The possibility is of particular interest to humans as it is easily inferred that the more similar a planet is to Earth, the more likely it is of sustaining Earth-like complex extraterrestrial life and, more importantly, civilization. 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".
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 the Earth should be common in an infinite universe, while the Rare Earth hypothesis suggests that they are extremely rare. Philosophers have pointed out that the size of the universe is such that a near identical planet must exist somewhere, such theories include the philosophy of Multiverse and the Twin Earth thought experiment.
Some scientific theories speculate that Earth analogs may have existed in our Solar System in the past. In the future, technology may be used by humans to artificially produce an Earth analog. In theory, terraforming, virtual reality or simulated reality could potentially create such a world. 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.
Technological advances in extrasolar planet detection are rapidly refining the probabilities of a real discovery, not just in our universe, but in the present Milky Way galaxy, though distribution and attributes are still very much unknown. Different studies on the frequency of Earth-like planets in the Milky Way have resulted in estimation varying from one (i.e. the Earth) to hundreds of billions. Current calculations tend to indicate that they may be relatively common in the universe.
The more recent serious scientific findings have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and Search for Extraterrestrial Intelligence. NASA and the SETI Institute have proposed categorising the increasing number of Earth-like planets found using a measure called the Earth Similarity Index (ESI) based on mass, radius and temperature. According to this measure, the planet currently thought to be most similar to Earth is Gliese 667C c (0.85).
Between 1858 and 1920, Mars was believed 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 civilisation 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 and Viking space probes, however, portrayed the planet as a dead cratered world. Although with continuing discoveries many 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 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 civilisation. 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 hellish world with the surface temperature around 900 °F (482 °C) under the atmosphere 92 times thicker the Earth's.
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, 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 (possibly through convergent evolution or parallel evolution). 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 things 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 is often thought to be an effective measure as planets of Earth's size are thought more likely to be terrestrial in nature and be capable of retaining a significant atmosphere.
The Earth Similarity Index uses both Mass and Radius as criteria.
The closest planets and natural satellites to Earth's size are:
|Name||Earth Masses (Me)||Earth Radii (Re)||Note|
|Kepler-22b||< 36||2.4||Much larger. Within the habitable zone (HZ) of solar analog.|
|Gliese 667 Cc||4.5||-||Within the HZ of red dwarf. Significantly less massive than Kepler-22b.|
|55 Cancri e||8.63||2||much larger|
|Kepler-20f||< 14.3||1.03||slightly larger and likely more massive|
|Tau Ceti f||6||Orbiting in an extended habitable zone|
|Tau Ceti e||4||Orbiting in the habitable zone|
|Tau Ceti d||4|
|Gliese 581 g||3.1||Orbiting in the habitable zone|
|Tau Ceti c||3|
|Tau Ceti b||2|
|Alpha Centauri Bb||1.1||Closest known mass to Earth but much hotter|
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 Bb (discovered 2012), Kepler-20 (discovered in 2011), COROT-7b and the three planets of KOI-961 (confirmed in 2012) are very hot and Mars, Ganymede and Titan are frigid worlds, resulting also in wide variety of surface and atmospheric conditions. The mass of the Solar System's moons are a tiny fraction of that of Earth whereas the mass of extrasolar planets are very difficult to accurately measure. However discoveries of Earth-sized exoplanets are important as they may indicate the probable frequency and distribution of Earth-like planets. One of the first steps to finding more Earth-like planets may be shortlisting candidates of similar size and then comparing their temperatures to that 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 organic 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 lifeforms.
There are several factors that can determine planetary temperatures and therefore several measures that can draw direct comparisons to that of the Earth in planets where atmospheric conditions are unknown. Equilibrium temperature is used for planets without atmospheres. With atmospheres, a greenhouse effect is assumed. Finally, surface temperature is used. Each of these temperatures is of course affected by climate, which is influenced by the orbit and rotation (or tidal locking) of the planet, each of which introduces further variabilities.
Below is a comparison of the confirmed planets with the closest known temperatures to Earth.
Solar Analog 
Another criteria of an Earth Analog is that it should orbit a Solar Analog, that is, a star much like our own 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 additionally many of these planetary systems have proven to be surprisingly different to our own.
Kepler-22, the parent star of Kepler-22b is slightly smaller and cooler than the Sun.
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 
The concept of the Liquid Water Zone (or habitable zone) defines a region where water can exist on the surface and 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. The 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 and it is speculated that Venus did have in their ancient pasts. Thus extrasolar planets (or moons) in the Goldilocks position with substantial atmospheres may possess oceans and water clouds like those on Earth.
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 the Earth.
The best candidates to date by these measure is Kepler-22b, it orbits a Sun-like star in a similar position within the habitable zone, however it is much larger than Earth and its composition is currently unknown. Our Solar System has shown that just being in the habitable zone does not guarantee that a planet is "Earth-like".
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.
However planets remarkably Earth-like may be found using less strict criteria. For example under certain conditions, such as strong greenhouse effect, atmospheric pressure or gravity, a planet could theoretically possess surface lakes and oceans without being strictly within the habitable zone. Indeed, Saturn's moon Titan is the only other body in the Solar System known to have surface liquid, however it is hydrocarbons rather than water. Other worlds may be similar to Earth, but have a largely dry or icy surface but with atmospheric water vapour, ground water or subsurface ocean, much like some arid and polar regions on Earth.
Candidates in the Solar System 
Early in the history of astronomy, Venus (and to a lesser extent Mars and Neptune) were thought to be Earth-like planets and some even conceptualised them to be home to extraterrestrial civilisation. Although these were later found to be misconceptions.
Still, scientists continue to find similarities with Mars and postulate that both ancient Venus and Mars could have been quite Earth-like.
Mars, the second closest planet, appears to have had and still have some similarities to Earth. Like Earth, Mars has an atmosphere with a greenhouse effect, geographical similarities including polar ice caps, similar rotation, volcanic activity and evidence of water. As such, Mars remains a candidate for extraterrestrial life. It also makes human colonization of Mars a subject of much research.
However Mars is much smaller and lacks a magnetosphere and its year is almost twice as long. Its freezing climate, lower gravity and thin but toxic carbon dioxide atmosphere all make it hostile to Earth life.
Ancient Mars 
Ancient Mars may have been quite Earth-like with a similar climate and liquid water.
Mars Ocean Hypothesis 
The Mars Ocean Hypothesis states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet’s geologic history. This primordial ocean, dubbed Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region that lies 4–5 km (2.5–3 miles) below the mean planetary elevation, at a time period of approximately 3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere. Early Mars would have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.
Venus is sometimes called Earth's "sister planet" (see below) due to the similar size, gravity, and bulk composition. Like Earth it has an atmosphere with a greenhouse effect and clouds, rain and is volcanically active. A younger Venus is believed to have possessed Earth-like oceans, but these evaporated as the temperature rose. This may be due to the fact that Venus, owing to its slow rotation, does not have a significant magnetic field, allowing the constituent atoms of the water to be blown away by the solar wind. However, the extreme heat on present day Venus, combined with the crushing atmosphere composed of toxic carbon dioxide and sulfuric acid rain makes the surface hostile to Earth life. The possibility that a habitable niche exists in the lower and middle cloud layers of Venus can not yet be excluded.
Saturn's moon Titan 
Saturn's moon Titan has some Earth-like properties. Titan's geography has similarities to Earth and is known to have a dense atmosphere, with clouds and rain and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found.
The possibility of life on Titan is a subject of ongoing research.
In billions of years, Titan may become Earth-like as the solar system's habitable zone moves farther out.
However Titan is much smaller than Earth, it has a lower gravity and its composition including its toxic methane atmosphere and anti-greenhouse effect is hostile to Earth life.
Extrasolar Earth analog 
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. However, 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 have theorised that a sufficiently massive natural satellite may form a habitable moon similar to Earth.
A 2011 estimate by the NASA's Jet Propulsion Laboratory (JPL), based on observations from the Kepler mission, determined that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earth-like planets "within the habitable zones of their stars". This would mean that there are two billion such worlds in the Milky Way galaxy, with the inference that many more exist in the known 50 billion galaxies.
Perhaps one of the most promising Earth analogs to date, Kepler-22b was confirmed December 5, 2011. orbiting the habitable zone of a sun-like main sequence star. At 2.4 times the radius of Earth  it has an estimated surface temperature around 22 degrees Celsius, however the nature of the planet is still unknown.
Estimated Frequency 
The frequency of Earth-like planets in both the Milky Way Galaxy and the larger universe is still very much unknown. It ranges from the extreme Rare Earth Hypothesis estimates - one (i.e. the Earth) to hundreds of billions.
Many equations, including the Drake equation use estimates as the basis for their calculations and predictions. Equations such as these are often based on the probability of each sun-like star possessing an Earth-like planet.
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 and sees this as a byproduct of the formation of rocky planets.
In 2009 Alan Boss of the Carnegie Institution of Science estimated that there are 100 billion terrestrial planets in our Milky Way Galaxy alone, many which could have lifeforms and thousands of civilisations.
In 2011 NASA's Jet Propulsion Laboratory (JPL) and based on observations from the Kepler Mission is that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars". This means there are "two billion" of them in our own 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 sextillion.
In 2013, a Harvard-Smithsonian Center for Astrophysics using statistical analysis of additional Kepler data concluded 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 (literally, "Earth-forming") of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth to make it habitable by terran organisms.
Due to proximity and similarity in size, Mars and to a lesser extent Venus are seen as the most likely candidates for terraforming.
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