Earth analog
An Earth analog, also called an Earth twin or second Earth, is a planet or moon 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 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 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 extrasolar planets, 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 planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way Galaxy. 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 contenders, from among more than 4000 confirmed exoplanets, based on astrophysical parameters, as well as the natural history 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.
History
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's deserts. Images and data from the Mariner and Viking space probes, 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 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.
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, 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 may also pose a problem in finding an Earth analog.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.
Size
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 are included.
According to the size criteria, the closest planetary mass objects by known radius or mass are:
| Name | Earth masses | Earth radii | Note |
| Kepler-69c | ≙2.14 | 1.7 | Originally thought to be in the circumstellar habitable zone, 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 g | >1.75 | Extremely hot. Not known to transit. | |
| Kepler-186f | 1.1 | Orbits in the habitable zone. | |
| Earth | 1 | 1 | Orbits in habitable zone. |
| Venus | 0.815 | 0.949 | Much hotter. |
| Kepler-20e | < 3.08 | 0.87 | Too hot to be Earth-like. |
| Proxima Centauri b | >1.07 | >0.94 | Closest exoplanet to Earth orbiting within the habitable zone. |
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, Kepler-20, COROT-7 and the three planets of Kepler-42 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
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 or as a byproduct of life, 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 of the planet, each of which introduces further variables.Below is a comparison of the confirmed planets with the closest known temperatures to Earth.