Astronomy:Superhabitable planet

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Short description: Hypothetical type of planet that may be better-suited for life than Earth
Artist's impression of one possible appearance of a superhabitable planet. The reddish hue is vegetation.[1]

A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search for habitable exoplanets and proposed clarifications. The authors argue that the mediocrity principle cannot explain why Earth should represent the typical conditions for planetary habitability, and that in order to identify a habitable planet, a new model of characterization is needed that is "biocentric rather than geo- or anthropocentric."[2]

Heller and Armstrong subsequently define a superhabitable world as a rocky planet or moon that could support more diverse flora and fauna than there are on Earth, as it would empirically show that its natural environment is more hospitable to life.[3] Stellar and planetary characteristics considered for "superhabitability" include the planet's age, geological activity, atmospheric composition, ocean coverage, and the star's spectral type, among other things. According to the authors, these worlds would likely be larger, warmer, wetter, and older than Earth, and orbiting K-type main-sequence stars.[4] In 2020, Dirk Schulze-Makuch and colleagues identified 24 potentially superhabitable planets based on measured characteristics that fit this criteria.[5]

Characteristics

Star

A star's characteristics largely determines the conditions present within a system and is a key consideration for planetary habitability.[6][7] The most massive stars—O, B, and A-type, respectively—have relatively short lives on the main sequence, ranging from roughly one billion years for A-type stars to only one million years for O-type stars, which are too short for complex life to emerge.[8][9] On the other hand, the less massive M-type stars (i.e., red dwarfs) are by far the most common and long-lived stars of the universe, although their potential for supporting life is still under study. Due to the low luminosity of red dwarfs, the habitable zone[lower-alpha 1] is in very close proximity to the star, causing any planet to become tidally locked and exposes them to frequent outbreaks of high-energy radiation.[6][12]

Dismissing both ends, late G to mid K-type stars—collectively known as orange dwarfs—arguably offer the best conditions for life.[3][12] These stars have a relatively long life expectancy on the main sequence (18 to 34 billion years, compared to 10 billion for Sun-like stars) while also providing stable habitable zones that are free of excessive radiation caused by close proximity.[13][12] The habitable zones of orange dwarfs also do not move very much during their lifetimes, allowing an Earth-like planet more time for complex life to emerge.[14][lower-alpha 2] Orange dwarfs are also believed to emit less ultraviolet radiation than Sun-like stars, which could allow complex life to develop without the need for a protective ozone layer.[3][16]

Habitable zone (HZ) position of some of the most similar and average surface temperature exoplanets.[17][lower-alpha 3]

Orbit

Because a main sequence star's luminosity gradually increases throughout its life, its habitable zone is not static but gradually moving outward as well.[20] This means that any planet will experience a limited time within this region, known as its habitable zone lifetime.[20] For example, studies suggest that Earth's orbit already lies near the inner edge of the Solar System's habitable zone,[21] which could harm its long-term livability as it will eventually leave the habitable zone altogether.[22][23] Therefore, superhabitable planets must be warmer than Earth, yet orbit further out and closer to the center of the habitable zone,[18][19] which would be possible through a stronger greenhouse effect in its atmosphere.[24][25]

The orbit of a superhabitable planet should ideally be located within the habitable zone,[26][24] but knowing whether or not a planet is in this region is insufficient to determine its habitability.[3] Not all rocky planets in the habitable zone may be habitable, while tidal heating can render planets or moons habitable beyond this region. For example, Jupiter's moon Europa is well beyond the outer limits of the Solar System's habitable zone, yet as a result of its orbital interactions with the other Galilean moons, it is believed to have an ocean of liquid water hidden beneath its icy surface.[27]

Age

The earliest stars in the universe were metal-free stars, which was initially believed to prevent the formation of rocky planets.

It was initially believed that since older stars contained little to no heavy elements (i.e., metallicity), they were incapable of forming rocky planets.[28][29] Early exoplanet discoveries supported this hypothesis, as they were mostly gas giants orbiting in close proximity to stars with a heavy metal abundance. However, in 2012, the Kepler space telescope challenged this assumption when it discovered many rocky exoplanets that were previously undetectable.[29] These findings suggested that the formation of rocky planets were not as dependent on stellar metallicity as gas giants were, and that the first Earth-sized planets likely appeared much earlier, at around 7 and 12 billion years ago.[29]

Heller and Armstrong argue that the optimal age for a planet should be greater than Earth's age (4.5 billion years) but less than the first formation of rocky planets (7 billion years).[14] This is based on the belief that planets older than Earth may have greater biodiversity, since native species have had more time to evolve, adapt, and stabilize the environmental conditions suitable for life.[14]

Mass and radius

Studies indicate that there is a natural radius limit, set at 1.6 R🜨, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures.[30] For the purpose of general habitability, it is expected that any exoplanet similar to Earth's density and with a radius under 2 R🜨 may be suitable for life.[31] Heller and Armstrong argue that the optimal mass and radius of a superhabitable world should be largely determined by geological activity; the more massive a planetary body, the longer time it will continuously generate internal heat–a major contributing factor to plate tectonics.[32] Too much mass, however, can slow plate tectonics by increasing the pressure of the mantle, while rocky planets with more than 7 M🜨 are understood to lack plate tectonics altogether.[32] It is believed that plate tectonics peak in bodies between 1 and 5 M🜨, with an ideal mass of approximately 2 M🜨.[33][lower-alpha 4] For this planet to provide a density similar to the Earth's, its radius should be between 1.2 and 1.3 R🜨.[33][32]

Kepler-62e, second from the left has a radius of 1.6 R🜨. Earth is on the far right; scaled.

Plate tectonics

An important geological process is plate tectonics, which appears to be common in terrestrial planets with a significant rotation speed and an internal heat source.[35] If large bodies of water are present on a planet, plate tectonics can maintain high levels of carbon dioxide (CO2) in its atmosphere and increase the global surface temperature through the greenhouse effect.[36][37] However, if tectonic activity is not significant enough to increase temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by another energy source like tidal heating or stellar irradiation.[38] On the other hand, if the effects of any of these processes are too strong, the amount of greenhouse gases in the atmosphere could cause a runaway greenhouse effect by trapping heat and preventing adequate cooling, similar to what is found on Venus.

Magnetosphere

The presence of a significant magnetosphere is important to the long-term survivability of life on a planet. A sufficiently strong magnetosphere effectively shields a planet's surface and atmosphere against ionizing radiation emanating from the interstellar medium and the host star.[39] A planet's magnetosphere is believed to be generated through a combination of dynamic processes that includes an internal heat source, an electrically conductive fluid like molten iron, and a significant rotation speed.

Experts have not reached a consensus on the optimal rotation speed for planetary habitability, but it should not be too fast or too slow. A planet with too slow of a rotation can cause problems similar to those observed with Venus; because it only completes a single rotation every 243 Earth days, it cannot generate an Earth-like magnetic field. Less massive bodies and those that are tidally locked can also have a weak to non-existent magnetic field, which over time can result in the loss of a significant portion of its atmosphere by hydrodynamic escape[32] and becoming a planet that resembles Mars. A more massive, slow-rotation planet could overcome this problem by hosting multiple moons, which through their combined gravitational effects, can boost the planet's magnetic field.[40][41]

Surface

Artistic impression of a possible Earth analog, Kepler-186f. Some superhabitable planets could have a similar appearance and may not have important differences with Earth.

The appearance of a superhabitable planet should be similar to the conditions found in the tropical climates of Earth.[42] Due to the denser atmosphere and less temperature variation across regions of the planet, such a planet would lack any major ice sheets,[24] have a higher concentration of clouds, and abundant rainfall.

As a result of these climatic conditions, plant life would potentially cover more of the planet's surface and be visible from space.[42] When also considering the differences in the peak wavelength of visible light for K-type stars and the assumed lower stellar flux of the planet, the vegetation may exhibit colors different than the typical green color found on Earth.[1][43] Instead, researchers speculate that vegetation on these worlds could have a red, orange, or even purple appearance.[44]

Moreover, it is hypothesized that the higher surface gravity of super-Earths would reduce the average ocean depth and create shallow ocean basins, providing the optimal environment for marine life to thrive.[45][46][47] For example, marine ecosystems found in the shallow areas of Earth's oceans and seas, given the amount of light and heat they receive, are observed to have greater biodiversity and are generally seen as being more comfortable for aquatic species. This has led researchers to speculate that shallow water environments on exoplanets should be similarly suitable for life.[48][49]

The climate of a warmer and wetter terrestrial exoplanet may resemble that of the tropical regions of Earth. In the picture, mangrove in Cambodia.

Climate

In general, the climate of a superhabitable planet would be warm, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences.[50][31] These characteristics are in contrast to those found on Earth, which has more variable and inhospitable regions that include frigid tundra and dry deserts. Deserts on superhabitable planets would be more limited in area and would likely support habitat-rich coastal environments.[51]

The optimum surface temperature for Earth-like life is unknown, although it appears that on Earth, organism diversity has been greater in warmer periods.[52] It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life.[51] The denser atmosphere of a superhabitable planet would naturally provide a greater average temperature than an Earth-like atmosphere. Ideally, the temperature should reach the optimal levels for plant life, which is 25 °C (77 °F). In addition, a large distributed ocean would have the ability to regulate a planet's surface temperature similar to Earth's ocean currents, and could allow it to maintain a moderate temperature within the habitable zone.[53][51]

There are no solid arguments to explain if Earth's atmosphere has the optimal composition to host life.[24] On Earth, during the period when coal was first formed, atmospheric oxygen (O2) levels were up to 35%, and coincided with the periods of greatest biodiversity.[54] It is therefore hypothesized that oxygen abundance in the atmosphere is essential for complex life on other worlds.[55][24]

A superhabitable planet with a mass greater than Earth would have a greater surface gravity, which would lead to a denser atmosphere and lower variability of the global climate.[56][57][58] If the atmosphere contains enough oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases. They would, however, need to develop adaptations to the increased gravity, such as an increase in muscle and bone density.[42][59][60]

Profile summary

A size comparison and artist's impression of Kepler-442b (1.34 R🜨) to the Earth (right).

Despite the scarcity of information available, the hypotheses presented above on superhabitable planets can be summarized as a preliminary profile.[56]

  • Star: Intermediate K-type star, i.e., an orange dwarf.
  • Age: 4.5–7 billion years old
  • Orbital distance: close to the midpoint of the habitable zone.
  • Mass: 2 M🜨
  • Radius: 1.2–1.3 R⊕
  • Oceans: Shallow ocean covering majority of surface with no large continuous land masses.
  • Surface temperature: ~25 °C (77 °F)[50]
  • Atmosphere: Denser than Earth with higher concentration of oxygen.
  • Abundance: The amount could exceed Earth analogs.

List of potentially superhabitable planets

In September 2020, Dirk Schulze-Makuch and colleagues identified 24 contenders for superhabitable planets out of more than 4000 confirmed exoplanets and exoplanet candidates.[5] Their criteria for superhabitability included measurable factors like the star’s spectral type and the planet’s age, mass, radius, and surface temperature. Additional factors that are more challenging to measure, such as the presence of abundant water, a large moon, and active geological processes like plate tectonics, were also considered.[5]

Among the identified contenders, Kepler-1126b (KOI-2162.01) and Kepler-69c (KOI-172.02) are the only ones that have been confirmed as exoplanets.[5] However, it is important to note that these planets may have characteristics of superhabitability without actually being superhabitable. For example, earlier research on Kepler-69c suggests that its atmospheric composition might be similar to that of a "super-Venus"—a term used to describe a super-Earth with extreme greenhouse conditions—which would not be conducive to support life.[61] The full list of identified superhabitable planets includes:

  • Kepler-1126b (confirmed)[62]
  • Kepler-69c (confirmed)[63]
  • KOI-5715.01
  • KOI-4878.01
  • KOI-456.04
  • KOI-5237.01
  • KOI-7711.01
  • KOI-5248.01
  • KOI-5176.01
  • KOI-7235.01
  • KOI-7223.01
  • KOI-7621.01
  • KOI-5135.01
  • KOI-5819.01
  • KOI-5554.01
  • KOI-7894.01
  • KOI-5276.01
  • KOI-8000.01
  • KOI-8242.01
  • KOI-5389.01
  • KOI-5130.01
  • KOI-5978.01
  • KOI-8047.01

See also

Notes

  1. ↑ The habitable zone (HZ) is a region present around each star where a terrestrial planet or moon, given the right physical conditions, could maintain liquid water on its surface.[10][11]
  2. ↑ Additional information on orange dwarfs, including quantitative estimates about their suitability to serve as hosts for superhabitable planets, has been given by Cuntz and Guinan.[15]
  3. ↑ The initials "HZD" or "Habitable Zone Distance" mark the position of a planet about the center of the habitable zone of the system (value 0). A negative HZD value means that the orbit of a planet is smaller near its star —the center of the habitable zone— while a positive value means a wider orbit around its star. The values 1 and −1 mark the boundary of the habitable zone.[18] A superhabitable planet should have a HZD of 0 (the optimal location within the habitable zone).[19]
  4. ↑ Other studies on the mass-radius relationship indicate that the transition point between a rocky planet and a planet with a volatile-rich atmosphere—known as mini-Neptunes—also occurs at about 2 M🜨[34]

References

  1. ↑ 1.0 1.1 Nancy Y. Kiang (April 2008). "The color of plants on other worlds". Scientific American 298 (4): 48–55. doi:10.1038/scientificamerican0408-48. PMID 18380141. Bibcode: 2008SciAm.298d..48K. 
  2. ↑ Heller & Armstrong 2014, p. 50
  3. ↑ 3.0 3.1 3.2 3.3 Heller & Armstrong 2014, p. 57
  4. ↑ Heller & Armstrong 2014, p. 55-59
  5. ↑ 5.0 5.1 5.2 5.3 Schulze-Makuch, Dirk; Heller, Rene; Guinan, Edward (18 September 2020). "In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World". Astrobiology 20 (12): 1394–1404. doi:10.1089/ast.2019.2161. PMID 32955925. Bibcode: 2020AsBio..20.1394S. 
  6. ↑ 6.0 6.1 Schirber, Michael (9 April 2009). "Can Life Thrive Around a Red Dwarf Star?". Space.com. http://www.space.com/6560-life-thrive-red-dwarf-star.html. 
  7. ↑ "Binary Star Systems: Classification and Evolution". Space.com. 23 August 2013. http://www.space.com/22509-binary-stars.html. 
  8. ↑ Naftilan, S. A.; Stetson, P. B. (13 July 2006). "How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?". Scientific American. http://www.scientificamerican.com/article.cfm?id=how-do-scientists-determi. 
  9. ↑ Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997). "The End of the Main Sequence". The Astrophysical Journal 482 (1): 420–432. doi:10.1086/304125. Bibcode: 1997ApJ...482..420L. 
  10. ↑ Mendez, Abel (10 August 2011). "Habitable Zone Distance (HZD): A habitability metric for exoplanets". PHL. http://phl.upr.edu/library/notes/habitablezonesdistancehzdahabitabilitymetricforexoplanets. Retrieved 22 July 2015. 
  11. ↑ "Planetary Habitability Laboratory". PHL de la UPRA. 2 April 2015. http://phl.upr.edu/projects/habitable-exoplanets-catalog/data. 
  12. ↑ 12.0 12.1 12.2 Perryman 2011, p. 285
  13. ↑ Rushby, A.J.; Claire, M.W.; Osborn, H.; Watson, A.J. (18 September 2013). "Habitable Zone Lifetimes of Exoplanets around Main Sequence". Astrobiology 13 (9): 833–849. doi:10.1089/ast.2012.0938. PMID 24047111. Bibcode: 2013AsBio..13..833R. 
  14. ↑ 14.0 14.1 14.2 Heller & Armstrong 2014, p. 56-57
  15. ↑ Cuntz, M.; Guinan, E. F. (2016). "About Exobiology: The Case for Dwarf K Stars". Astrophysical Journal 827 (1): 79 (9 pp.). doi:10.3847/0004-637X/827/1/79. Bibcode: 2016ApJ...827...79C. 
  16. ↑ Rushby, A.J.; Claire, M.W.; Osborn, H.; Watson, A.J. (2013). "Habitable Zone Lifetimes of Exoplanets around Main Sequence". Astrobiology 13 (9): 833–849. doi:10.1089/ast.2012.0938. PMID 24047111. Bibcode: 2013AsBio..13..833R. http://iopscience.iop.org/0004-637X/765/2/131/. 
  17. ↑ PHL. "HEC: Graphical Catalog Results". http://phl.upr.edu/projects/habitable-exoplanets-catalog/results. 
  18. ↑ 18.0 18.1 Mendez, Abel (30 July 2012). "Habitable Zone Distance (HZD): A habitability metric for exoplanets". PHL. http://phl.upr.edu/library/notes/habitablezonesdistancehzdahabitabilitymetricforexoplanets. 
  19. ↑ 19.0 19.1 Heller & Armstrong 2014, p. 56
  20. ↑ 20.0 20.1 Rushby, Andrew J.; Claire, Mark W.; Osborn, Hugh; Watson, Andrew J. (18 Sep 2013). "Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars". Astrobiology 13 (9): 833–849. doi:10.1089/ast.2012.0938. PMID 24047111. Bibcode: 2013AsBio..13..833R. 
  21. ↑ Kopparapu, R.K.; Ramirez, R.; Kasting, J.; Eymet, V. (2013). "Habitable Zones Around Main-Sequence Stars: New Estimates". Astrophysical Journal 765 (2): 131. doi:10.1088/0004-637X/765/2/131. Bibcode: 2013ApJ...765..131K. http://iopscience.iop.org/0004-637X/765/2/131/. 
  22. ↑ Perryman 2011, p. 283-284
  23. ↑ Cain, Fraser (30 September 2013). "How Long Will Life Survive on Earth?". Universe Today. http://www.universetoday.com/25367/how-long-will-life-survive-on-earth/. 
  24. ↑ 24.0 24.1 24.2 24.3 24.4 Heller & Armstrong 2014, p. 58
  25. ↑ Perryman 2011, p. 269
  26. ↑ Tate, Karl (11 December 2013). "How Habitable Zones for Alien Planets and Stars Work". http://www.space.com/23910-habitable-zones-alien-planets-stars-infographic.html. 
  27. ↑ Reynolds, R.T.; McKay, C.P.; Kasting, J.F. (1987). "Europa, tidally heated oceans, and habitable zones around giant planets". Advances in Space Research 7 (5): 125–132. doi:10.1016/0273-1177(87)90364-4. PMID 11538217. Bibcode: 1987AdSpR...7e.125R. 
  28. ↑ Sanders, Ray (9 April 2012). "When Stellar Metallicity Sparks Planet Formation". Astrobiology Magazine. http://www.astrobio.net/news-exclusive/when-stellar-metallicity-sparks-planet-formation/. 
  29. ↑ 29.0 29.1 29.2 Cooper, Keith (4 September 2012). "When Did the Universe Have the Right Stuff for Planets?". http://www.space.com/17441-universe-heavy-metals-planet-formation.html. 
  30. ↑ Rogers, Leslie A. (2015). "Most 1.6 Earth-radius Planets are Not Rocky". The Astrophysical Journal 801 (1): 41. doi:10.1088/0004-637X/801/1/41. Bibcode: 2015ApJ...801...41R. 
  31. ↑ 31.0 31.1 Moyer, Michael (31 January 2014). "Faraway Planets May Be Far Better for Life". Scientific American. http://www.scientificamerican.com/podcast/episode/superhabitable-planets/. Retrieved 20 April 2015. 
  32. ↑ 32.0 32.1 32.2 32.3 Heller & Armstrong 2014, p. 55
  33. ↑ 33.0 33.1 Noack, L.; Breuer, D. (2011). "Plate Tectonics on Earth-like Planets". EPSC Abstracts (6): pp. 890–891. Bibcode: 2011epsc.conf..890N. 
  34. ↑ Chen, Jingjing; Kipping, David (2016). "Probabilistic Forecasting of the Masses and Radii of Other Worlds". The Astrophysical Journal 834 (1): 17. doi:10.3847/1538-4357/834/1/17. 
  35. ↑ Riguzzi, F.; Panza, G.; Varga, P.; Doglioni, C. (19 March 2010). "Can Earth's rotation and tidal despinning drive plate tectonics?". Tectonophysics 484 (1): 60–73. doi:10.1016/j.tecto.2009.06.012. Bibcode: 2010Tectp.484...60R. 
  36. ↑ Van Der Meer, Douwe G.; Zeebe, Richard E.; van Hinsbergen, Douwe J. J.; Sluijs, Appy; Spakman, Wim; Torsvik, Trond H. (25 March 2014). "Plate tectonic controls on atmospheric CO2 levels since the Triassic". PNAS 111 (12): 4380–4385. doi:10.1073/pnas.1315657111. PMID 24616495. Bibcode: 2014PNAS..111.4380V. 
  37. ↑ NASA. "Climate change: How do we know?". https://climate.nasa.gov/evidence/. 
  38. ↑ Walker, J.C.G.; Hays, P.B.; Kasting, J.F. (1981). "A negative feedback mechanism for the long-term stabilization of the earth's surface temperature". Journal of Geophysical Research 86 (86): 9776–9782. doi:10.1029/JC086iC10p09776. Bibcode: 1981JGR....86.9776W. 
  39. ↑ Baumstark-Khan, Christa; Facius, Rainer (2002). "Life under Conditions of Ionizing Radiation". Astrobiology. pp. 261–284. doi:10.1007/978-3-642-59381-9_18. ISBN 978-3-642-63957-9. 
  40. ↑ Choi, Charles Q. (4 November 2014). "Planet Venus Facts: A Hot, Hellish & Volcanic Planet". http://www.space.com/44-venus-second-planet-from-the-sun-brightest-planet-in-solar-system.html. 
  41. ↑ Heller & Armstrong 2014, p. 57-58
  42. ↑ 42.0 42.1 42.2 Heller & Armstrong 2014, p. 54-59
  43. ↑ Than, Ker (11 April 2007). "Colorful Worlds: Plants on Other Planets Might Not Be Green". http://www.space.com/3680-colorful-worlds-plants-planets-green.html. 
  44. ↑ Skibba, Ramin (2 October 2023). "What Will Plants Be Like on Alien Worlds?". Wired. https://www.wired.com/story/what-will-plants-be-like-on-alien-worlds/. Retrieved 24 January 2024. 
  45. ↑ Lewis, Tanya (9 January 2014). "Super-Earth Planets May Have Watery Earthlike Climates". http://www.space.com/24216-super-earth-exoplanets-earthlike-climate-aas223.html. 
  46. ↑ Heller & Armstrong 2014, p. 54-56
  47. ↑ Pierrehumbert, Raymond T. (2 December 2010). Principles of Planetary Climate. Cambridge University Press. ISBN 9780521865562. http://www.cambridge.org/gb/academic/subjects/earth-and-environmental-science/climatology-and-climate-change/principles-planetary-climate?format=HB. 
  48. ↑ Heller & Armstrong 2014, p. 54
  49. ↑ Gray, John S. (1997). "Marine biodiversity: patterns, threats, and conservation needs". Biodiversity & Conservation 6 (6): 153–175. doi:10.1023/A:1018335901847. 
  50. ↑ 50.0 50.1 Heller & Armstrong 2014, p. 55-58
  51. ↑ 51.0 51.1 51.2 Heller & Armstrong 2014, p. 55-56
  52. ↑ Mayhew, P.J.; Bell, M.A.; Benton, T.G.; McGowan, A.J. (2012). "Biodiversity tracks temperature over time". Proceedings of the National Academy of Sciences 109 (38): 15141–15145. doi:10.1073/pnas.1200844109. PMID 22949697. Bibcode: 2012PNAS..10915141M. 
  53. ↑ O'Neill, Ian (21 July 2014). "Oceans Make Exoplanets Stable for Alien Life". Discovery News. http://news.discovery.com/space/alien-life-exoplanets/oceans-make-exoplanets-stable-for-alien-life-140721.htm. 
  54. ↑ Falcon-Lang, H. J. (1999). "156". Fire ecology of a Late Carboniferous floodplain, Joggins, Nova Scotia. London: Journal of the Geological Society. pp. 137–148. 
  55. ↑ Harrison, J.F.; Kaiser, A.; VandenBrooks, J.M. (26 May 2010). "Atmospheric oxygen level and the evolution of insect body size". Proceedings of the Royal Society B 277: pp. 1937–1946. http://rspb.royalsocietypublishing.org/content/277/1690/1937. 
  56. ↑ 56.0 56.1 Heller & Armstrong 2014, p. 59
  57. ↑ PHL. "Habitable Zone Atmosphere". PHL University of Puerto Rico at Arecibo. http://phl.upr.edu/library/notes/habitablezoneatmospherehzaahabitabilitymetricforexoplanets. 
  58. ↑ Pierrehumbert, Raymond T. (2 December 2010). Principles of Planetary Climate. Cambridge University Press. ISBN 9780521865562. http://www.cambridge.org/gb/academic/subjects/earth-and-environmental-science/climatology-and-climate-change/principles-planetary-climate?format=HB. 
  59. ↑ Howell, Elizabeth (2 January 2014). "Kepler-62f: A Possible Water World". http://www.space.com/24142-kepler-62f.html. 
  60. ↑ Wall, Mike (18 April 2013). "What Might Alien Life Look Like on New 'Water World' Planets?". Space.com. http://www.space.com/20728-new-alien-planets-oceans-life.html. 
  61. ↑ Kane, Stephen R.; Barclay, Thomas; Gelino, Dawn M. (2013-05-30). "A Potential Super-Venus in the Kepler-69 System". The Astrophysical Journal 770 (2): L20. doi:10.1088/2041-8205/770/2/L20. ISSN 2041-8205. Bibcode: 2013ApJ...770L..20K. 
  62. ↑ Morton, Timothy D.; Bryson, Stephen T.; Coughlin, Jeffrey L.; Rowe, Jason F.; Ravichandran, Ganesh; Petigura, Erik A.; Haas, Michael R.; Batalha, Natalie M. (May 2016). "False Positive Probabilities for all Kepler Objects of Interest: 1284 Newly Validated Planets and 428 Likely False Positives". The Astrophysical Journal 822 (2): 86. doi:10.3847/0004-637X/822/2/86. Bibcode: 2016ApJ...822...86M. 
  63. ↑ Barclay, Thomas; Burke, Christopher J.; Howell, Steve B.; Rowe, Jason F.; Huber, Daniel (May 2013). "A Super-Earth-sized Planet Orbiting in or Near the Habitable Zone around a Sun-like Star". The Astrophysical Journal 768 (2). doi:10.1088/0067-0049/217/2/31. https://resolver.caltech.edu/CaltechAUTHORS:20150518-092232452. 

Bibliography

Further reading

Newly confirmed, false probabilities for Kepler objects of interest.

In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World

External links



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