Earth analog

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Earth_analog 

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. The thousands of exoplanetary star systems discovered so far are profoundly different from the Solar system, supporting the Rare Earth Hypothesis.

Philosophers have pointed out that the size of the universe is such that a near-identical planet must exist somewhere. In the far 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 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 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 planet (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, 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 (SETI).

History

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

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 but with a desert like landscape. Images and data from the Mariner (1965) and Viking space probes (1975–1980), 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 462 °C (864 °F) under an acidic atmosphere with a surface pressure of 9.2 MPa (1,330 psi).

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.

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.

Size

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.

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:

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	1.64	Extremely hot.
COROT-7b	<9	1.58
Kepler-20f	< 14.3	1.03	Slightly larger and likely more massive, far too hot to be Earth-like.
Tau Ceti b	2		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 b	>1.27	>1.1	Closest exoplanet to Earth.

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), 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

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. 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 effect	737 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	Almost	No	Unknown	No
Global Bond albedo	0.9	0.29		0.25

Solar analog

Main article: 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 our Sun. However, this criteria 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 our sun, yet it harbors at least 6 earth-like planets in its habitable zone. While these conditions may seem unfavorable to life as we know it, TRAPPIST-1 is expected to continue burning for 12 trillion years (compared to our suns remaining 5 billion year lifetime) which is time enough for life to arise by abiogenesis. For comparison, life evolved on earth in a mere 1 billion years.

Surface water and hydrological cycle

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

 

Main article: Habitable zone

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, and it is speculated that the same was the case for Venus. 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 continually habitable like Earth.

Extrasolar Earth analog

See also: List of potentially habitable exoplanets

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

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. 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 of Science speculated that there could be 100 billion terrestrial planets in our Milky Way galaxy alone.

In 2011 NASA's Jet Propulsion Laboratory (JPL) and based on observations from the Kepler Mission is that about 1.4% to 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 two billion of them in the 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 hundred quintillion. 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. 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.

Terraforming

Main article: Terraforming

 

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, and to a lesser extent Venus, have been cited as the most likely candidates for terraforming.

Superhabitable planet

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Stellar_population 

 

Artist's impression of one possible appearance of a superhabitable planet. The reddish hue is vegetation.

A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong, who have criticized the language used in the search for habitable planets and proposed clarifications. According to Heller and Armstrong, knowing whether or not a planet is in its host star's habitable zone (HZ) is insufficient to determine its habitability: it is not clear why Earth should offer the most suitable physicochemical parameters to living organisms, as "planets could be non-Earth-like, yet offer more suitable conditions for the emergence and evolution of life than Earth did or does." While still assuming that life requires water, they hypothesize that Earth may not represent the optimal planetary habitability conditions for maximum biodiversity; in other words, they define a superhabitable world as a terrestrial planet or moon that could support more diverse flora and fauna than there are on Earth, as it would empirically show that its environment is more hospitable to life.

Heller and Armstrong also point out that not all rocky planets in a habitable zone (HZ) may be habitable, and that tidal heating can render terrestrial or icy worlds habitable beyond the stellar HZ, such as in Europa's internal ocean. The authors propose that in order to identify a habitable—or superhabitable—planet, a characterization concept is required that is biocentric rather than geo- or anthropocentric. Heller and Armstrong proposed to establish a profile for exoplanets according to stellar type, mass and location in their planetary system, among other features. According to these authors, such superhabitable worlds would likely be larger, warmer, and older than Earth, and orbiting K-type main-sequence stars.

General characteristics

Heller and Armstrong proposed that a series of basic characteristics are required to classify an exoplanet or exomoon as superhabitable; for size, it is required to be about 2 Earth masses, and 1.3 Earth radii will provide an optimal size for plate tectonics. In addition, it would have a greater gravitational attraction that would increase retention of gases during the planet's formation. It is therefore likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life to about 25 °C (77 °F). A denser atmosphere may also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which would improve diversity of marine life in shallow waters.

Other factors to consider are the type of star in the system. K-type stars are less massive than the Sun, and are stable on the main sequence for a very long time (18 to 34 billion years, compared to 10 billion for the Sun, a G-class star), giving more time for the emergence of life and evolution. In addition, K-type stars emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than G-type stars like the Sun.

Surface, size and composition

Kepler-62e, second from the left has a radius of 1.6 R_⊕. Earth is on the far right; scaled.

An exoplanet with a larger volume than that of Earth, or with a more complex terrain, or with a larger surface covered with liquid water, could be more hospitable for life than Earth. Since the volume of a planet tends to be directly related to its mass, the more massive it is, the greater its gravitational pull, which can result in a denser atmosphere.

Some studies indicate that there is a natural radius limit, set at R_⊕, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures. Generally, objects with a mass below 8 M_⊕ are very likely to be of similar composition as Earth. Above this limit, the density of the planets decreases with increasing size, the planet will become a "water world" and finally a gas giant. In addition, for most super-Earths with masses 7 times Earth's, their high masses may cause them to lack plate tectonics. Thus, it is expected that any exoplanet similar to Earth's density and with a radius under 2 R_⊕ may be suitable for life. However, other studies indicate that water worlds represent a transitional stage between mini-Neptunes and the terrestrial planets, especially if they belong to red dwarfs or K dwarfs. Although water planets may be habitable, the average depth of the water and the absence of land area would not make them superhabitable as defined by Heller and Armstrong. From a geological perspective, the optimal mass of a planet is about 2 M_⊕, so it must have a radius that keeps the density of the Earth among 1.2 and 1.3R_⊕.

The average depth of the oceans also affects the habitability of a planet. The shallow areas of the sea, given the amount of light and heat they receive, usually are more comfortable for known aquatic species, so it is likely that exoplanets with a lower average depth are more suitable for life. More massive exoplanets would tend to have a regular surface gravity, which can mean shallower—and more hospitable—ocean basins.

Geology

Plate tectonics, in combination with the presence of large bodies of water on a planet, is able to maintain high levels of carbon dioxide (CO
₂) in its atmosphere. This process appears to be common in geologically active terrestrial planets with a significant rotation speed. The more massive a planetary body, the longer time it will generate internal heat, which is a major contributing factor to plate tectonics. However, excessive mass can also slow plate tectonics because of increased pressure and viscosity of the mantle, which hinders the sliding of the lithosphere. Research suggests that plate tectonics peaks in activity in bodies with a mass between 1 and 5M_⊕, with an optimum mass of approximately 2M_⊕.

If the geological activity is not strong enough to generate a sufficient amount of greenhouse gases to increase global temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by an intense internal heat source such as tidal heating or stellar irradiation.

Magnetosphere

Another feature favorable to life is a planet's potential to develop a strong magnetosphere to protect its surface and atmosphere from cosmic radiation and stellar winds, especially around red dwarf stars. Less massive bodies and those with a slow rotation, or those that are tidally locked, have a weak or no magnetic field, which over time can result in the loss of a significant portion of its atmosphere, especially hydrogen, by hydrodynamic escape.

The climate of a warmer and wetter terrestrial exoplanet may resemble that of the tropical regions of Earth. In the picture, mangrove in Cambodia.

Temperature and climate

The optimum temperature for Earth-like life in general is unknown, although it appears that on Earth organism diversity has been greater in warmer periods. It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life. The thermoregulatory effect of large oceans on exoplanets located in a habitable zone may maintain a moderate temperature range. In this case, deserts would be more limited in area and would likely support habitat-rich coastal environments.

However, studies suggest that Earth already lies near to the inner edge of the habitable zone of the Solar System, and that may harm its long-term livability as the luminosities of main-sequence stars steadily increase over time, pushing the habitable zone outwards. Therefore, superhabitable exoplanets must be warmer than Earth, yet orbit further out than Earth does and closer to the center of the system's habitable zone. This would be possible with a thicker atmosphere or with a higher concentration of greenhouse gases.

Star

Habitable zone (HZ) position of some of the most similar and average surface temperature exoplanets.

The star's type largely determines the conditions present in a system. The most massive stars O, B, and A have a very short life cycle, quickly leaving the main sequence. In addition, O-type stars produce a photoevaporation effect that prevents the accretion of planets around the star.

On the opposite side, the less massive M-and K-types are by far the most common and long-lived stars of the universe, but their potential for supporting life is still under study. Their low luminosity reduces the size of the habitable zone, which are exposed to ultraviolet radiation outbreaks that occur frequently, especially during their first billion year of existence. When a planet's orbit is too short, it can cause tidal locking of the planet, where it always presents the same hemisphere to the star, known as day hemisphere. Even if the existence of life were possible in a system of this type, it is unlikely that any exoplanet belonging to a red dwarf star would be considered "superhabitable".

Dismissing both ends, systems with a K-type stars offer the best habitable zones for life. K-type stars allow the formation of planets around them, have a long life expectancy, and provide a stable habitable zone free of the effects of excessive proximity to its star. Furthermore, the radiation produced by a K-type star is low enough to allow complex life without the need for an atmospheric ozone layer. They are also the most stable and their habitable zone does not move very much during its lifetime, so a terrestrial analog located near a K-type star may be habitable for almost all of the main sequence.

Orbit and rotation

Artistic impression of a possible Earth analog, Kepler-186f. Some superhabitable planets could have a similar appearance and may not have important differences with Earth.

Experts have not reached a consensus about what the optimal rotation speed for an exoplanet is, but it can't be too fast or slow. The latter case can cause problems similar to those observed in Venus, which completes one rotation every 243 Earth days, and as a result, cannot generate an Earth-like magnetic field. A more massive slow-rotation-planet could overcome this problem by having multiple moons due to its higher gravity that can boost the magnetic field.

Ideally, the orbit of a superhabitable world would be at the midpoint of the habitable zone of its star system.

Atmosphere

There are no solid arguments to explain if Earth's atmosphere has the optimal composition to host life. On Earth, during the period when coal was first formed, atmospheric oxygen (O
₂) levels were up to 35%, and coincided with the periods of greatest biodiversity. So, assuming that the presence of a significant amount of oxygen in the atmosphere is essential for exoplanets to develop complex life forms, the percentage of oxygen relative to the total atmosphere appears to limit the maximum size of the planet for optimum superhabitability and ample biodiversity.

Also, the atmospheric density should be higher in more massive planets, which reinforces the hypothesis that super-Earths can provide superhabitable conditions.

Age

The first stars that formed in the universe were metal-free stars, which probably prevented planet formation.

In a biological context, older planets than Earth may have greater biodiversity, since native species have had more time to evolve, adapt and stabilize the environmental conditions to sustain a suitable environment for life that can benefit their descendants.

However, for many years it was thought that since older star systems have lower metallicity, they should display low planet formation, and thus such old planets may have been scant in the beginning, but the number of metallic items in the universe must have grown steadily since its inception. The first exoplanetary discoveries, mostly gas giants orbiting very close to their stars, known as Hot Jupiters, suggest that planets were rare in systems with low metallicity, which invited suspicion of a time limit on the appearance of the first objects landmass. Later, in 2012, the Kepler telescope's observations allowed experts to find out that this relationship is much more restrictive in systems with Hot Jupiters, and that terrestrial planets could form in stars of much lower metallicity, to some extent. It is now thought that the first Earth-mass objects should appear sometime between 7 and 12 billion years. Given the greater stability of the orange dwarfs (K-type) compared to the Sun (G-type) and longer life expectancy, it is possible that superhabitable exoplanets belonging to K-type stars, orbiting within its habitable zone, could provide a longer, steadier, and better environment for life than Earth.

Profile summary

A size comparison and artist's impression of Kepler-442b (1.34 R_⊕) to the Earth (right).

Despite the scarcity of information available, the hypotheses presented above on superhabitable planets can be summarized as a preliminary profile, even if there is no scientific consensus.

• Mass: approximately 2M_⊕.
• Radius: to maintain a similar density to Earth, its radius should be close to 1.2 or 1.3R_⊕.
• Oceans: percentage of surface area covered by oceans should be Earth-like but more distributed, without large continuous land masses. The oceans should be shallow; the light then will penetrate easier through the water and will reach the fauna and flora, stimulating an abundance of life down in the ocean.
• Distance: shorter distance from the center of the habitable zone of the system than Earth.
• Temperature: average surface temperature of about 25 °C (77 °F).
• Star and age: belonging to an intermediate K-type star with an older age than the Sun (4.5 billion years) but younger than 7 billion years.
• Atmosphere: somewhat denser than Earth's and with a higher concentration of oxygen. That will make life larger and more abundant.

There is no confirmed exoplanet that meets all these requirements. After updating the database of exoplanets on 23 July 2015, the one that comes closest is Kepler-442b, belonging to an orange dwarf star, with a radius of 1.34R_⊕ and a mass of 2.36M_⊕, but with an estimated surface temperature of 4 °C (39 °F).

Appearance

The appearance of a superhabitable planet should be, in general, very similar to Earth. The main differences, in compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere may prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet. A superhabitable world would also have a higher concentration of clouds, and abundant rainfall.

The vegetation of such a planet would be very different due to the increased air density, precipitation, temperature, and stellar flux compared to Earth. As the peak wavelength of light differs for K-type stars compared to the Sun, plants may be a different colour than the green vegetation present on Earth. Plant life would also cover more of the surface of the planet, which would be visible from space.

In general, the climate of a superhabitable planet would be warm, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences in contrast to Earth, which has inhospitable areas such as glaciers, deserts and some tropical regions. If the atmosphere contains enough oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases, but they would need to develop adaptations to the increased gravity, such as an increase in muscle and bone density.

Abundance

Set and subsets of terrestrial worlds.

Heller and Armstrong speculate that the number of superhabitable planets around Kepler 442-like stars can far exceed that of Earth analogs: less massive stars in the main sequence are more abundant than the larger and brighter stars, so there are more orange (K) dwarfs than solar analogues. It is estimated that about 9% of stars in the Milky Way are K-type stars.

Another point favoring the predominance of superhabitable planets in regard to Earth analogs is that, unlike the latter, most of the requirements of a superhabitable world can occur spontaneously and jointly simply by having a higher mass. A planetary body close to 2 or 3M_⊕ should have longer-lasting plate tectonics and also will have a larger surface area in comparison to Earth. Similarly, it is likely that its oceans are shallower by the effect of gravity on the planet's crust, its gravitational field more intense and, a denser atmosphere.

By contrast, Earth-mass planets may have a wider range of conditions. For example, some may sustain active tectonics for a shorter time period and will therefore end up with lower air density than Earth, increasing the probability of developing global ice coverage, or even a permanent Snowball Earth scenario. Another negative effect of lower atmospheric density can be manifested in the form of thermal oscillations, which can lead to high variability in the global climate and increase the chance for catastrophic events. In addition, by having a weaker magnetosphere, such planets may lose their atmospheric hydrogen by hydrodynamic escape easier and become a desert planet. Any of these examples could prevent the emergence of life on a planet's surface. In any case, the multitude of scenarios that can turn an Earth-mass planet located in the habitable zone of a solar analogue into an inhospitable place are less likely on a planet that meets the basic features of a superhabitable world, so that the latter should be more common.

In September 2020, astronomers identified 24 superhabitable planet contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.

Superhabitable planets discovered so far

Researchers have identified 24 planets that are "superhabitable", i.e. that offer conditions more suitable for life than Earth does.

• Kepler-452b
• Kepler-442b
• Kepler-22b

See also

• Earth analog
• Earth Similarity Index
• List of potentially habitable exoplanets

Stellar population

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Stellar_population 

 

Artist's conception of the spiral structure of the Milky Way showing Baade's general population categories. The blue regions in the spiral arms comprise the younger Population I stars, while the yellow stars in the central bulge are the older Population II stars. In reality, many Population I stars are also found mixed in with the older Population II stars.

During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.

In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926:

[...] The two types of stellar populations had been recognized among the stars of our own galaxy by Oort as early as 1926.

Baade noticed that bluer stars were strongly associated with the spiral arms and yellow stars dominated near the central galactic bulge and within globular star clusters. Two main divisions were defined as

• Population I and
• Population II,

with another newer division called

• Population III added in 1978;

they are often simply abbreviated as Pop. I, Pop. II, and Pop. III.

Between the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important, and were possibly related to star formation, observed kinematics, stellar age, and even galaxy evolution in both spiral or elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or metallicity.

By definition, each population group shows the trend where decreasing metal content indicates increasing age of stars. Hence, the first stars in the universe (very low metal content) were deemed Population III, old stars (low metallicity) as Population II, and recent stars (high metallicity) as Population I. The Sun is considered Population I, a recent star with a relatively high 1.4 percent metallicity. Note that astrophysics nomenclature considers any element heavier than helium to be a "metal", including chemical non-metals such as oxygen.

Stellar development

Observation of stellar spectra has revealed that stars older than the Sun have fewer heavy elements compared to the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar nucleosynthesis.

Formation of the first stars

Under current cosmological models, all matter created in the Big Bang was mostly hydrogen (75%) and helium (25%), with only a very tiny fraction consisting of other light elements such as lithium and beryllium. When the universe had cooled sufficiently, the first stars were born as Population III stars without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their nucleosynthetic processes created the first 26 elements (up to iron in the periodic table).

Many theoretical stellar models show that most high-mass Population III stars rapidly exhausted their fuel and likely exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass Population III stars should be observable. However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe. None have been discovered, however, scientists have found evidence of an extremely small ultra metal-poor star, slightly smaller than the Sun, found in a binary system of the spiral arms in the Milky Way. The discovery opens up the possibility of observing even older stars.

Stars too massive to produce pair-instability supernovae would have likely collapsed into black holes through a process known as photodisintegration. Here some matter may have escaped during this process in the form of relativistic jets, and this could have distributed the first metals into the universe.

Formation of the observable stars

The oldest observed stars, known as Population II, have very low metallicities; as subsequent generations of stars were born they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via planetary nebulae and supernovae, enriching further the nebulae out of which the newer stars formed. These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

Chemical classification by Baade

Population I stars

Population I star Rigel with reflection nebula IC 2118

Population I, or metal-rich, stars are young stars with the highest metallicity out of all three populations, and are more commonly found in the spiral arms of the Milky Way galaxy. The Earth's Sun is an example of a metal-rich star and is considered as an intermediate Population I star, while the solar-like Mu Arae is much richer in metals.

Population I stars usually have regular elliptical orbits of the galactic centre, with a low relative velocity. It was earlier hypothesized that the high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, because planets, particularly terrestrial planets, are thought to be formed by the accretion of metals. However, observations of the Kepler Space Telescope data have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity—a finding that has implications for theories of gas giant formation. Between the intermediate Population I and the Population II stars comes the intermediary disc population.

Population II stars

Schematic profile of the Milky Way. Population II stars appear in the galactic bulge and within the globular clusters

Population II, or metal-poor, stars are those with relatively little of the elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate Population II stars are common in the bulge near the centre of the Milky Way, whereas Population II stars found in the galactic halo are older and thus more metal-dGlobular clusters also contain high numbers of Population II stars.

A characteristic of Population II stars is that despite their lower overall metallicity, they often have a higher ratio of "alpha elements" (elements produced by the alpha process, namely O, Ne, etc.) relative to Fe as compared to Population I stars; current theory suggests this is the result of Type II supernovas being more important contributors to the interstellar medium at the time of their formation, whereas Type Ia supernova metal-enrichment came at a later stage in the universe's development.

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten ultra metal poor (UMP) stars (such as Sneden's Star, Cayrel's Star, BD +17° 3248) and three of the oldest stars known to date: HE0107-5240, HE1327-2326 and HE 1523-0901. Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data. However, in February 2014 the discovery of an even lower metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant) and HD 140283 (a subgiant).

Population III stars

Possible glow of Population III stars imaged by NASA's Spitzer Space Telescope

Population III stars are a hypothetical population of extremely massive, luminous and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby Population III supernovae. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.

The existence of Population III stars is inferred from physical cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe. Their existence may account for the fact that heavy elements – which could not have been created in the Big Bang – are observed in quasar emission spectra. They are also thought to be components of faint blue galaxies. These stars likely triggered the universe's period of reionization, a major phase transition of gases leading to the lack of opacity observed today. Observations of the galaxy UDFy-38135539 suggest it may have played a role in this reionization process. The European Southern Observatory discovered a bright pocket of early population stars in the very bright galaxy Cosmos Redshift 7 from the reionization period around 800 million years after the Big Bang, at z = 6.60. The rest of the galaxy has some later redder Population II stars. Some theories hold that there were two generations of Population III stars.

Artist's impression of the first stars, 400 million years after the Big Bang

Current theory is divided on whether the first stars were very massive or not. One possibility is that these stars were much larger than current stars: several hundred solar masses, and possibly up to 1,000 solar masses. Such stars would be very short lived and last only 2-5 million years. Such large stars may have been possible due to the lack of heavy elements and a much warmer interstellar medium from the Big Bang. Conversely, theories proposed in 2009 and 2011 suggest the first star groups might have consisted of a massive star surrounded by several smaller stars. The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses (M_☉) or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy.

Analysis of data of extremely low-metallicity Population II stars such as HE0107-5240, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses. On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae, which are typically associated with very massive stars, were responsible for their metallic composition. This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Population III stars. Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae) have been proposed as dark matter candidates, but searches for these types of MACHOs through gravitational microlensing have produced negative results.

Detection of Population III stars is a goal of NASA's James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Population III stars.