Earth analog

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Evolutionary paths of Earth and Venus. Venus has been the prime example for a planet resembling Earth and how such a planet can differ. 11214 2023 956 Fig1 HTML.webp
Evolutionary paths of Earth and Venus. Venus has been the prime example for a planet resembling Earth and how such a planet can differ.

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.

Contents

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. [1] [2] The nearest such planet could be expected to be within 12 light-years of the Earth, statistically. [1] [2] In September 2020, astronomers identified 24 superhabitable planets (planets better than Earth) 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. [3]

Artist's representation of a hypothetical habitable exoplanet with three natural satellites Exoplaneta habitable.jpg
Artist's representation of a hypothetical habitable exoplanet with three natural satellites

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).

History

Percival Lowell depicted Mars as a dry but Earth-like planet and habitable for an extraterrestrial civilisation. Lowell Mars channels.jpg
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 Titan dunes crop.png
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's deserts. Images and data from the Mariner (1965) and Viking space probes (1975–1980), however, revealed the planet as a barren cratered world. [4] [5] [6] [7] [8] [9] 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. [10] 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. [11] 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 462 °C (864 °F) [12] under an acidic atmosphere with a surface pressure of 9.2 MPa (1,330 psi). [12]

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. [13] [14] Further observations, including weather phenomena, have aided the understanding of geological processes that may operate on Earth-like planets. [15]

The Kepler space telescope began observing the transits of potential terrestrial planets in the habitable zone from 2011. [16] [17] 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. [18] 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. [19] [20]

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. [21]

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 (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 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.

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, [22] 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. [23]

Size

Size comparisons: Kepler-20e and Kepler-20f with Venus and Earth Kepler 20 - planet lineup.jpg
Size comparisons: Kepler-20e and Kepler-20f 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.

According to the size criteria, the closest planetary mass objects by known radius or mass are:

NameEarth masses (ME)Earth radii (R🜨)Note
Kepler-69c ≙2.141.7Originally 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 <91.58
Kepler-20f < 14.3 [25] 1.03 [25] Slightly larger and likely more massive, far too hot to be Earth-like.
Tau Ceti g >1.75Extremely hot. Not known to transit.
Kepler-186f 1.1 [28] Orbits in the habitable zone.
Earth 11Orbits in habitable zone.
Venus 0.8150.949Much hotter.
Kepler-20e < 3.08 [24] 0.87 [24] Too hot to be Earth-like.
Proxima Centauri b >1.07>0.94 [29] 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 (discovered in 2012), Kepler-20 (discovered in 2011 [30] [31] ), 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.

Terrestrial

Surfaces like this of Saturn's moon Titan (taken by Huygens probe) bear superficial similarities to the floodplains of Earth. Huygens surface color.jpg
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

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 comparisons Venus Earth Kepler-22b Mars
Global equilibrium temperature 307 K
34 °C
93 °F		255 K
−18 °C
−0.4 °F		262 K
−11 °C
22.2 °F		206 K
−67 °C
−88.6 °F
+ Greenhouse gas effect737 K
464 °C
867 °F		288 K
15 °C
59 °F		295 K
22 °C
71.6 °F		210 K
−63 °C
−81 °F
Tidally locked [32] AlmostNoUnknownNo
Global Bond albedo 0.90.290.25
Refs. [33] [34] [35]

Solar analog

Another criterion of an ideal life-harboring earth analog is that it should orbit a solar analog; that is, a star much like the Sun. However, this criterion may not be entirely valid as many different types of stars can provide a local environment hospitable to life. For example, in the Milky Way, most stars are smaller and dimmer than the Sun. One such star, TRAPPIST-1, is located 12 parsecs (39 light years) away and is roughly 10 times smaller and 2,000 times dimmer than the Sun, yet it harbors at least six Earth-like planets in its habitable zone. While these conditions may seem unfavorable to known life, TRAPPIST-1 is expected to continue burning for 12 trillion years (compared to the Suns remaining 5 billion year lifetime) which is time enough for life to arise by abiogenesis. [36] For comparison, life evolved on Earth in a mere one billion years.[ citation needed ]

Surface water and hydrological cycle

Water covers 70% of Earth's surface and is required by all known life. Eckert4.jpg
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. Kepler-22 diagram.jpg
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.

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 currently 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, [37] [38] [39] and it is speculated that the same was the case for Venus. [11] 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 continuously habitable like Earth.[ citation needed ]

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. 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.

History

Estimated frequency

Artist's concept of Earth-like planets Artist's Concept of Earth-Like Planets in the Future Universe.jpg
Artist's concept of Earth-like planets

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 hypothesis estimates – 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. [41] 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. [42]

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 stars are expected to have Earth-size planets within the habitable zones 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. [43] This would correspond to around 20 earth analogs per square centimeter of the Earth. [44]

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. [45] 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. [46]

Terraforming

Artist's conception of a terraformed Venus, a potential Earth analog TerraformedVenus.jpg
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, [47] [48] [49] and to a lesser extent Venus, [50] [51] [52] [53] [54] have been cited as the most likely candidates for terraforming.

See also

Related Research Articles

<span class="mw-page-title-main">Exoplanet</span> Planet outside the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 7 November 2024, there are 5,787 confirmed exoplanets in 4,320 planetary systems, with 969 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Terraforming</span> Hypothetical planetary engineering process

Terraforming or terraformation ("Earth-shaping") is the hypothetical process of deliberately modifying the atmosphere, temperature, surface topography or ecology of a planet, moon, or other body to be similar to the environment of Earth to make it habitable for humans to live on.

<span class="mw-page-title-main">Terrestrial planet</span> Planet that is composed primarily of silicate rocks or metals

A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.

<span class="mw-page-title-main">Kepler space telescope</span> NASA space telescope for exoplanetology (2009–2018)

The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine and a half years of operation, the telescope's reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018.

<span class="mw-page-title-main">Habitable zone</span> Orbits where planets may have liquid surface water

In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

<span class="mw-page-title-main">Planetary habitability</span> Known extent to which a planet is suitable for life

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain an environment hospitable to life. Life may be generated directly on a planet or satellite endogenously. Research suggests that life may also be transferred from one body to another, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.

<span class="mw-page-title-main">Super-Earth</span> Type of exoplanet

A Super-Earth or super-terran or super-tellurianis a type of exoplanet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.

Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and is thus surmised to be essential for extraterrestrial life.

<span class="mw-page-title-main">Habitability of natural satellites</span> Measure of the potential of natural satellites to have environments hospitable to life

The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.

<span class="mw-page-title-main">Discoveries of exoplanets</span> Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016; no planet discovery has yet come from that evidence. What turned out to be the first detection of an exoplanet was published among a list of possible candidates in 1988, though not confirmed until 2003. The first confirmed detection came in 1992, with the discovery of terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems, with 1007 systems having more than one planet. This is a list of the most notable discoveries.

<span class="mw-page-title-main">Kepler-22b</span> Super-Earth exoplanet orbiting Kepler-22

Kepler-22b is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about 640 light-years from Earth in the constellation of Cygnus. It was discovered by NASA's Kepler Space Telescope in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star, where liquid water could exist on the planet's surface. Kepler-22 is too dim to be seen with the naked eye.

<span class="mw-page-title-main">Kepler-69c</span> Super-Earth orbiting Kepler-69

Kepler-69c is a confirmed super-Earth exoplanet, likely rocky, orbiting the Sun-like star Kepler-69, the outermore of two such planets discovered by NASA's Kepler spacecraft. It is located about 2,430 light-years from Earth.

<span class="mw-page-title-main">Kepler-62f</span> Super-Earth orbiting Kepler-62

Kepler-62f is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kepler space telescope. It is located about 982 light-years from Earth in the constellation of Lyra.

<span class="mw-page-title-main">Kepler-69</span> Star in the constellation Cygnus

Kepler-69 is a G-type main-sequence star similar to the Sun in the constellation Cygnus, located about 2,390 ly (730 pc) from Earth. On April 18, 2013 it was announced that the star has two planets. Although initial estimates indicated that the terrestrial planet Kepler-69c might be within the star's habitable zone, further analysis showed that the planet very likely is interior to the habitable zone and is far more analogous to Venus than to Earth and thus completely inhospitable.

<span class="mw-page-title-main">Kepler-438b</span> Super-Earth orbiting Kepler-438

Kepler-438b is a confirmed near-Earth-sized exoplanet. It is likely rocky. It orbits on the inner edge of the habitable zone of a red dwarf, Kepler-438, about 460.2 light-years from Earth in the constellation Lyra. It receives 1.4 times our solar flux. The planet was discovered by NASA's Kepler spacecraft using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. NASA announced the confirmation of the exoplanet on 6 January 2015.

<span class="mw-page-title-main">Kepler-452b</span> Super-Earth exoplanet candidate orbiting Kepler-452

Kepler-452b is a candidate super-Earth exoplanet orbiting within the inner edge of the habitable zone of the sun-like star Kepler-452 and is the only planet in the system discovered by the Kepler space telescope. It is located about 1,800 light-years (550 pc) from Earth in the constellation of Cygnus.

<span class="mw-page-title-main">Kepler-1649c</span> Earth-size exoplanet orbiting Kepler-1649

Kepler-1649c is an Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star Kepler-1649, the outermost planet of the planetary system discovered by Kepler’s space telescope. It is located about 301 light-years (92 pc) away from Earth, in the constellation of Cygnus.

Kepler-737b is a super-Earth exoplanet 669 light years away. There is a chance it could be on the inner edge of the habitable zone.

<span class="mw-page-title-main">Habitability of yellow dwarf systems</span> Likelihood of finding extraterrestrial life in yellow dwarf systems

Habitability of yellow dwarf systems defines the suitability for life of exoplanets belonging to yellow dwarf stars. These systems are the object of study among the scientific community because they are considered the most suitable for harboring living organisms, together with those belonging to K-type stars.

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