Earth analog I am going to talk about this first, then describe planets.
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 principlesuggests 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 starsand red dwarf stars within the Milky Way galaxy.[1][2] 11 billion of these estimated planets may be orbiting Sun-like stars.[3] The nearest such planet may be 12 light-years away.[1][2]
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.[4][5]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.[6] Scientists estimate that there may be billions of Earth-size planets within the Milky Way galaxy alone.[2][7]
Contents 1History 2Attributes and criteria 2.1Size 2.2Terrestrial 2.3Temperature 2.4Solar analog 2.5Surface water and hydrological cycle 3Extrasolar Earth analog 3.1Estimated frequency 4Terraforming 5See also 6References History[edit]
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.[8][9][10][11][12][13] 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.[14] 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.[15] 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.[citation needed]
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.[16][17] Further observations, including weather phenomena, have aided the understanding of geological processes that may operate on Earth-like planets.[18]
The Kepler space telescope began observing the transits of potential terrestrial planets in the habitable zone from 2011.[19][20] 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.[21] 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.[22][23]
Attributes and criteria[edit] 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 weatherconditions, 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[edit]
Size Comparisons: Kepler-20e[24] and Kepler-20f[25] 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.[26]
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:
NameEarth masses (M⊕)Earth radii (R⊕)Note
Kepler-69c0.981.7Originally thought to be in the circumstellar habitable zone (CHZ), now thought to be too hot.
Kepler-9d>1.5[27]1.64Extremely hot.
COROT-7b<91.58Extremely hot.
Kepler-20f< 14.3[25]1.03[25]Slightly larger and likely more massive, far too hot to be Earth-like.
Tau Ceti b2Extremely hot. Not known to transit.
Kepler-186f1.1[28]Orbits in CHZ.
Earth11Orbits in habitable zone.
Venus0.8150.949Much hotter.
Kepler-20e< 3.08[24]0.87[24]Too hot to be Earth-like.
Proxima b>1.27>1.1Closest 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[29][30]), 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 planetsare 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[edit]
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[edit] 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.[citation needed] 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 comparisonsVenusEarthKepler 22bMars
Global equilibrium temperature307 K 34 °C 93 °F255 K −18 °C −0.4 °F262 K −11 °C 22.2 °F206 K −67 °C −88.6 °F
+ Greenhouse gas effect737 K 464 °C 867 °F288 K 15 °C 59 °F295 K 22 °C 71.6 °F210 K −63 °C −81 °F
Tidally locked[31]AlmostNoUnknownNo
Global Bond albedo0.90.29Unknown0.25
Refs.[32][33][34]
Solar analog[edit] 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.[citation needed] 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[edit]
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,[35][36][37] and it is speculated that the same was the case for Venus.[15] 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.[citation needed]
Extrasolar Earth analog[edit] 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[edit]
Artist's concept of Earth-like planets[38]
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 hypothesisestimates – 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.[39] 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.[40]
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.[41] This would correspond to around 20 earth analogs per square centimeter of the Earth.[42]
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.[43] This, however, says nothing of their position in relation to the habitable zone.
Terraforming[edit] 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,[44][45][46] and to a lesser extent Venus,[47][48][49][50][51] have been cited as the most likely candidates for terraforming.
Gliese 581, extrasolar planetary system containing four planets. One of them, Gliese 581d, was the first planet to be found within the habitable zone of an extrasolar planetary system, the orbital region around a star in which an Earth-like planet could possess liquid water on its surface and possibly support life. Another, Gliese 581e, is the smallest planet seen in orbit around an ordinary main sequence star other than the Sun.
Gliese 581d was discovered in 2007 and has a mass at least 5.6 times that of Earth. Gliese 581d is the third planet discovered around the red dwarf star Gliese 581, which has a mass 0.31 times that of the Sun and is 20.4 light-years from Earth. Gliese 581d orbits its star every 66.64 days at a distance of 32.9 million km (20.5 million miles), which is within the habitable zone. Since it is so close to its star, the tidal forces on Gliese 581d have likely made its rotational period the same as its orbital period. Thus, on one side of the planet it is always day, and on the other side it is always night. However, the planet’s mass is such that, if it has an atmosphere with sufficient amounts of carbon dioxide, the atmosphere would be thick enough to avoid freezing on the night side.
Gliese 581e, discovered in 2009, has a mass 1.94 times that of Earth and is the smallest planet in the Gliese 581 system. It is also the closest to its star, orbiting once every 3.15 days at a distance from Gliese 581 of 4.5 million km (2.8 million miles). Gliese 581e is a rocky planet like Earth, but it probably could not sustain life; it is too close to its star for liquid water to survive on its surface.
Kepler-452b, the first approximately Earth-sized planet to be found in a Sun-like star’s habitable zone—the orbital region where an Earth-like planet could possess liquid water on its surface and thus possibly support life. Kepler-452b was discovered in 2015, in data that the Kepler satellite had gathered—before the first phase of its mission ended in 2013, when a reaction wheel, which kept Kepler pointed at the same spot in the sky for four years, failed. The star that Kepler-452b orbits has the same spectral type (G2) as the Sun and is about 1,400 light-years from Earth. The planet’s radius is 1.63 times that of Earth. If the planet has a rocky composition, its mass would be about 5 times that of Earth. However, it is equally likely that the planet’s composition is like that of Neptune, a small rocky core surrounded by a thick mantle of ice and a gaseous envelope, and therefore its mass may be less than that. Kepler-452b’s orbital period is 384.8 days, and it orbits at a distance of 156.5 million km (97.2 million miles). Since that distance is about the same as the distance between Earth and the Sun and the star is about 20 percent brighter than the Sun, Kepler-452b receives just 10 percent more solar energy than Earth. The star is 6 billion years old, 1.5 billion years older than the Sun, and is in the stage of its life when its radius and luminosity begin to increase, suggesting that Kepler-452b may have undergone or will soon undergo a runaway greenhouse effect in which its surface water will boil away, similar to what happened on Venus...
Earth analog I am going to talk about this first, then describe planets.
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 principlesuggests 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 starsand red dwarf stars within the Milky Way galaxy.[1][2] 11 billion of these estimated planets may be orbiting Sun-like stars.[3] The nearest such planet may be 12 light-years away.[1][2]
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.[4][5]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.[6] Scientists estimate that there may be billions of Earth-size planets within the Milky Way galaxy alone.[2][7]
Contents 1History 2Attributes and criteria 2.1Size 2.2Terrestrial 2.3Temperature 2.4Solar analog 2.5Surface water and hydrological cycle 3Extrasolar Earth analog 3.1Estimated frequency 4Terraforming 5See also 6References History[edit]
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.[8][9][10][11][12][13] 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.[14] 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.[15] 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.[citation needed]
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.[16][17] Further observations, including weather phenomena, have aided the understanding of geological processes that may operate on Earth-like planets.[18]
The Kepler space telescope began observing the transits of potential terrestrial planets in the habitable zone from 2011.[19][20] 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.[21] 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.[22][23]
Attributes and criteria[edit] 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 weatherconditions, 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[edit]
Size Comparisons: Kepler-20e[24] and Kepler-20f[25] 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.[26]
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:
NameEarth masses (M⊕)Earth radii (R⊕)Note
Kepler-69c0.981.7Originally thought to be in the circumstellar habitable zone (CHZ), now thought to be too hot.
Kepler-9d>1.5[27]1.64Extremely hot.
COROT-7b<91.58Extremely hot.
Kepler-20f< 14.3[25]1.03[25]Slightly larger and likely more massive, far too hot to be Earth-like.
Tau Ceti b2Extremely hot. Not known to transit.
Kepler-186f1.1[28]Orbits in CHZ.
Earth11Orbits in habitable zone.
Venus0.8150.949Much hotter.
Kepler-20e< 3.08[24]0.87[24]Too hot to be Earth-like.
Proxima b>1.27>1.1Closest 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[29][30]), 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 planetsare 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[edit]
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[edit] 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.[citation needed] 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 comparisonsVenusEarthKepler 22bMars
Global equilibrium temperature307 K 34 °C 93 °F255 K −18 °C −0.4 °F262 K −11 °C 22.2 °F206 K −67 °C −88.6 °F
+ Greenhouse gas effect737 K 464 °C 867 °F288 K 15 °C 59 °F295 K 22 °C 71.6 °F210 K −63 °C −81 °F
Tidally locked[31]AlmostNoUnknownNo
Global Bond albedo0.90.29Unknown0.25
Refs.[32][33][34]
Solar analog[edit] 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.[citation needed] 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[edit]
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,[35][36][37] and it is speculated that the same was the case for Venus.[15] 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.[citation needed]
Extrasolar Earth analog[edit] 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[edit]
Artist's concept of Earth-like planets[38]
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 hypothesisestimates – 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.[39] 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.[40]
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.[41] This would correspond to around 20 earth analogs per square centimeter of the Earth.[42]
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.[43] This, however, says nothing of their position in relation to the habitable zone.
Terraforming[edit] 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,[44][45][46] and to a lesser extent Venus,[47][48][49][50][51] have been cited as the most likely candidates for terraforming.
Gliese 581, extrasolar planetary system containing four planets. One of them, Gliese 581d, was the first planet to be found within the habitable zone of an extrasolar planetary system, the orbital region around a star in which an Earth-like planet could possess liquid water on its surface and possibly support life. Another, Gliese 581e, is the smallest planet seen in orbit around an ordinary main sequence star other than the Sun.
Gliese 581d was discovered in 2007 and has a mass at least 5.6 times that of Earth. Gliese 581d is the third planet discovered around the red dwarf star Gliese 581, which has a mass 0.31 times that of the Sun and is 20.4 light-years from Earth. Gliese 581d orbits its star every 66.64 days at a distance of 32.9 million km (20.5 million miles), which is within the habitable zone. Since it is so close to its star, the tidal forces on Gliese 581d have likely made its rotational period the same as its orbital period. Thus, on one side of the planet it is always day, and on the other side it is always night. However, the planet’s mass is such that, if it has an atmosphere with sufficient amounts of carbon dioxide, the atmosphere would be thick enough to avoid freezing on the night side.
Gliese 581e, discovered in 2009, has a mass 1.94 times that of Earth and is the smallest planet in the Gliese 581 system. It is also the closest to its star, orbiting once every 3.15 days at a distance from Gliese 581 of 4.5 million km (2.8 million miles). Gliese 581e is a rocky planet like Earth, but it probably could not sustain life; it is too close to its star for liquid water to survive on its surface.
Kepler-452b, the first approximately Earth-sized planet to be found in a Sun-like star’s habitable zone—the orbital region where an Earth-like planet could possess liquid water on its surface and thus possibly support life. Kepler-452b was discovered in 2015, in data that the Kepler satellite had gathered—before the first phase of its mission ended in 2013, when a reaction wheel, which kept Kepler pointed at the same spot in the sky for four years, failed. The star that Kepler-452b orbits has the same spectral type (G2) as the Sun and is about 1,400 light-years from Earth. The planet’s radius is 1.63 times that of Earth. If the planet has a rocky composition, its mass would be about 5 times that of Earth. However, it is equally likely that the planet’s composition is like that of Neptune, a small rocky core surrounded by a thick mantle of ice and a gaseous envelope, and therefore its mass may be less than that. Kepler-452b’s orbital period is 384.8 days, and it orbits at a distance of 156.5 million km (97.2 million miles). Since that distance is about the same as the distance between Earth and the Sun and the star is about 20 percent brighter than the Sun, Kepler-452b receives just 10 percent more solar energy than Earth. The star is 6 billion years old, 1.5 billion years older than the Sun, and is in the stage of its life when its radius and luminosity begin to increase, suggesting that Kepler-452b may have undergone or will soon undergo a runaway greenhouse effect in which its surface water will boil away, similar to what happened on Venus...