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An Earth analog, also called an Earth twin or second Earth, is a Exoplanet or Exomoon with environmental conditions similar to those found on Earth. The term Earth-like planet is also used, but this term may refer to any terrestrial planet.
The possibility is of particular interest to Astrobiology 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 and space and survival have long sought an Earth analog for settlement. In the far future, humans might artificially produce an Earth analog by terraforming.
Before the scientific search for and study of exoplanet, the possibility was argued through philosophy and science fiction. Philosophers have suggested that the size of the universe is such that a near-identical planet must exist somewhere. 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 the Solar System, supporting the Rare Earth Hypothesis.
On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized exoplanet orbiting in the habitable zones of Solar analog and red dwarf within the Milky Way. The nearest such planet could be expected to be within 12 light-years of the Earth, statistically. In September 2020, astronomers identified 24 superhabitable planets (planets better than Earth) contenders, from among more than 4000 confirmed , based on Astrophysics, as well as the Life of known life forms on the Earth.
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).
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 Venusians 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 under an acidic atmosphere with a surface pressure of .
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.
On 11 January 2023, NASA scientists reported the detection of LHS 475 b, an Earth-like exoplanet – and the first exoplanet discovered by the James Webb Space Telescope.
The process of determining Earth analogs often involves reconciling several registers of uncertainty quantification. As anthropologist Vincent Ialenti's work on the epistemology of analogical reasoning has shown, some planetary scientists are "more comfortable making the leap of faith to bridge time and space and pull together two disparate objects" than others are.
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.
According to the size criteria, the closest planetary mass objects by known radius or mass are:
| Originally thought to be in the circumstellar habitable zone (CHZ), now thought to be too hot. |
| Extremely hot. |
| 1.58 |
| Slightly larger and likely more massive, far too hot to be Earth-like. |
| Extremely hot. Not known to transit. |
| Orbits in the habitable zone. |
| Orbits in habitable zone. |
| Much hotter. |
| Too hot to be Earth-like. |
| Closest exoplanet to Earth orbiting within the habitable zone. |
This comparison indicates that size alone is a poor measure, particularly in terms of Habitable zone. 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.
Many of Earth's surface materials and landforms are formed as a result of interaction with water (such as clay and ) 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.
Below is a comparison of the confirmed planets with the closest known temperatures to Earth.
| Temperature comparisons | Venus | Earth | Kepler-22b | Mars |
| 206 K −67 °C −88.6 °F | ||||
| 210 K −63 °C −81 °F | ||||
| No | ||||
| 0.25 | ||||
| Refs. | ||||
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 continuously habitable like Earth.
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.
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 for Science speculated that there could be 100 billion terrestrial planets just in the Milky Way galaxy.
In 2011 NASA's Jet Propulsion Laboratory (JPL), based on observations from the Kepler Mission suggested that between 1.4% and 2.7% of all Sun-like are expected to have Earth-size planets within the of their stars. This means there could be as many as two billion Earth-sized planets in the Milky Way galaxy alone, and assuming that all galaxies have number of such planets similar to the Milky Way, in the 50 billion galaxies in the observable universe, there may be as many as a hundred quintillion Earth-like planets. 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. 17 Billion Earth-Size Alien Planets Inhabit Milky Way SPACE.com 07 January 2013 This, however, says nothing of their position in relation to the habitable zone.
A 2019 study determined that Earth-size planets may circle 1 in 6 Sun-like stars.
Due to proximity and similarity in size, Mars, and to a lesser extent Venus, have been cited as the most likely candidates for terraforming.
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