest discoveries by the
K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.
In September 2020, astronomers identified 24
Notable Kepler Space Telescope
|
---|
Confirmed small exoplanets in habitable zones.
(Kepler-62e, Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b) (Kepler Space Telescope; January 6, 2015).[165] |
Habitability outside the HZ
Liquid-water environments have been found to exist in the absence of atmospheric pressure and at temperatures outside the HZ temperature range. For example,
Outside the HZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.[16][17] 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.[19]
With some theorising that life on Earth may have actually originated in stable, subsurface habitats,[180][181] it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.[182] On Earth itself, living organisms may be found more than 6 km (3.7 mi) below the surface.[183]
Another possibility is that outside the HZ organisms may use
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.[22] 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.[22] In addition, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.[184]
Significance for complex and intelligent life
The
On Earth, several complex multicellular life forms (or
Species, including
Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.
Because the HZ is considered the most likely habitat for intelligent life,
See also
- Habitability of binary star systems
- Habitability of F-type main-sequence star systems
- Habitability of K-type main-sequence star systems
- Habitability of neutron star systems
- Habitability of red dwarf systems
- Habitability of yellow dwarf systems
- Habitable zone for complex life
References
- ^ Su-Shu Huang, American Scientist 47, 3, pp. 397–402 (1959)
- ^ a b c d e Dole, Stephen H. (1964). Habitable Planets for Man. Blaisdell Publishing Company. p. 103.
- ^ a b J. F. Kasting, D. P. Whitmire, R. T. Reynolds, Icarus 101, 108 (1993).
- ^ S2CID 119103101.
- PMID 23641107.
- ^ ISBN 978-0-415-08689-9.
- ^ Overbye, Dennis (January 6, 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times. Retrieved January 6, 2015.
- S2CID 17080374.
- ^ Overbye, Dennis (November 4, 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Retrieved November 5, 2013.
- PMID 24191033.
- ^ Khan, Amina (November 4, 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved November 5, 2013.
- S2CID 4451513.
- ^ Schirber, Michael (26 Oct 2009). "Detecting Life-Friendly Moons". Astrobiology Magazine. NASA. Archived from the original on 29 October 2009. Retrieved 9 May 2013.
- S2CID 123220355. Archived from the original(PDF) on 2016-06-02. Retrieved 2016-05-03.
- ISSN 0084-6597.
- ^ a b Cowen, Ron (2008-06-07). "A Shifty Moon". Science News. Archived from the original on 2011-11-04. Retrieved 2013-04-22.
- ^ a b Bryner, Jeanna (24 June 2009). "Ocean Hidden Inside Saturn's Moon". Space.com. TechMediaNetwork. Retrieved 22 April 2013.
- S2CID 73631942.
- ^ a b "Rogue Planets Could Harbor Life in Interstellar Space, Say Astrobiologists". MIT Technology Review. MIT Technology Review. 9 February 2011. Archived from the original on 7 October 2015. Retrieved 24 June 2013.
- ^ Wall, Mike (28 September 2015). "Salty Water Flows on Mars Today, Boosting Odds for Life". Space.com. Retrieved 2015-09-28.
- PMID 26315260.
- ^ a b c d Villard, Ray (November 18, 2011). "Alien Life May Live in Various Habitable Zones: Discovery News". News.discovery.com. Discovery Communications LLC. Retrieved April 22, 2013.
- ^ Newton, Isaac (1729). "Book III - Section I - Proposition VIII - Corol. 4". Philosophiae Naturalis Principia Mathematica (PDF) (3rd ed.). p. 739. Archived (PDF) from the original on 13 November 2023.
- ISBN 978-1108471541.
- S2CID 219930646.
- ^ Strughold, Hubertus (1953). The Green and Red Planet: A Physiological Study of the Possibility of Life on Mars. University of New Mexico Press.
- ISBN 978-0-691-13805-3. Retrieved 4 May 2013.
- ^ PMID 11536936.
- Bibcode:1966elab.book.....S.
- doi:10.1086/127489.
- ISBN 978-0-387-00436-5.
- ^ "The Goldilocks Zone" (Press release). NASA. October 2, 2003. Archived from the original on August 29, 2011. Retrieved April 22, 2013.
- S2CID 206546351.
- ^ ISBN 978-0-387-95289-5.
- S2CID 18179704.
- ^ a b c d Hadhazy, Adam (April 3, 2013). "The 'Habitable Edge' of Exomoons". Astrobiology Magazine. NASA. Archived from the original on May 2, 2013. Retrieved April 22, 2013.
- ^ S2CID 118952886.
- ^ a b c No one agrees what it means for a planet to be "habitable". Neel V. Patel, MIT Technology Review. 2 October 2019. Quote: surface conditions are dependent on a host of different individual properties of that planet, such as internal and geological processes, magnetic field evolution, climate, atmospheric escape, rotational effects, tidal forces, orbits, star formation and evolution, unusual conditions like binary star systems, and gravitational perturbations from passing bodies.
- ^ Tan, Joshua (8 February 2017). "Until we get better tools, excited reports of 'habitable planets' need to come back down to Earth". The Conversation. Retrieved 2019-10-21.
- ^ a b "Why just being in the habitable zone doesn't make exoplanets livable". Science News. 2019-10-04. Retrieved 2019-10-21.
- ^ No, the Exoplanet K2-18b Is Not Habitable. News outlets that said otherwise are just crying wolf—but they're not the only ones at fault. Laura Kreidberg, Scientific American. 23 September 2019.
- ^ Tasker, Elizabeth. "Let's Lose the Term "Habitable Zone" for Exoplanets". Scientific American Blog Network. Retrieved 2019-10-21.
- ^ Ruher, Hugo (2019-10-20). "Exoplanètes: faut-il en finir avec la "zone d'habitabilité"? - Sciences". Numerama (in French). Retrieved 2019-10-21.
- ^ PMID 11539465.
- PMID 11538226.
- ^ S2CID 27805994.
- ^ S2CID 7404376.
- ^ S2CID 119333468.
- ^ "Stellar habitable zone calculator". University of Washington. Retrieved 17 December 2015.
- ^ "Venus". Case Western Reserve University. 13 September 2006. Archived from the original on 2012-04-26. Retrieved 2011-12-21.
- ^ Sharp, Tim. "Atmosphere of the Moon". Space.com. TechMediaNetwork. Retrieved April 23, 2013.
- ISBN 978-3-642-03629-3.
- ^ ISSN 0148-0227.
- ^ Mann, Adam (February 18, 2014). "Strange Dark Streaks on Mars Get More and More Mysterious". Wired. Retrieved February 18, 2014.
- ^ "NASA Finds Possible Signs of Flowing Water on Mars". voanews.com. 3 August 2011. Archived from the original on September 17, 2011. Retrieved August 5, 2011.
- ^ "Is Mars Weeping Salty Tears?". news.sciencemag.org. Archived from the original on August 14, 2011. Retrieved August 5, 2011.
- ^ Webster, Guy; Brown, Dwayne (December 10, 2013). "NASA Mars Spacecraft Reveals a More Dynamic Red Planet". NASA. Retrieved December 10, 2013.
- .
- S2CID 135136696.
- ^ "Flashback: Water on Mars Announced 10 Years Ago". SPACE.com. June 22, 2000. Retrieved December 19, 2010.
- ^ "Flashback: Water on Mars Announced 10 Years Ago". SPACE.com. June 22, 2010. Retrieved May 13, 2018.
- ^ "Science@NASA, The Case of the Missing Mars Water". Archived from the original on March 27, 2009. Retrieved March 7, 2009.
- ISSN 0012-821X.
- PMID 29546238.
- ^ NASA.gov PIA21471: Landslides on Ceres
- .
- .
- .
- (PDF) from the original on 14 November 2023.
- .
- S2CID 15899053.
- PMID 21707386.
- ^ S2CID 49553651.
- S2CID 76651902.
- S2CID 2115695.
- S2CID 119209241.
- S2CID 118610856.
- PMID 9360920.
- PMID 11543507.
- ^ Vu, Linda. "Planets Prefer Safe Neighborhoods" (Press release). Spitzer.caltech.edu. NASA/Caltech. Retrieved April 22, 2013.
- S2CID 2241081.
- ^ PMID 23537137.
- ^ S2CID 14119086.
- S2CID 118739494.
- S2CID 119276912.
- ^ Carroll, Bradley W.; Ostlie, Dale A. (2007). An Introduction to Modern Astrophysics (2nd ed.).
- ^ Richmond, Michael (November 10, 2004). "Late stages of evolution for low-mass stars". Rochester Institute of Technology. Retrieved 2007-09-19.
- S2CID 118500534.
- PMID 11539665.
- ^ ISBN 1-58381-109-5. Retrieved April 26, 2013.
- ^ Croswell, Ken (January 27, 2001). "Red, willing and able". New Scientist. Retrieved August 5, 2007. Full reprint
- .
- ^ PMID 16318021.
- ^ Research Corporation (December 19, 2006). "Andrew West: 'Fewer flares, starspots for older dwarf stars'". EarthSky. Retrieved April 27, 2013.
- ^ Cain, Fraser; Gay, Pamela (2007). "AstronomyCast episode 40: American Astronomical Society Meeting, May 2007". Universe Today. Archived from the original on 2007-09-26. Retrieved 2007-06-17.
- ^ Ray Villard (27 July 2009). "Living in a Dying Solar System, Part 1". Astrobiology. Archived from the original on 24 April 2016. Retrieved 8 April 2016.
- ^ Christensen, Bill (April 1, 2005). "Red Giants and Planets to Live On". Space.com. TechMediaNetwork. Retrieved April 27, 2013.
- ^ S2CID 119225201.
- ^ S2CID 17075384.
- S2CID 14172341.
- ^ Voisey, Jon (February 23, 2011). "Plausibility Check – Habitable Planets around Red Giants". Universe Today. Retrieved April 27, 2013.
- ^ Alien Life More Likely on 'Dune' Planets Archived December 2, 2013, at the Wayback Machine, 09/01/11, Charles Q. Choi, Astrobiology Magazine
- PMID 21707386.
- S2CID 12808812.
- ISBN 978-0-521-85200-5.
- S2CID 15999168.
- S2CID 4360404.
- S2CID 8369390.
- ^ Vastag, Brian (December 5, 2011). "Newest alien planet is just the right temperature for life". The Washington Post. Retrieved April 27, 2013.
- S2CID 54997095.
- S2CID 7852371.
- ^ Chaplin, Martin (April 8, 2013). "Water Phase Diagram". Ices. London South Bank University. Retrieved April 27, 2013.
- .
- ^ Becquerel P. (1950). "La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve". C. R. Acad. Sci. Paris (in French). 231: 261–263.
- ISBN 978-94-007-1895-1.
- S2CID 10551100.
- ^ Paul Gilster; Andrew LePage (2015-01-30). "A Review of the Best Habitable Planet Candidates". Centauri Dreams, Tau Zero Foundation. Retrieved 2015-07-24.
- ISBN 978-3-319-17004-6.
- ^ Wethington, Nicholos (September 16, 2008). "How Many Stars are in the Milky Way?". Universe Today. Retrieved April 21, 2013.
- ^ a b Torres, Abel Mendez (April 26, 2013). "Ten potentially habitable exoplanets now". Habitable Exoplanets Catalog. University of Puerto Rico. Archived from the original on October 21, 2019. Retrieved April 29, 2013.
- ^ Borenstein, Seth (19 February 2011). "Cosmic census finds crowd of planets in our galaxy". Associated Press. Archived from the original on 27 September 2011. Retrieved 24 April 2011.
- ^ Choi, Charles Q. (21 March 2011). "New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone". Space.com. Retrieved 2011-04-24.
- S2CID 119290692.
- S2CID 37593615.
- ^ "70 Virginis b". Extrasolar Planet Guide. Extrasolar.net. Archived from the original on 2012-06-19. Retrieved 2009-04-02.
- S2CID 37593615.
- S2CID 16004653.
- S2CID 119078585.
- S2CID 119067572.
- S2CID 14508244.
- S2CID 4327454.
- S2CID 4343578.
- ^ a b "Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri". Sciencedaily.com. November 6, 2007. Archived from the original on 26 September 2008. Retrieved 2008-09-14.
- S2CID 55779685.
- ^ Ian Sample, science correspondent (7 November 2007). "Could this be Earth's near twin? Introducing planet 55 Cancri f". The Guardian. London. Archived from the original on 2 October 2008. Retrieved 17 October 2008.
- ^ Than, Ker (2007-02-24). "Planet Hunters Edge Closer to Their Holy Grail". space.com. Retrieved 2007-04-29.
- S2CID 206556796.
- ^ "Researchers find potentially habitable planet" (in French). maxisciences.com. 2011-08-30. Retrieved 2011-08-31.
- ^ "Kepler 22-b: Earth-like planet confirmed". BBC. December 5, 2011. Retrieved May 2, 2013.
- ^ Scharf, Caleb A. (2011-12-08). "You Can't Always Tell an Exoplanet by Its Size". Scientific American. Retrieved 2012-09-20.: "If it [Kepler-22b] had a similar composition to Earth, then we're looking at a world in excess of about 40 Earth masses".
- S2CID 16531923.
- ^ Staff (September 20, 2012). "LHS 188 – High proper-motion Star". Centre de données astronomiques de Strasbourg (Strasbourg astronomical Data Center). Retrieved September 20, 2012.
- ^ Méndez, Abel (August 29, 2012). "A Hot Potential Habitable Exoplanet around Gliese 163". University of Puerto Rico at Arecibo (Planetary Habitability Laboratory). Archived from the original on October 21, 2019. Retrieved September 20, 2012.
- ^ Redd (September 20, 2012). "Newfound Alien Planet a Top Contender to Host Life". Space.com. Retrieved September 20, 2012.
- ^ "A Hot Potential Habitable Exoplanet around Gliese 163". Spacedaily.com. Retrieved 2013-02-10.
- S2CID 7424216.
- ^ Aron, Jacob (December 19, 2012). "Nearby Tau Ceti may host two planets suited to life". New Scientist. Reed Business Information. Retrieved April 1, 2013.
- S2CID 2390534.
- ^ Torres, Abel Mendez (May 1, 2013). "The Habitable Exoplanets Catalog". University of Puerto Rico. Retrieved May 1, 2013.
- ^ Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"
- ^ "Solar Variability and Terrestrial Climate". NASA Science. 2013-01-08.
- ^ "Stellar Luminosity Calculator". University of Nebraska-Lincoln astronomy education group.
- ISBN 978-0-309-26564-5.
- ^ Most of Earth's twins aren't identical, or even close!, By Ethan. June 5, 2013.
- ^ "Are there oceans on other planets?". National Oceanic and Atmospheric Administration. 6 July 2017. Retrieved 2017-10-03.
- ^ Moskowitz, Clara (January 9, 2013). "Most Earth-Like Alien Planet Possibly Found". Space.com. Retrieved January 9, 2013.
- S2CID 51490784.
- ^ a b Johnson, Michele; Harrington, J.D. (18 April 2013). "NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date". NASA. Archived from the original on 8 May 2020. Retrieved 18 April 2013.
- ^ a b Overbye, Dennis (18 April 2013). "Two Promising Places to Live, 1,200 Light-Years from Earth". The New York Times. Retrieved 18 April 2013.
- S2CID 21029755.
- ^ Chang, Kenneth (17 April 2014). "Scientists Find an 'Earth Twin,' or Maybe a Cousin". The New York Times. Retrieved 17 April 2014.
- AP News. Retrieved 17 April 2014.
- ^ Morelle, Rebecca (17 April 2014). "'Most Earth-like planet yet' spotted by Kepler". BBC News. Retrieved 17 April 2014.
- ^ Wall, Mike (3 June 2014). "Found! Oldest Known Alien Planet That Might Support Life". Space.com. Retrieved 10 January 2015.
- ^ a b Clavin, Whitney; Chou, Felicia; Johnson, Michele (6 January 2015). "NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones". NASA. Retrieved 6 January 2015.
- Science Daily. Retrieved 25 July 2015.
- S2CID 26447864.
- ^ "NASA telescope discovers Earth-like planet in star's habitable zone". BNO News. 23 July 2015. Retrieved 23 July 2015.
- ^ "Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star". European Southern Observatory. 2 May 2016.
- S2CID 13419148.
- S2CID 2718408.
- ^ Bradley, Sian (2017-11-16). "Astronomers are beaming techno into space for aliens to decode". Wired UK.
- ^ "In Earth's Backyard: Newfound Alien Planet May be Good Bet for Life". Space.com. 15 November 2017.
- ^ "K2-155 d". Exoplanet Exploration. 2018.
- ^ Mack, Eric (March 13, 2018). "A super-Earth around a red star could be wet and wild". CNET.
- ^ Whitwam, Ryan (March 14, 2018). "Kepler Spots Potentially Habitable Super-Earth Orbiting Nearby Star". ExtremeTech.
- ISSN 0004-6361.
- PMID 32955925.
- ^ Torres, Abel (2012-06-12). "Liquid Water in the Solar System". Archived from the original on 2013-11-18. Retrieved 2013-12-15.
- ^ Munro, Margaret (2013), "Miners deep underground in northern Ontario find the oldest water ever known", National Post, retrieved 2013-10-06
- PMID 11382135.
- ^ Taylor, Geoffrey (1996), "Life Underground" (PDF), Planetary Science Research Discoveries, retrieved 2013-10-06
- ^ Doyle, Alister (4 March 2013), "Deep underground, worms and "zombie microbes" rule", Reuters, retrieved 2013-10-06
- ^
Nicholson, W. L.; Moeller, R.; Horneck, G.; PROTECT Team (2012). "Transcriptomic Responses of Germinating Bacillus subtilis Spores Exposed to 1.5 Years of Space and Simulated Martian Conditions on the EXPOSE-E Experiment PROTECT". Astrobiology. 12 (5): 469–86. PMID 22680693.
- ISBN 978-3-642-13178-3.
- ^ ISBN 978-0-09-187927-3.
- ^
Goldsmith, Donald; Owen, Tobias (1992). The Search for Life in the Universe (2 ed.). ISBN 978-0-201-56949-0.
- ISBN 978-0-262-69298-4.
- PMID 11538217.
- .
- ^ Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Archived from the original on 28 May 2012. Retrieved 27 April 2012.
- ^ de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). European Geosciences Union. Archived from the original (PDF) on 4 May 2012. Retrieved 27 April 2012.
- ^ PMID 26684504.
- PMID 17148287.
- ^ Palca, Joe (September 29, 2010). "'Goldilocks' Planet's Temperature Just Right For Life". NPR. NPR. Retrieved April 5, 2011.
- ^ "Project Cyclops: A design study of a system for detecting extraterrestrial intelligent life" (PDF). NASA. 1971. Retrieved June 28, 2009.
- ISBN 978-1-4381-0892-6. Retrieved 26 June 2013.
- S2CID 14734094.
- S2CID 119302350.
- ^ Wall, Mike (2011). "HabStars: Speeding Up In the Zone". Space.com. Retrieved 2013-06-26.
- ^ Zaitsev, A. L. (June 2004). "Transmission and reasonable signal searches in the Universe" (PDF). Horizons of the Universe Передача и поиски разумных сигналов во Вселенной. Plenary presentation at the National Astronomical Conference WAC-2004 "Horizons of the Universe", Moscow, Moscow State University, June 7, 2004 (in Russian). Moscow. Archived from the original on 2019-05-30. Retrieved 2013-06-30.
- ^ David Grinspoon (July 13, 2012) [December 12, 2007]. "Who Speaks for Earth?". Seed. Archived from the original on 2012-07-13. Retrieved 2021-06-24.
- ^
P. C. Gregory; D. A. Fischer (2010). "A Bayesian periodogram finds evidence for three planets in 47 Ursae Majoris". S2CID 16722873.
- ^
B. Jones; Underwood, David R.; et al. (2005). "Prospects for Habitable "Earths" in Known Exoplanetary Systems". S2CID 119089227.
- ^ Moore, Matthew (October 9, 2008). "Messages from Earth sent to distant planet by Bebo". London: .telegraph.co.uk. Archived from the original on 11 October 2008. Retrieved 2008-10-09.
External links
- "Circumstellar Habitable Zone Simulator". Astronomy Education at the University of Nebraska-Lincoln.
- "The Habitable Exoplanets Catalog". PHL/University of Puerto Rico at Arecibo.
- "The Habitable Zone Gallery".
- "Stars and Habitable Planets". SolStation. Archived from the original on 2011-06-28.
- Nikos Prantzos (2006). "On the Galactic Habitable Zone". Space Science Reviews. 135 (1–4): 313–322. S2CID 119441813.
- Interstellar Real Estate: Location, Location, Location – Defining the Habitable Zone
- Shiga, David (November 19, 2009). "Why the universe may be teeming with aliens". New Scientist.
- Simmons; et al. "The New Worlds Observer: a mission for high-resolution spectroscopy of extra-solar terrestrial planets" (PDF). New Worlds.
- Cockell, Charles S.; Herbst, Tom; Léger, Alain; Absil, O.; Beichman, Charles; Benz, Willy; Brack, Andre; Chazelas, Bruno; Chelli, Alain (2009). "Darwin – an experimental astronomy mission to search for extrasolar planets" (PDF). Experimental Astronomy. 23 (1): 435–461. S2CID 32204693.
- Atkinson, Nancy (March 19, 2009). "JWST Will Provide Capability to Search for Biomarkers on Earth-like Worlds". Universe Today. Archived from the original on March 27, 2009. Retrieved February 6, 2011.