Twin Earth

An Earth analog (also referred to as a Twin Earth, Earth Twin, or Earth-like planet, though this latter term may refer to any terrestrial planet) is another planetor moon with environmental conditions similar to those found on the planet Earth.

The possibility is of particular interest to humans under reasoning that the more similar a planet is to Earth, the more likely it is 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 colonizationhave 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 mankind.

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. Philosophers have pointed out that the size of the universe is such that a near-identical planet must exist somewhere. In the 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 Waygalaxy.^[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, according to the scientists.^[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.

Attributes and criteria[edit]
Size[edit]
Size Comparisons: Kepler-20e^[24] and Kepler-20f^[25] with Venus and Earth.

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[27]	1.64	Extremely hot
COROT-7b	<9	1.58	Extremely hot
Kepler-20f	< 14.3[25]	1.03[25]	Slightly larger and likely more massive, far too hot to be Earth-like
Tau Ceti b	2		Extremely hot. Not known to transit.
Alpha Centauri Bb	1.1[28]		Closest known mass to Earth but much hotter NOTE: May not exist (NYT, June 10, 2013).[29]
Kepler-186f		1.1[30]	Orbits in CHZ
Earth	1	1	Orbits in habitable zone
Venus	0.815	0.949	Much hotter
Kepler-20e	< 3.08[24]	0.87[24]	Too hot to be Earth-like

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 forestscovering 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 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 assuper-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:

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^[31][32]), 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 mass of theSolar 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 terrestrial planets are important as they may indicate the probable frequency and distribution of Earth-like planets.