The idea of the existence of planets around other stars of the solar type has excited the minds of the humankind for at least four centuries. The discovery of the first extrasolar planet (or exoplanet) in 1995 created a new area of research and raised many issues within the astronomical research community (Kitchin, 2012). These issues are matters of an independent scientific value, as well as a part of other scientific problems. One of these problems is the possibility of extraterrestrial life in the universe. Currently, six extrasolar planets, namely Gliese 667 Cc, Kepler-22b, Kepler-69c, Kepler-62f, Kepler-186f, and Kepler-452b are regarded as possible extrasolar planets for human’s life.
About twenty-three years have passed since the discovery of the first extrasolar planet orbiting the star. During this period, the number of known exoplanets has been continuously growing and has already exceeded 2,000. About 500 planetary systems have two or more planets. The history of these achievements started in 1992 when astronomers Aleksander Wolszczan and Dale Frail found the first two exoplanets – Draugr and Poltergeist – orbiting the pulsar called Lich (Schilling, 2011). Then, in 1995, astronomers Michel Mayorand and Didier Queloz found a wiggling of the star Helvetius using the ultra-precise spectrometer (Kitchin, 2012). The planet that caused those wiggling is located in immediate proximity to the star and thereby it was detected. Subsequently, several hundreds of exoplanets have been discovered by using this method, namely measuring the radial velocity of stars and finding their Doppler periodical shift. Today, there are several methods for detection of exoplanets.
Doppler spectroscopy or the radial-velocity method implies the spectrometric measurement of the radial velocity of the star. It can be used to detect planets with a mass of at least few Earth masses that are located closely to the star and giant planets with periods of about ten years. As noted above, a planet revolving around a star wiggles it, making it possible to observe the Doppler shift of the star’s spectrum. This method allows determining the amplitude of the oscillation of a radial velocity, the orbital period, the eccentricity, and the lower limit of the mass of the exoplanet (Townsend, 2009). As of February of 2016, 651 planets were discovered via this method (Exoplanet.eu, 2016a).
The next is the transit method. It is associated with the passage of a planet on the background of a star. At this point, the luminosity of the star decreases. The method allows determining the size of the planet and density planet if combined with the Doppler method (Townsend, 2009). It gives information on the availability and composition of the atmosphere. It should be noted that this method helps to detect only those planets the orbit of which lies in the same plane as the observation point. As of February of 2016, 1,296 planets were discovered via this method (Exoplanet.eu, 2016b).
In addition, one can use the gravitational microlensing method if there is a star between the observed object (stars, galaxies) and an observer on the Earth. A star acts as a lens, focusing light on the observed object by its gravitational field (Townsend, 2009). The method has very limited application. As of February of 2016, 44 planets were discovered via this method (Exoplanet.eu, 2016c).
A name of each discovered exoplanet consists of a name of the star a planet goes around and a lower-case letter of the alphabet starting with letter ‘b’ (e.g., 51 Pegasi b.) The next planet is assigned letter ‘c’, then ‘d’, and so on through the alphabet. The letter ‘a’ is not used in the title of exoplanets because such a name would imply the very star a planet goes around. Moreover, one should pay attention to the fact that planets are named in the order of their discovery, but not the order of their distance from the star. It means that planet ‘c’ may be closer to the star than planet ‘b’ just because it was discovered later (International Astronomical Union, n.d.). In December of 2015, some extrasolar planets got new names such as above-mentioned Draugr and Poltergeist and many others (International Astronomical Union, 2015).
It is estimated that at least 22% of Sun-like stars have, at least, one potentially habitable planet (Templeton, 2013). Their share grows along with the data acquisition and improvement of observation techniques. Initially, the majority of discovered exoplanets were giant planets because it was more difficult to detect planets of other types. However, as of February of 2014 astronomers discovered many planets with masses of Neptune and even less (NASA, 2014).
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There is a correlation between the amount of giant planets and the content of heavy elements in the stars. Systems with giant planets are found in solar-type stars predominantly (classes K5-F5), while their share in red dwarf stars is much smaller. Planets formed around the stars that are as metal-rich as the Sun have small cores, whereas planets formed around the stars that contain two to three times more metals have much larger cores (Astrobio, 2006).
Some exoplanets move in orbits with high eccentricity and include several layers of matter such as layers of crust, mantle, and core material. Under such conditions, tidal forces can release heat energy, which can help to create and maintain favorable living conditions on the planets, and their orbits can evolve on near-circular ones over time. They are close to the Earth’s mass, as well as being composed mostly of silicate rock and not shrouded in a strong envelope of hydrogen and helium. In other words, they have a potential to develop and sustain life. Possible extrasolar planets for extraterrestrial life are Gliese 667 Cc, Kepler-22b, Kepler-69c, Kepler-62f, Kepler-186f, and Kepler-452b.
Gliese 667 Cc is the second exoplanet of the Gliese 667 C star in a triple Gliese 667 system. The planet orbits the red dwarf Gliese 667 C at a distance of 0.12 AU (Anglada-Escude et al., 2013). Its orbital period is about 28 terrestrial days (Anglada-Escude et al., 2013). Given the fact that the effective Earth’s radius in this system is only 0.114 AU, the temperature regime of the planet may be very close to the temperature regime of the Earth (Anglada-Escude et al., 2013). Simulation of the planetary habitability shows that the average surface temperature of the atmosphere should be about 300 K (80 °F) if the planet has an atmosphere similar to Earth’s, the greenhouse effect due to the presence of 1% of carbon dioxide and albedo of 0.3 (Anglada-Escude et al., 2013). According to calculations, the effective temperature will be 246 K (-17 °F), while the Earth’s effective temperature is 249 K (-11.5 °F) (Anglada-Escude et al., 2013). The planet receives about 90% of the energy that the Earth receives from the Sun (Anglada-Escude et al., 2013). If the inclination of its orbit is not too small and, accordingly, the mass is not too large, the greenhouse effect created by a dense atmosphere possibly creates quite comfortable conditions for the existence of primitive life forms on the planet surface.
Kepler-22b is an extrasolar planet in the Cygnus constellation. The radius of the planet is about 2.4 times greater than the radius of the Earth (Borucki et al., 2012).
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It is located at a distance of about 620 light years from the Earth in orbit around the star Kepler-22 of G5 spectral type (Borucki et al., 2012). Distance from Kepler-22b to Kepler-22 is about 15% less than distance from the Earth to the Sun (Borucki et al., 2012). In this case, the light flux from the parent star is about 25% less than that of the Sun (Borucki et al., 2012). The combination of a smaller distance from the star and less light flux implies a moderate temperature on the surface of the planet. Scientists estimate that the equilibrium surface temperature would be about -12 °F in the absence of an atmosphere (Borucki et al., 2012). If there is the greenhouse effect caused by the presence of an atmosphere similar to the Earth, it corresponds to an average surface temperature of about 72 °F. Kepler-22b is similar to Neptune, which consists mainly of an extended atmosphere, the ocean, and a small solid core. However, the life inside the ocean of Kepler-22b has every chance to exist.
Kepler-62f is an extrasolar planet in the planetary system of star Kepler-62. Kepler-62 is a dwarf star in two-thirds of the size of and one-fifth of the brightness of the Sun. It is two billion years older than the Sun. According to the currently available information, it is surrounded by five planets, two of which are located at a distance that allows the presence of water on them in liquid form (Borucki et al., 2013). The other three are so much closer to the star that their orbital period is no more than 20 days (Borucki et al., 2013). This fact makes them unsuitable for life.
The planets in the habitable zone are Kepler-62e and Kepler-62f. The latter has the greatest chances to be habitable because it is just 40% larger than the Earth and its orbital period is 267 terrestrial days (Borucki et al., 2013). The small size makes it the most exact copy of the Earth today. Moreover, the planet has a good chance of being rocky like the Earth. Since it is the farthest known planet in the Kepler-62 system, it may require quite a bit of cloud to insulate the planet and keep water above freezing. If that were the case, water would likely be there in abundance. Thus, Kepler-62f presumably has all the necessary conditions for the presence of water in liquid form, which, in turn, is the most favorable condition for the emergence of life forms.
The planetary system Kepler-69 is a small system, which includes only two planets, namely Kepler-69b and Kepler-69c (Tate, 2013). At the first glance, this system is a more suitable candidate for the existence of life than the Kepler-62 system. This is because Kepler-69 is a star of the G-type, i.e. the same class the Sun belongs to.
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The Kepler-69 star is 10% smaller than the Sun and has approximately 80% of the brightness of the Sun (Tate, 2013). The Kepler-69b planet is closer to the star. However, it is twice larger than the Earth and too close to the star and, therefore, it is very hot (Tate, 2013). The second planet called Kepler-69c is 70% larger than the Earth. It orbits the star in 242 terrestrial days. Kepler-69c is located in a habitable zone (Tate, 2013). The star from its surface must look like the Sun. However, the planet is too close to the star to have any forms of life though it may actually lie in the innermost region of the habitable zone (Kane, Barclay, & Gelino, 2013).
Kepler-186f is an extrasolar planet in a planetary system of the red dwarf Kepler-186 in the Cygnus constellation (Quintana et al., 2014). It is the first planet in the habitable zone with the radius similar to the Earth’s. A full orbital rotation takes about 130 terrestrial days around Kepler-186, which has only 4% the luminosity of the Sun (Quintana et. al., 2014). The semi-major axis of the orbit of the planet is 0.393 AU, which is close to Mercury (Quintana et al., 2014). However, the habitable zone of this planetary system is located at a very close distance. The illumination that Kepler-186f receives is enough to be certainly within the habitable zone though it is closer to the outer edge, being similar to the position of Mars in the solar system. Mass, density, and composition of the planet are unknown. The mass can vary from 0.32 of the weight of the Earth if the planet is fully composed of water and ice to 3.77 if it is composed of iron (Quintana et al., 2014). If the mass of Kepler-186f is greater than the Earth’s one, it probably has a dense atmosphere. This fact allows the planet to absorb energy from its star more efficiently, avoiding freezing. Finally, such stars as Kepler-186 are the most long-lived stars in the universe. If Kepler-186f were habitable, life would appear there billions of years ago.
Kepler-452b is an exoplanet orbiting the yellow dwarf star Kepler-452 in the constellation Cygnus (Jenkins et al., 2015). This is the first near-Earth-sized planet discovered in the habitable zone of a sun-like star of the G2 spectral type.
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The orbiting period of the planet is about 385 terrestrial days (Jenkins et al., 2015). Kepler-452b is 63% larger and 1.5 billion years older than the Earth, but orbits in the habitable zone of the star (Jenkins et al., 2015). The mass of Kepler-452b is unknown, but the simulation results have estimated that it is five times larger than the Earth’s mass. If it has such a mass, the planet’s gravity is approximately 1.88 times greater than the Earth’s. Its size is too large to be almost entirely composed of heavy elements like the Earth, but it can be a solid planet with a probability from 49% to 62% (Jenkins et al., 2015). In this case, it is highly unlikely that its metallic core has a large mass.
The assumption that the new planet can support life is based on the nature of the star Kepler-452b goes around. The star is almost identical in its characteristics to the Sun, while the planet’s orbital period is just 5% greater than the Earth’s (Jenkins et al., 2015). If its surface is rocky, the planet must have an active volcanic activity and the gravity force on it should be approximately two times greater than on the Earth. Since the planet is 1.5 billion years older than the Earth, a possible extraterrestrial civilization had more time to develop than the humankind. However, the existence of intelligent life or even the simplest form of life on Kepler-452b is still a hypothesis. Kepler-452b is regarded as the most likely candidate for being a habitable planet in comparison with other five above-mentioned planets.
Primarily, the energy source is absolutely necessary for the existence of living organisms. Besides, a planet must have a number of other conditions, namely geophysical, geochemical, and astrophysical ones. The NASA Astrobiology Program defines criteria of planetary habitability as follows: large bodies of water and conditions that contribute to the synthesis of complex organic substances as well as the presence of a source of energy to maintain metabolism. Moreover, the planetary habitability depends on the properties of a star a planet goes around.
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A star must have a stable luminosity for a long time for the emergence and evolution of life with no strong variability and must contain many heavy elements, which enable the formation of Earth-like planets. Rocky planets and satellites are of the greatest interest because they can have carbon-based life forms. However, the existence of life with an entirely different biochemistry, which is possible on other celestial bodies, is also possible.
A key factor for the emergence of life on a planet is liquid water. Liquid water is a universal solvent unlike most other fluids. It is the ideal environment for the emergence of increasingly complex molecules. Carbon is the next key factor since it is tetravalent and can bind to four other atoms, creating complex molecules. These general conditions and many others are necessary for the development of life on a planet. It is clear that not every planet could observe such strict conditions. To date, possible extrasolar planets for human’s life are Gliese 667 Cc, Kepler-22b, Kepler-69c, Kepler-62f, Kepler-186f, and Kepler-452b. The latter, namely Kepler-452b, is the most Earth-like planet from these six.
Some of the last trends of observing planets for extraterrestrial life include the orbital insertion of the Gaia space observatory, as well as the future launch of the TESS space telescope, the EChO space telescope, and the ATLAST space telescope. Each of them should discover hundreds of thousands of new extrasolar planets in the nearest future. In addition to the space missions, it is planned to develop ground-based instruments of observation. For example, the European Extremely Large Telescope, which is currently under construction, will have equipment capable of exploring the atmosphere of exoplanets.
Despite all efforts made for finding any life forms in the universe, the humanity has no direct evidence that life exists on other planets and their satellites, as well as in the interstellar space. Nevertheless, there are compelling and very convincing reasons to believe that the humankind will find extraterrestrial life forms. For example, life on the Earth most likely originated from chemical reactions that eventually formed the cell membranes. However, these primary chemical reactions could have started in the atmosphere and the ocean within complex organic compounds. Some satellites in the Solar system contain these compounds. Thus, if these components are common throughout the universe, it is quite possible that life emerged not only on the Earth, but also in other places.