Earth analog - wikipedia Earth analog For the idea of a planet orbiting the sun directly opposite Earth, see Counter-Earth. The planet Earth An Earth analog ( also referred to as an Earth twin or Earth - like planet, though this latter term may refer to any terrestrial planet ) is a planet or moon with environmental conditions similar to those found on Earth. The possibility is of particular interest to astrobiologists and astronomers under reasoning that the more similar a planet is to Earth, the more likely it is to be capable of sustaining complex extraterrestrial life. As such, it has long been speculated and the subject expressed in science, philosophy, science fiction and popular culture. Advocates of space colonization have long sought an Earth analog as a `` second home '', while advocates for space and survival would regard such a planet as a potential `` new home '' for humankind. Before the scientific search for and study of extrasolar planets, the possibility was argued through philosophy and science fiction. The mediocrity principle suggests that planets like Earth should be common in the universe, while the Rare Earth hypothesis suggests that they are extremely rare. The thousands of exoplanetary star systems discovered so far are profoundly different from our solar system, supporting the Rare Earth hypothesis. Philosophers have pointed out that the size of the universe is such that a near - identical planet must exist somewhere. In the far future, technology may be used by humans to artificially produce an Earth analog by terraforming. The multiverse theory suggests that an Earth analog could exist in another universe or even be another version of the Earth itself in a parallel universe. On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth - sized planets orbiting in the habitable zones of Sun - like stars and red dwarf stars within the Milky Way galaxy. 11 billion of these estimated planets may be orbiting Sun - like stars. The nearest such planet may be 12 light - years away. Scientific findings since the 1990s have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and the search for extraterrestrial intelligence ( SETI ). NASA and the SETI Institute have proposed categorising the increasing number of planets found using a measure called the Earth Similarity Index ( ESI ) based on mass, radius and temperature. According to this measure, as of 23 July 2015, the confirmed planet currently thought to be most similar to Earth on mass, radius and temperature is Kepler - 438b. Scientists estimate that there may be billions of Earth - size planets within the Milky Way galaxy alone. Contents 1 History 2 Attributes and criteria 2.1 Size 2.2 Terrestrial 2.3 Temperature 2.4 Solar analog 2.5 Surface water and hydrological cycle 3 Extrasolar Earth analog 3.1 Estimated frequency 4 Terraforming 5 See also 6 References History Percival Lowell depicted Mars as a dry but Earth - like planet and habitable for an extraterrestrial civilisation Sand dunes in the Namib Desert on Earth ( top ), compared with dunes in Belet on Titan Between 1858 and 1920, Mars was thought by many, including some scientists, to be very similar to Earth, only drier with a thick atmosphere, similar axial tilt, orbit and seasons as well as a Martian civilization that had built great Martian canals. These theories were advanced by Giovanni Schiaparelli, Percival Lowell and others. As such Mars in fiction portrayed the red planet as similar to Earth but with a desert like landscape. Images and data from the Mariner ( 1965 ) and Viking space probes ( 1975 -- 1980 ), however, revealed the planet as a barren cratered world. However, with continuing discoveries, other Earth comparisons remained. For example, the Mars Ocean Hypothesis had its origins in the Viking missions and was popularised during the 1980s. With the possibility of past water, there was the possibility that life could have begun on Mars and it was once again perceived to be more Earth - like. Likewise, until the 1960s, Venus was believed by many, including some scientists, to be a warmer version of Earth with a thick atmosphere and either hot and dusty or humid with water clouds and oceans. Venus in fiction was often portrayed as having similarities to Earth and many speculated about Venusian civilization. These beliefs were dispelled in the 1960s as the first space probes gathered more accurate scientific data on the planet and found that Venus is a very hot world with the surface temperature around 900 ° F ( 482 ° C ) under an acidic atmosphere 92 times thicker the Earth 's. From 2004, Cassini -- Huygens began to reveal Saturn 's moon Titan to be one of the most Earth - like worlds outside of the habitable zone. Though having a dramatically different chemical makeup, discoveries such as the confirmation of Titanian lakes, rivers and fluvial processes in 2007, advanced comparisons to Earth. Further observations, including weather phenomena, have aided the understanding of geological processes that may operate on Earth - like planets. The Kepler space telescope began observing the transits of potential terrestrial planets in the habitable zone from 2011. Though the technology provided a more effective means for detecting and confirming planets, it was unable to conclude definitively how Earth - like the candidate planets actually are. In 2013, several Kepler candidates less than 1.5 Earth radii were confirmed orbiting in the habitable zone of stars. It was not until 2015 that the first near - Earth sized candidate orbiting a solar candidate, Kepler - 452b, was announced. Attributes and criteria The probability of finding an Earth analog depends mostly on the attributes that are expected to be similar, and these vary greatly. Generally it is considered that it would be a terrestrial planet and there have been several scientific studies aimed at finding such planets. Often implied but not limited to are such criteria as planet size, surface gravity, star size and type ( i.e. Solar analog ), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air and weather conditions, strong magnetosphere and even the presence of Earth - like complex life. If there is complex life, there could be some forests covering much of the land. If there is intelligent life, some parts of land could be covered in cities. Some factors that are assumed of such a planet may be unlikely due to Earth 's own history. For instance the Earth 's atmosphere was not always oxygen - rich and this is a biosignature from the emergence of photosynthetic life. The formation, presence, influence on these characteristics of the Moon ( such as tidal forces ) may also pose a problem in finding an Earth analog. Size Size Comparisons : Kepler - 20e and Kepler - 20f with Venus and Earth Size is often thought to be a significant factor, as planets of Earth 's size are thought more likely to be terrestrial in nature and be capable of retaining an Earth - like atmosphere. The list includes planets within the range of 0.8 -- 1.9 Earth masses, below which are generally classed as sub-Earth and above classed as super-Earth. In addition, only planets known to fall within the range of 0.5 -- 2.0 Earth radius ( between half and twice the radius of the Earth ) are included. In contrast, the Earth Similarity Index uses both mass and radius as criteria. According to the size criteria, the closest planetary mass objects by known radius or mass are : Name Earth masses ( M ) Earth radii ( R ) Note Kepler - 69c 0.98 1.7 Originally thought to be in the circumstellar habitable zone ( CHZ ), now thought to be too hot. Kepler - 9d > 1.5 1.64 Extremely hot. COROT - 7b < 9 1.58 Extremely hot. Kepler - 20f < 14.3 1.03 Slightly larger and likely more massive, far too hot to be Earth - like. Tau Ceti b Extremely hot. Not known to transit. Kepler - 186f Orbits in CHZ. Earth Orbits in habitable zone. Venus 0.815 0.949 Much hotter. Kepler - 20e < 3.08 0.87 Too hot to be Earth - like. Proxima b > 1.27 > 1.1 Closest exoplanet to Earth. This comparison indicates that size alone is a poor measure, particularly in terms of habitability. Temperature must also be considered as Venus and the planets of Alpha Centauri B ( discovered in 2012 ), Kepler - 20 ( discovered in 2011 ), COROT - 7 ( discovered in 2009 ) and the three planets of Kepler - 42 ( all discovered in 2011 ) are very hot, and Mars, Ganymede and Titan are frigid worlds, resulting also in wide variety of surface and atmospheric conditions. The masses of the Solar System 's moons are a tiny fraction of that of Earth whereas the masses of extrasolar planets are very difficult to accurately measure. However discoveries of Earth - sized terrestrial planets are important as they may indicate the probable frequency and distribution of Earth - like planets. Terrestrial Surfaces like this of Saturn 's moon Titan ( taken by Huygens probe ) bear superficial similarities to the floodplains of Earth Another criterion often cited is that an Earth analog must be terrestrial, that is, it should possess a similar surface geology -- a planetary surface composed of similar surface materials. The closest known examples are Mars and Titan and while there are similarities in their types of landforms and surface compositions, there are also significant differences such as the temperature and quantities of ice. Many of Earth 's surface materials and landforms are formed as a result of interaction with water ( such as clay and sedimentary rocks ) or as a byproduct of life ( such as limestone or coal ), interaction with the atmosphere, volcanically or artificially. A true Earth analog therefore might need to have formed through similar processes, having possessed an atmosphere, volcanic interactions with the surface, past or present liquid water and life forms. Temperature There are several factors that can determine planetary temperatures and therefore several measures that can draw comparisons to that of the Earth in planets where atmospheric conditions are unknown. Equilibrium temperature is used for planets without atmospheres. With atmosphere, a greenhouse effect is assumed. Finally, surface temperature is used. Each of these temperatures is affected by climate, which is influenced by the orbit and rotation ( or tidal locking ) of the planet, each of which introduces further variables. Below is a comparison of the confirmed planets with the closest known temperatures to Earth. Temperature comparisons Venus Earth Kepler 22b Mars Global equilibrium temperature 307 K 34 ° C 93 ° F 255 K − 18 ° C − 0.4 ° F 262 K − 11 ° C 22.2 ° F 206 K − 67 ° C − 88.6 ° F + Greenhouse gas effect 737 K 464 ° C 867 ° F 288 K 15 ° C 59 ° F 295 K 22 ° C 71.6 ° F 210 K − 63 ° C − 81 ° F Tidally locked Almost No Unknown No Global Bond albedo 0.9 0.29 Unknown 0.25 Refs. Solar analog Main article : Solar analog Another criteria of an Earth analog is that it should orbit a solar analog, that is, a star much like our Sun, similar photometrically or in terms of spectral type. It follows that the composition of a planet around a solar twin with similar metallicity might have a similar composition to Earth. Also, this would help eliminate stellar extremes and variability that may be applicable to other types of stars. While planets have been discovered orbiting similar stars to the Sun, most are gas giant or super-Earth sized, and many of these planetary systems have shown to be surprisingly different from our own. This measure is not entirely reliable as Mars and Venus also orbit the Sun but have different compositions and properties. Surface water and hydrological cycle Water covers 70 % of Earth 's surface and is required by all known life Kepler - 22b, located in the habitable zone of a Sun - like star may be the best exoplanetary candidate for extraterrestrial surface water discovered to date, but is significantly larger than Earth and its actual composition is unknown Main article : Habitable zone The concept of the habitable zone ( or Liquid Water Zone ) defining a region where water can exist on the surface, is based on the properties of both the Earth and Sun. Under this model, Earth orbits roughly at the centre of this zone or in the `` Goldilocks '' position. Earth is the only planet in the universe confirmed to possess large bodies of surface water. Venus is on the hot side of the zone while Mars is on the cold side. Neither are known to have persistent surface water, though evidence exists that Mars did have in its ancient past, and it is speculated that the same was the case for Venus. Thus extrasolar planets ( or moons ) in the Goldilocks position with substantial atmospheres may possess oceans and water clouds like those on Earth. In addition to surface water, a true Earth analog would require a mix of oceans or lakes and areas not covered by water, or land. Some argue that a true Earth analog must not only have a similar position of its planetary system but also orbit a solar analog and have a near circular orbit such that it remains continually habitable like Earth. Extrasolar Earth analog See also : List of potentially habitable exoplanets The mediocrity principle suggests that there is a chance that serendipitous events may have allowed an Earth - like planet to form elsewhere that would allow the emergence of complex, multi-cellular life. In contrast, the Rare Earth hypothesis asserts that if the strictest criteria are applied, such a planet, if it exists, may be so far away that humans may never locate it. Because the Solar System proved to be devoid of an Earth analog, the search has widened to extrasolar planets. Astrobiologists assert that Earth analogs would most likely be found in a stellar habitable zone, in which liquid water could exist, providing the conditions for supporting life. Some astrobiologists, such as Dirk Schulze - Makuch, estimated that a sufficiently massive natural satellite may form a habitable moon similar to Earth. Estimated frequency Artist 's concept of Earth - like planets The frequency of Earth - like planets in both the Milky Way and the larger universe is still unknown. It ranges from the extreme Rare Earth hypothesis estimates -- one ( i. e., Earth ) -- to innumerable. Several current scientific studies, including the Kepler mission, are aimed at refining estimates using real data from transiting planets. A 2008 study by astronomer Michael Meyer from the University of Arizona of cosmic dust near recently formed Sun - like stars suggests that between 20 % and 60 % of solar analogs have evidence for the formation of rocky planets, not unlike the processes that led to those of Earth. Meyer 's team found discs of cosmic dust around stars and sees this as a byproduct of the formation of rocky planets. In 2009, Alan Boss of the Carnegie Institution of Science speculated that there could be 100 billion terrestrial planets in our Milky Way galaxy alone. In 2011 NASA 's Jet Propulsion Laboratory ( JPL ) and based on observations from the Kepler Mission is that about 1.4 % to 2.7 % of all Sun - like stars are expected to have Earth - size planets within the habitable zones of their stars. This means there could be two billion of them in the Milky Way galaxy alone, and assuming that all galaxies have a similar number as the Milky Way, in the 50 billion galaxies in the observable universe, there may be as many as a hundred quintillion. This would correspond to around 20 earth analogs per square centimeter of the Earth. In 2013, a Harvard - Smithsonian Center for Astrophysics using statistical analysis of additional Kepler data suggested that there are at least 17 billion Earth - sized planets in the Milky Way. This, however, says nothing of their position in relation to the habitable zone. Terraforming Main article : Terraforming Artist 's conception of a terraformed Venus, a potential Earth analog Terraforming ( literally, `` Earth - shaping '' ) of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecosystems to be similar to those of Earth to make it habitable to humans. Due to proximity and similarity in size, Mars, and to a lesser extent Venus, have been cited as the most likely candidates for terraforming. See also Astrobiology Earth Similarity Index List of potentially habitable moons References ^ Jump up to : Overbye, Dennis ( November 4, 2013 ). `` Far - Off Planets Like the Earth Dot the Galaxy ''. New York Times. Retrieved November 5, 2013. ^ Jump up to : Petigura, Eric A. ; Howard, Andrew W. ; Marcy, Geoffrey W. ( October 31, 2013 ). `` Prevalence of Earth - size planets orbiting Sun - like stars ''. Proceedings of the National Academy of Sciences of the United States of America. 110 ( 48 ) : 19273. arXiv : 1311.6806. Bibcode : 2013PNAS... 11019273P. doi : 10.1073 / pnas. 1319909110. PMC 3845182. PMID 24191033. Retrieved November 5, 2013. Jump up ^ Khan, Amina ( November 4, 2013 ). `` Milky Way may host billions of Earth - size planets ''. Los Angeles Times. Retrieved November 5, 2013. Jump up ^ Brown, Mark ( 21 November 2011 ). `` Exoplanet hunters propose system to find life - supporting worlds ''. Wired Science. Retrieved 2015 - 09 - 18. Jump up ^ Stuart Gary New approach in search for alien life ABC Online. November 22, 2011 Jump up ^ `` HEC : Data of Potentially Habitable Worlds ''. Planetary Habitability Laboratory. University of Puerto Rico. January 13, 2016. Retrieved 2016 - 03 - 13. Jump up ^ `` Study : 8.8 billion Earth - size, just - right planets '', ap.org Jump up ^ O'Gallagher, J.J. ; Simpson, J.A. ( September 10, 1965 ). `` Search for Trapped Electrons and a Magnetic Moment at Mars by Mariner IV ''. Science, New Series. 149 ( 3689 ) : 1233 -- 1239. Bibcode : 1965Sci... 149.1233 O. doi : 10.1126 / science. 149.3689. 1233. PMID 17747452. Jump up ^ Smith, Edward J. ; Davis Jr., Leverett ; Coleman Jr., Paul J. ; Jones, Douglas E. ( September 10, 1965 ). `` Magnetic Field Measurements Near Mars ''. Science, New Series. 149 ( 3689 ) : 1241 -- 1242. Bibcode : 1965Sci... 149.1241 S. doi : 10.1126 / science. 149.3689. 1241. PMID 17747454. Jump up ^ Leighton, Robert B. ; Murray, Bruce C. ; Sharp, Robert P. ; Allen, J. Denton ; Sloan, Richard K. ( August 6, 1965 ). `` Mariner IV Photography of Mars : Initial Results ''. Science, New Series. 149 ( 3684 ) : 627 -- 630. Bibcode : 1965Sci... 149... 627L. doi : 10.1126 / science. 149.3684. 627. PMID 17747569. Jump up ^ Kliore, Arvydas ; Cain, Dan L. ; Levy, Gerald S. ; Eshleman, Von R. ; Fjeldbo, Gunnar ; Drake, Frank D. ( September 10, 1965 ). `` Occultation Experiment : Results of the First Direct Measurement of Mars 's Atmosphere and Ionosphere ''. Science, New Series. 149 ( 3689 ) : 1243 -- 1248. Bibcode : 1965Sci... 149.1243 K. doi : 10.1126 / science. 149.3689. 1243. PMID 17747455. Jump up ^ Salisbury, Frank B. ( April 6, 1962 ). `` Martian Biology ''. Science, New Series. 136 ( 3510 ) : 17 -- 26. Bibcode : 1962Sci... 136... 17S. doi : 10.1126 / science. 136.3510. 17. PMID 17779780. Jump up ^ Kilston, Steven D. ; Drummond, Robert R. ; Sagan, Carl ( 1966 ). `` A Search for Life on Earth at Kilometer Resolution ''. Icarus. 5 ( 1 -- 6 ) : 79 -- 98. Bibcode : 1966Icar... 5... 79K. doi : 10.1016 / 0019 - 1035 ( 66 ) 90010 - 8. Jump up ^ NASA -- Mars Ocean Hypothesis Archived 2012 - 02 - 20 at the Wayback Machine. ^ Jump up to : Hashimoto, G.L. ; Roos - Serote, M. ; Sugita, S. ; Gilmore, M.S. ; Kamp, L.W. ; Carlson, R.W. ; Baines, K.H. ( 2008 ). `` Felsic highland crust on Venus suggested by Galileo Near - Infrared Mapping Spectrometer data ''. Journal of Geophysical Research : Planets. 113 : E00B24. Bibcode : 2008JGRE... 113.0 B24H. doi : 10.1029 / 2008JE003134. Jump up ^ `` Cassini Reveals Titan 's Xanadu Region To Be An Earth - Like Land ''. Science Daily. July 23, 2006. Retrieved 2007 - 08 - 27. Jump up ^ `` Seeing, touching and smelling the extraordinarily Earth - like world of Titan ''. ESA News, European Space Agency. January 21, 2005. Retrieved 2005 - 03 - 28. Jump up ^ `` Cassini - Huygens : News ''. Saturn.jpl.nasa.gov. Archived from the original on 2008 - 05 - 08. Retrieved 2011 - 08 - 20. Jump up ^ `` NASA -- NASA 's Kepler Confirms Its First Planet in Habitable Zone of Sun - like Star ''. NASA Press Release. Retrieved 6 December 2011. Jump up ^ Kepler - 22b : Facts About Exoplanet in Habitable Zone Jump up ^ Petigura, E.A. ; Howard, A.W. ; Marcy, G.W. ( 2013 ). `` Prevalence of Earth - size planets orbiting Sun - like stars ''. Proceedings of the National Academy of Sciences. 110 ( 48 ) : 19273 -- 19278. arXiv : 1311.6806. Bibcode : 2013PNAS... 11019273P. doi : 10.1073 / pnas. 1319909110. ISSN 0027 - 8424. PMC 3845182. PMID 24191033. Jump up ^ Jenkins, Jon M. ; Twicken, Joseph D. ; Batalha, Natalie M. ; Caldwell, Douglas A. ; Cochran, William D. ; Endl, Michael ; Latham, David W. ; Esquerdo, Gilbert A. ; Seader, Shawn ; Bieryla, Allyson ; Petigura, Erik ; Ciardi, David R. ; Marcy, Geoffrey W. ; Isaacson, Howard ; Huber, Daniel ; Rowe, Jason F. ; Torres, Guillermo ; Bryson, Stephen T. ; Buchhave, Lars ; Ramirez, Ivan ; Wolfgang, Angie ; Li, Jie ; Campbell, Jennifer R. ; Tenenbaum, Peter ; Sanderfer, Dwight ; Henze, Christopher E. ; Catanzarite, Joseph H. ; Gilliland, Ronald L. ; Borucki, William J. ( 23 July 2015 ). `` Discovery and Validation of Kepler - 452b : A 1.6 R ⨁ Super Earth Exoplanet in the Habitable Zone of a G2 Star ''. The Astronomical Journal. 150 ( 2 ) : 56. arXiv : 1507.06723. Bibcode : 2015AJ... 150... 56J. doi : 10.1088 / 0004 - 6256 / 150 / 2 / 56. ISSN 1538 - 3881. Retrieved 24 July 2015. Jump up ^ `` NASA telescope discovers Earth - like planet in star 's ' habitable zone ''. BNO News. 23 July 2015. Retrieved 23 July 2015. ^ Jump up to : NASA Staff ( 20 December 2011 ). `` Kepler : A Search For Habitable Planets -- Kepler - 20e ''. NASA. Retrieved 2011 - 12 - 23. ^ Jump up to : NASA Staff ( 20 December 2011 ). `` Kepler : A Search For Habitable Planets -- Kepler - 20f ''. NASA. Retrieved 2011 - 12 - 23. Jump up ^ Erkaev, N.V. ; Lammer, H. ; Elkins - Tanton, L.T. ; Stökl, A. ; Odert, P. ; Marcq, E. ; Dorfi, E.A. ; Kislyakova, K.G. ; Kulikov, Yu. N. ; Leitzinger, M. ; Güdel, M. ( 2014 ). `` Escape of the martian protoatmosphere and initial water inventory ''. Planetary and Space Science. 98 : 106 -- 119. arXiv : 1308.0190. Bibcode : 2014P&SS... 98... 106E. doi : 10.1016 / j. pss. 2013.09. 008. ISSN 0032 - 0633. PMC 4375622. PMID 25843981. Jump up ^ Torres, Guillermo ; Fressin, François ( 2011 ). `` Modeling Kepler transit light curves as false positives : Rejection of blend scenarios for Kepler - 9, and validation of Kepler - 9d, a super-Earth - size planet in a multiple system '' ( PDF ). Astrophysical Journal. 727 ( 24 ) : 24. arXiv : 1008.4393. Bibcode : 2011ApJ... 727... 24T. doi : 10.1088 / 0004 - 637X / 727 / 1 / 24. Archived from the original ( PDF ) on 5 May 2011. Retrieved 20 March 2011. Jump up ^ Johnson, Michele ; Harrington, J.D. ( 17 April 2014 ). `` NASA 's Kepler Discovers First Earth - Size Planet In The ' Habitable Zone ' of Another Star ''. NASA. Retrieved 17 April 2014. Jump up ^ Johnson, Michele ( 20 December 2011 ). `` NASA Discovers First Earth - size Planets Beyond Our Solar System ''. NASA. Retrieved 2011 - 12 - 20. Jump up ^ Hand, Eric ( 20 December 2011 ). `` Kepler discovers first Earth - sized exoplanets ''. Nature. doi : 10.1038 / nature. 2011.9688. Jump up ^ To stellar primary Jump up ^ `` NASA, Mars : Facts & Figures ''. Retrieved 2010 - 01 - 28. Jump up ^ Mallama, A. ; Wang, D. ; Howard, R.A. ( 2006 ). `` Venus phase function and forward scattering from H SO ''. Icarus. 182 ( 1 ) : 10 -- 22. Bibcode : 2006Icar... 182... 10M. doi : 10.1016 / j. icarus. 2005.12. 014. Jump up ^ Mallama, A. ( 2007 ). `` The magnitude and albedo of Mars ''. Icarus. 192 ( 2 ) : 404 -- 416. Bibcode : 2007Icar... 192... 404M. doi : 10.1016 / j. icarus. 2007.07. 011. Jump up ^ Cabrol, N. and E. Grin ( eds. ). 2010. Lakes on Mars. Elsevier. NY Jump up ^ Clifford, S.M. ; Parker, T.J. ( 2001 ). `` The Evolution of the Martian Hydrosphere : Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains ''. Icarus. 154 : 40 -- 79. Bibcode : 2001Icar... 154... 40C. doi : 10.1006 / icar. 2001.6671. Jump up ^ Villanueva, G. ; Mumma, M. ; Novak, R. ; Käufl, H. ; Hartogh, P. ; Encrenaz, T. ; Tokunaga, A. ; Khayat, A. ; Smith, M. ( 2015 ). `` Strong water isotopic anomalies in the martian atmosphere : Probing current and ancient reservoirs ''. Science. 348 ( 6231 ) : 218 -- 221. Bibcode : 2015Sci... 348... 218V. doi : 10.1126 / science. aaa3630. PMID 25745065. Jump up ^ `` Artist 's Concept of Earth - Like Planets in the Future Universe ''. Retrieved 22 October 2015. Jump up ^ Planet - hunters set for big bounty, BBC Jump up ^ Galaxy may be full of ' Earths, ' alien life Jump up ^ Choi, Charles Q. ( 21 March 2011 ). `` New Estimate for Alien Earths : 2 Billion in Our Galaxy Alone ''. Space.com. Retrieved 2011 - 04 - 24. Jump up ^ `` Wolfram Alpha : Making the world 's knowledge computable ''. www.wolframalpha.com. Retrieved 2018 - 03 - 19. Jump up ^ 17 Billion Earth - Size Alien Planets Inhabit Milky Way Archived October 6, 2014, at the Wayback Machine. SPACE.com 07 January 2013 Jump up ^ Robert M. Zubrin ( Pioneer Astronautics ), Christopher P. McKay. NASA Ames Research Center ( c. 1993 ). `` Technological Requirements for Terraforming Mars ''. Jump up ^ Mat Conway ( 2007 - 02 - 27 ). `` Now We 're There : Terraforming Mars ''. Aboutmyplanet.com. Retrieved 2011 - 08 - 20. Jump up ^ Peter Ahrens. `` The Terraformation of Worlds '' ( PDF ). Nexial Quest. Retrieved 2007 - 10 - 18. Jump up ^ Sagan, Carl ( 1961 ). `` The Planet Venus ''. Science. 133 ( 3456 ) : 849 -- 58. Bibcode : 1961Sci... 133... 849S. doi : 10.1126 / science. 133.3456. 849. PMID 17789744. Jump up ^ Zubrin, Robert ( 1999 ). Entering Space : Creating a Spacefaring Civilization. Jump up ^ Fogg, Martyn J. ( 1995 ). Terraforming : Engineering Planetary Environments. SAE International, Warrendale, PA. ISBN 1 - 56091 - 609 - 5. Jump up ^ Birch, Paul ( 1991 ). `` Terraforming Venus Quickly '' ( PDF ). Journal of the British Interplanetary Society. Jump up ^ Landis, Geoffrey A. ( Feb 2 -- 6, 2003 ). `` Colonization of Venus ''. Conference on Human Space Exploration, Space Technology & Applications International Forum, Albuquerque NM. Exoplanetology Planet Definition IAU Planetary science Main topics Exoplanet Methods of detecting exoplanets Planetary system Sizes and types Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Ice planet Iron planet Lava planet Mega-Earth Ocean planet Sub-Earth Super-Earth Gaseous Chthonian planet Eccentric Jupiter Gas dwarf Helium planet Hot Jupiter Hot Neptune Ice giant Mini-Neptune Super-Neptune Super-Jupiter Other types Brown dwarf Circumbinary planet Double planet Eyeball planet Mesoplanet Planemo Planet / Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Ultra-cool dwarf Ultra-short period planets ( USP ) Formation and evolution Accretion Merging stars Nebular hypothesis Planetary migration Systems Exocomet Exomoon Interstellar comet Mean - motion resonances Retrograde planet Rogue planet Titius -- Bode laws Trojan planet Host stars Binary star Brown dwarfs Extragalactic planet F / Yellow - white dwarfs G / Yellow dwarfs Herbig Ae / Be K / Orange dwarfs M / Red dwarfs Planets in globular clusters Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giant Detection Astrometry Direct imaging list Microlensing list Polarimetry Pulsar timing list Radial velocity list Transit method list Transit - timing variation Habitability Astrobiology Circumstellar habitable zone Earth analog Extraterrestrial liquid water Habitability of natural satellites Superhabitable planet Catalogues Catalog of Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Lists Exoplanetary systems Host stars Multiplanetary systems Stars with proplyds Exoplanets List of exoplanets Discoveries Extremes Firsts Nearest Largest Most massive Terrestrial candidates Kepler Potentially habitable Discovered exoplanets by year before 2000 2000 -- 2009 2011 2012 2013 2014 2015 2016 2017 2018 Other Carl Sagan Institute Exoplanet phase curves Nexus for Exoplanet System Science Planets in science fiction Sudarsky 's gas giant classification Discoveries of exoplanets Search projects Astrobiology Disciplines Astrochemistry Astrophysics Atmospheric sciences Biochemistry Evolutionary biology Exoplanetology Geomicrobiology Microbiology Paleontology Planetary science Main topics Allan Hills 84001 Biomolecule Biosignature Drake equation Earliest known life forms Earth analog Extraterrestrial life Extremophiles Hypothetical types of biochemistry List of microorganisms tested in outer space Molecules detected in outer space Ocean planet Origin of life Panspermia Planetary protection Search for extraterrestrial intelligence ( SETI ) Yamato meteorite Planetary habitability Habitability of binary star systems Habitability of K - type main - sequence star systems Habitability of natural satellites Habitability of red dwarf systems Circumstellar habitable zone Earth analog List of potentially habitable exoplanets Tholin Extraterrestrial liquid water Galactic habitable zone Space missions Earth orbit BIO BIOCORE Biolab Bion BIOPAN Biosatellite program E-MIST ERA EXOSTACK EXPOSE O / OREOS OREOcube Tanpopo VEGGIE Mars missions Beagle 2 ExoMars ExoMars Trace Gas Orbiter Schiaparelli EDM lander Fobos - Grunt Mars Science Laboratory Curiosity Phoenix Viking Planned BioSentinel Europa Clipper ExoMars ExoMars rover ExoMars 2020 surface platform Mars 2020 Mars Global Remote Sensing Orbiter and Small Rover Proposed CAESAR Dragonfly Enceladus Explorer Enceladus Life Finder ‎ Enceladus Life Signatures and Habitability Europa Lander ExoLance Explorer of Enceladus and Titan Journey to Enceladus and Titan Laplace - P Life Investigation For Enceladus Mars sample return mission Oceanus THEO Cancelled and undeveloped Astrobiology Field Laboratory Beagle 3 Biological Oxidant and Life Detection Icebreaker Life Living Interplanetary Flight Experiment Mars Astrobiology Explorer - Cacher MELOS Northern Light Red Dragon Terrestrial Planet Finder Institutions and programs Astrobiology Society of Britain Astrobiology Science and Technology for Exploring Planets Breakthrough Initiatives Breakthrough Listen Breakthrough Message Breakthrough Starshot Carl Sagan Institute European Astrobiology Network Association NASA Astrobiology Institute Nexus for Exoplanet System Science Ocean Worlds Exploration Program Portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Earth_analog&oldid=844570460 '' Categories : Planetary habitability Types of planet Hidden categories : All articles with dead external links Articles with dead external links from November 2017 Webarchive template wayback links All articles with unsourced statements Articles with unsourced statements from July 2015 Talk Contents About Wikipedia Català Español Esperanto Français Bahasa Indonesia Italiano עברית Bahasa Melayu Norsk Polski Português Română Русский Slovenščina Svenska Українська 8 more Edit links This page was last edited on 5 June 2018, at 18 : 53 ( UTC ). 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Question: which planet is most similar in temperature to earth
Answer: Kepler-438b