soaked earth

In January 2020, NASA reported that the TESS spacecraft had discovered its first potentially habitable Earth-sized exoplanet orbiting a star about 100 light-years away.

The planet is part TOI 700 system (TOI stands for TESS Objects of Interest) is a small, relatively cold star, i.e., a dwarf of spectral class M, in the constellation Goldfish, having only about 40% of the mass and size of our Sun and half the temperature of its surface.

Object named TOI 700 d and is one of three planets revolving around its center, the farthest from it, passing a path around a star every 37 days. It is located at such a distance from TOI 700 as to theoretically be able to keep liquid water afloat, located in the habitable zone. It receives about 86% of the energy that our Sun gives to the Earth.

However, environmental simulations created by the researchers using data from the Transiting Exoplanet Survey Satellite (TESS) showed that TOI 700 d could behave very differently from Earth. Because it rotates in sync with its star (meaning one side of the planet is always in daylight and the other in darkness), the way clouds form and the wind blows can be a little exotic for us.

1. Comparison of the Earth and TOI 700 d, with visualization of the Earth's system of continents on an exoplanet

Astronomers confirmed their discovery with the help of NASA. Spitzer Space Telescopewhich has just completed its activity. Initially, Toi 700 was misclassified as being much hotter, leading astronomers to believe that all three planets were too close together and therefore too hot to support life.

Emily Gilbert, a member of the University of Chicago team, said during the presentation of the discovery. -

The researchers hope that in the future, tools such as James Webb Space Telescopethat NASA plans to place in space in 2021, they will be able to determine if the planets have an atmosphere and can study its composition.

The researchers used computer software to hypothetical climate modeling planet TOI 700 d. Since it is not yet known what gases may be in its atmosphere, various options and scenarios have been tested, including options that assume the modern Earth's atmosphere (77% nitrogen, 21% oxygen, methane and carbon dioxide), the likely composition Earth's atmosphere 2,7 billion years ago (mostly methane and carbon dioxide) and even the Martian atmosphere (lots of carbon dioxide), which probably existed there 3,5 billion years ago.

From these models, it was found that if TOI 700 d's atmosphere contains a combination of methane, carbon dioxide, or water vapor, the planet could be habitable. Now the team has to confirm these hypotheses using the aforementioned Webb telescope.

At the same time, climate simulations conducted by NASA show that both the Earth's atmosphere and gas pressure are not enough to hold liquid water on its surface. If we put the same amount of greenhouse gases on TOI 700 d as on Earth, the surface temperature would still be below zero.

Simulations by all participating teams show that the climate of planets around small and dark stars such as TOI 700, however, is very different from what we experience on our Earth.

Interesting news

Most of what we know about exoplanets, or planets orbiting the solar system, comes from space. It scanned the skies from 2009 to 2018 and found over 2600 planets outside of our solar system.

NASA then passed the baton of discovery to the TESS(2) probe, launched into space in April 2018 in its first year of operation, as well as nine hundred unconfirmed objects of this type. In search of planets unknown to astronomers, the observatory will scour the entire sky, having seen enough of 200 XNUMX. the brightest stars.

2. Transit satellite for exoplanet exploration

TESS uses a series of wide angle camera systems. It is capable of studying the mass, size, density and orbit of a large group of minor planets. The satellite works according to the method remote search for brightness dips potentially pointing to planetary transits - the passage of objects in orbit in front of the faces of their parent stars.

The last few months have been a series of extremely interesting discoveries, partly thanks to the still relatively new space observatory, partly with the help of other instruments, including ground-based ones. In the weeks leading up to our meeting with the Earth's twin, word got out about the discovery of a planet orbiting two suns, just like Tatooine from Star Wars!

TOI planet 1338 b found XNUMX light years away, in the constellation of the Artist. Its size is between the sizes of Neptune and Saturn. The object experiences regular mutual eclipses of its stars. They revolve around each other on a fifteen-day cycle, one slightly larger than our Sun and the other much smaller.

In June 2019, information appeared that two terrestrial-type planets were discovered literally in our space backyard. This is reported in an article published in the journal Astronomy and Astrophysics. Both facilities are located in an ideal zone where water can form. They probably have a rocky surface and orbit the Sun, known as star of Tigarden (3), located just 12,5 light years from Earth.

- said the main author of the discovery, Matthias Zechmeister, Research Fellow, Institute of Astrophysics, University of Göttingen, Germany. -

3. Teegarden star system, visualization

In turn, the intriguing unknown worlds discovered by TESS last July revolve around UCAC stars4 191-004642, seventy-three light-years from Earth.

Planetary system with a host star, now labeled as TOI 270, contains at least three planets. One of them, TOI 270 p, slightly larger than the Earth, the other two are mini-Neptunes, belonging to a class of planets that do not exist in our solar system. The star is cold and not very bright, about 40% smaller and less massive than the Sun. Its surface temperature is about two-thirds warmer than that of our own stellar companion.

The solar system TOI 270 is located in the constellation of the Artist. The planets that make it up orbit so close to the star that their orbits can fit into Jupiter's companion satellite system (4).

4. Comparison of the TOI 270 system with the Jupiter system

Further exploration of this system may reveal additional planets. Those orbiting farther from the Sun than TOI 270 d could be cold enough to hold liquid water and eventually give rise to life.

TESS worth a closer look

Despite the relatively large number of discoveries of small exoplanets, most of their parent stars are between 600 and 3 meters away. light-years from Earth, too far and too dark for detailed observation.

Unlike Kepler, TESS's main focus is to find planets around the sun's nearest neighbors that are bright enough to be observed now and later with other instruments. From April 2018 to the present, TESS has already discovered over 1500 candidate planets. Most of them are more than twice the size of the Earth and take less than ten days to orbit. As a result, they receive much more heat than our planet, and they are too hot for liquid water to exist on their surface.

It is liquid water that is needed in order for the exoplanet to become habitable. It serves as a breeding ground for chemicals that can interact with each other.

Theoretically, it is believed that exotic life forms could exist in conditions of high pressure or very high temperatures - as is the case with extremophiles found near hydrothermal vents, or with microbes hidden almost a kilometer under the West Antarctic ice sheet.

However, the discovery of such organisms was made possible by the fact that people were able to directly study the extreme conditions in which they live. Unfortunately, they could not be detected in deep space, especially from a distance of many light years.

The search for life and even habitation outside our solar system is still entirely dependent on remote observation. Visible liquid water surfaces that create potentially favorable conditions for life can interact with the atmosphere above, creating remotely detectable biosignatures visible with ground-based telescopes. These can be gas compositions known from Earth (oxygen, ozone, methane, carbon dioxide and water vapor) or components of the ancient Earth’s atmosphere, for example, 2,7 billion years ago (mainly methane and carbon dioxide, but not oxygen). ).

In search of a place "just right" and the planet that lives there

Since the discovery of 51 Pegasi b in 1995, over XNUMX exoplanets have been identified. Today we know for sure that most of the stars in our galaxy and the universe are surrounded by planetary systems. But only a few dozen exoplanets found are potentially habitable worlds.

What makes an exoplanet habitable?

The main condition is the already mentioned liquid water on the surface. In order for this to be possible, we first of all need this solid surface, i.e. rocky groundbut also atmosphere, and dense enough to create pressure and influence the temperature of the water.

You also need right starwhich does not unleash too much radiation on the planet, which blows away the atmosphere and destroys living organisms. Every star, including our Sun, constantly emits huge doses of radiation, so it would undoubtedly be beneficial for the existence of life to protect itself from it. a magnetic fieldas produced by the Earth's liquid metal core.

However, since there may be other mechanisms to protect life from radiation, this is only a desirable element, not a necessary condition.

Traditionally, astronomers are interested in life zones (ecospheres) in star systems. These are regions around the stars where the prevailing temperature prevents water from constantly boiling or freezing. This area is often talked about. «Zlatovlaski Zone»because “just right for life”, which refers to the motifs of a popular children's fairy tale (5).

5. The zone of life around the star

And what do we know so far about exoplanets?

The discoveries made to date show that the diversity of planetary systems is very, very large. The only planets that we knew anything about about three decades ago were in the solar system, so we thought that small and solid objects revolve around stars, and only further from them there is space reserved for large gaseous planets.

It turned out, however, that there are no "laws" regarding the location of the planets at all. We encounter gas giants that almost rub against their stars (so-called hot Jupiters), as well as compact systems of relatively small planets such as TRAPPIST-1 (6). Sometimes planets move in very eccentric orbits around binary stars, and there are also "wandering" planets, most likely ejected from young systems, floating freely in the interstellar void.

6. Visualization of the planets of the TRAPPIST-1 system

Thus, instead of close similarities, we see great diversity. If this happens at the system level, then why should exoplanet conditions resemble everything we know from the immediate environment?

And, going even lower, why should the forms of hypothetical life be similar to those known to us?

Super category

Based on the data collected by Kepler, in 2015 a NASA scientist calculated that our galaxy itself has billion Earth-like planetsI. Many astrophysicists have emphasized that this was a conservative estimate. Indeed, further research has shown that the Milky Way could be home to 10 billion earth planets.

Scientists didn't want to rely solely on the planets found by Kepler. The transit method used in this telescope is better suited for detecting large planets (such as Jupiter) than Earth-sized planets. This means that Kepler's data is probably falsifying the number of planets like ours a bit.

The famous telescope observed tiny dips in the brightness of a star caused by a planet passing in front of it. Larger objects understandably block more light from their stars, making them easier to spot. Kepler's method was focused on small, not the brightest stars, the mass of which was about one third of the mass of our Sun.

The Kepler telescope, although not very good at finding minor planets, has found a fairly large number of so-called super-Earths. This is the name of exoplanets with a mass greater than the Earth, but much less than Uranus and Neptune, which are 14,5 and 17 times heavier than our planet, respectively.

Thus, the term "super-Earth" only refers to the mass of the planet, meaning it does not refer to surface conditions or habitability. There is also an alternative term "gas dwarfs". According to some, it may be more accurate for objects in the upper part of the mass scale, although another term is more commonly used - the already mentioned "mini-Neptune".

The first super-Earths were discovered Alexander Volshchan i Dalea Fraila around pulsar PSR B1257+12 in 1992. The two outer planets of the system are poltergeysyou fobetor - they have a mass of about four times the mass of the Earth, which is too small to be gas giants.

The first super-Earth around a main sequence star has been identified by a team led by Eugenio Rivery in 2005. It revolves around Gliese 876 and received the designation Gliese 876 d (Earlier, two Jupiter-sized gas giants were discovered in this system). Its estimated mass is 7,5 times the mass of the Earth, and the period of revolution around it is very short, about two days.

There are even hotter objects in the super-Earth class. For example, discovered in 2004 55 Kankri is, located forty light-years away, revolves around its star in the shortest cycle of any known exoplanet - only 17 hours and 40 minutes. In other words, a year at 55 Cancri e takes less than 18 hours. The exoplanet orbits about 26 times closer to its star than Mercury.

The proximity to the star means that the surface of 55 Cancri e is like the inside of a blast furnace with a temperature of at least 1760°C! New observations from the Spitzer Telescope show that 55 Cancri e has a mass 7,8 times greater and a radius slightly more than twice that of Earth. The Spitzer results suggest that about one-fifth of the planet's mass should be made up of elements and light compounds, including water. At this temperature, this means that these substances would be in a "supercritical" state between liquid and gas and could leave the surface of the planet.

But super-Earths aren't always so wild. Last July, an international team of astronomers using TESS discovered a new exoplanet of its kind in the constellation Hydra, about thirty-one light-years from Earth. Item marked as GJ 357 d (7) twice the diameter and six times the mass of the Earth. It is located on the outer edge of the star's residential area. Scientists believe that there may be water on the surface of this super-Earth.

she said Diana Kosakovskand Research Fellow at the Max Planck Institute for Astronomy in Heidelberg, Germany.

7. Planet GJ 357 d - visualization

A system in orbit around a dwarf star, about one-third the size and mass of our own Sun and 40% colder, is being supplemented by terrestrial planets. GJ 357 b and another super earth GJ 357 s. The study of the system was published on July 31, 2019 in the journal Astronomy and Astrophysics.

Last September, researchers reported that a newly discovered super-Earth, 111 light-years away, is "the best habitat candidate known so far." Discovered in 2015 by the Kepler telescope. K2-18b (8) very different from our home planet. It has more than eight times its mass, meaning it is either an ice giant like Neptune or a rocky world with a dense, hydrogen-rich atmosphere.

The orbit of K2-18b is seven times closer to its star than the Earth's distance from the Sun. However, since the object is orbiting a dark red M dwarf, this orbit is in a zone potentially favorable for life. Preliminary models predict that temperatures on K2-18b range from -73 to 46°C, and if the object has about the same reflectivity as the Earth, its average temperature should be similar to ours.

– said an astronomer from University College London during a press conference, Angelos Ciaras.

It's hard to be like the earth

An Earth analog (also called an Earth twin or Earth-like planet) is a planet or moon with environmental conditions similar to those found on Earth.

The thousands of exoplanetary star systems discovered so far are different from our solar system, confirming the so-called rare earth hypothesisI. However, philosophers point out that the universe is so huge that somewhere there must be a planet almost identical to ours. It is possible that in the distant future it will be possible to use the technology to artificially obtain analogues of the Earth by the so-called. . Fashionable now multitheory theory they also suggest that an earthly counterpart could exist in another universe, or even be a different version of Earth itself in a parallel universe.

In November 2013, astronomers reported that, based on data from the Kepler telescope and other missions, there could be up to 40 billion Earth-sized planets in the habitable zone of sun-like stars and red dwarfs in the Milky Way galaxy.

The statistical distribution showed that the closest of them can be removed from us no more than twelve light years. In the same year, several candidates discovered by Kepler with diameters less than 1,5 times the radius of the Earth were confirmed to be orbiting stars in the habitable zone. However, it wasn’t until 2015 that the first close-to-Earth candidate was announced – egzoplanetę Kepler-452b.

The probability of finding an Earth analog depends mainly on the attributes you want to be like. Standard but not absolute conditions: planet size, surface gravity, parent star size and type (i.e. solar analog), orbital distance and stability, axial tilt and rotation, similar geography, presence of oceans, atmosphere and climate, strong magnetosphere . .

If complex life existed there, forests could cover most of the planet's surface. If intelligent life existed, some areas could be urbanized. However, the search for exact analogies with the Earth can be misleading due to very specific circumstances on and around the Earth, for example, the existence of the Moon affects many phenomena on our planet.

The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo recently compiled a list of candidates for Earth analogues (9). Most often, this type of classification starts with size and mass, but this is an illusory criterion, given, for example, Venus, which is close to us, which is almost the same size as the Earth, and what conditions prevail on it. , it is known.

9. Promising exoplanets - potential analogues of the Earth, according to the Planetary Habitability Laboratory

Another frequently cited criterion is that the Earth analogue must have similar surface geology. The closest known examples are Mars and Titan, and while there are similarities in terms of topography and composition of the surface layers, there are also significant differences, such as temperature.

Indeed, many surface materials and landforms arise only as a result of interaction with water (for example, clay and sedimentary rocks) or as a by-product of life (for example, limestone or coal), interaction with the atmosphere, volcanic activity, or human intervention.

Thus, a true analogue of the Earth must be created through similar processes, having an atmosphere, volcanoes interacting with the surface, liquid water, and some form of life.

In the case of the atmosphere, the greenhouse effect is also assumed. Finally, the surface temperature is used. It is influenced by climate, which is in turn influenced by the planet's orbit and rotation, each of which introduces new variables.

Another criterion for an ideal analogue of the life-giving earth is that it must orbit around the solar analog. However, this element cannot be fully justified, since a favorable environment is capable of providing the local appearance of many different types of stars.

For example, in the Milky Way, most stars are smaller and darker than the Sun. One of them was mentioned earlier TRAPPIST-1, is located at a distance of 10 light years in the constellation of Aquarius and is about 2 times smaller and is 1. times less bright than our Sun, but there are at least six terrestrial planets in its habitable zone. These conditions may seem unfavorable for life as we know it, but TRAPPIST-XNUMX likely has a longer life ahead of us than our star, so life still has plenty of time to develop there.

Water covers 70% of the Earth's surface and is considered one of the iron conditions for the existence of life forms known to us. Most likely, the water world is a planet Kepler-22b, located in the habitable zone of a sun-like star but much larger than Earth, its actual chemical composition remains unknown.

Conducted in 2008 by an astronomer Michaela Meyerand from the University of Arizona, studies of cosmic dust in the vicinity of newly formed stars like the Sun show that 20 to 60% of the Sun's analogs we have evidence of the formation of rocky planets in processes similar to those that led to the formation of the Earth.

In 2009 was Alan Boss from the Carnegie Institute of Science suggested that only in our galaxy the Milky Way can exist 100 billion earth-like planetsh.

In 2011, NASA's Jet Propulsion Laboratory (JPL), also based on observations from the Kepler mission, concluded that approximately 1,4 to 2,7% of all sun-like stars should orbit Earth-sized planets in habitable zones. This means that there could be 2 billion galaxies in the Milky Way galaxy alone, and assuming this estimate is true for all galaxies, there could even be 50 billion galaxies in the observable universe. 100 quintillion.

In 2013, the Harvard-Smithsonian Center for Astrophysics, using a statistical analysis of additional Kepler data, suggested that there is at least 17 billion planets the size of the Earth - without taking into account their location in residential areas. A 2019 study found that Earth-sized planets could orbit one of six sun-like stars.

Pattern on likeness

The Earth Similarity Index (ESI) is a suggested measure of the similarity of a planetary object or natural satellite to the Earth. It was designed on a scale from zero to one, with the Earth assigned a value of one. The parameter is intended to facilitate the comparison of planets in large databases.

ESI, proposed in 2011 in the journal Astrobiology, combines information about a planet's radius, density, velocity, and surface temperature.

Website maintained by one of the authors of the 2011 article, Abla Mendes from the University of Puerto Rico, gives his index calculations for various exoplanetary systems. ESI Mendesa is calculated using the formula shown in illustration 10where x_i their_i0 are the properties of the extraterrestrial body in relation to the Earth, v_i the weighted exponent of each property and the total number of properties. It was built on the basis Bray-Curtis similarity index.

The weight assigned to each property, w_i, is any option that can be selected to highlight certain features over others, or to achieve desired index or ranking thresholds. The website also categorizes what it describes as the possibility of living on exoplanets and exo-moons according to three criteria: location, ESI, and suggestion of the possibility of keeping organisms in the food chain.

As a result, it was shown, for example, that the second largest ESI in the solar system belongs to Mars and is 0,70. Some of the exoplanets listed in this article exceed this figure, and some recently discovered Tigarden b it has the highest ESI of any confirmed exoplanet, at 0,95.

When we talk about Earth-like and habitable exoplanets, we must not forget the possibility of habitable exoplanets or satellite exoplanets.

The existence of any natural extrasolar satellites has yet to be confirmed, but in October 2018 Prof. David Kipping announced the discovery of a potential exomoon orbiting the object Kepler-1625b.

Large planets in the solar system, such as Jupiter and Saturn, have large moons that are viable in some respects. Consequently, some scientists have suggested that large extrasolar planets (and binary planets) may have similarly large potentially habitable satellites. A moon of sufficient mass is capable of supporting a Titan-like atmosphere as well as liquid water on the surface.

Of particular interest in this regard are massive extrasolar planets known to be in the habitable zone (such as Gliese 876 b, 55 Cancer f, Upsilon Andromedae d, 47 Ursa Major b, HD 28185 b, and HD 37124 c) because they potentially have natural satellites with liquid water on the surface.

Life around a red or white star?

Armed with nearly two decades of discoveries in the world of exoplanets, astronomers have already begun to form a picture of what a habitable planet might look like, although most have focused on what we already know: an Earth-like planet orbiting a yellow dwarf like ours. The Sun, classified as a G-type main-sequence star. What about smaller red M stars, of which there are many more in our galaxy?

What would our home be like if it were orbiting a red dwarf? The answer is a bit Earth-like, and largely not Earth-like.

From the surface of such an imaginary planet, we would first of all see a very large sun. It would seem that one and a half to three times more than what we have before our eyes, given the proximity of the orbit. As the name suggests, the sun will glow red due to its cooler temperature.

Red dwarfs are twice as warm as our Sun. At first, such a planet may seem a little alien to Earth, but not shocking. The real differences only become apparent when we realize that most of these objects rotate in sync with the star, so one side always faces its star, like our Moon does to Earth.

This means that the other side remains really dark, as it does not have access to a light source - unlike the Moon, which is slightly illuminated by the Sun from the other side. In fact, the general assumption is that the part of the planet that remained in eternal daylight would burn out, and that which plunged into eternal night would freeze. However... it shouldn't be like that.

For years, astronomers ruled out the red dwarf region as an Earth hunting ground, believing that dividing the planet into two completely different parts would not make either of them uninhabitable. However, some note that atmospheric worlds will have a specific circulation that will cause thick clouds to accumulate on the sunny side to prevent intense radiation from burning the surface. Circulating currents would also distribute heat throughout the planet.

In addition, this thickening of the atmosphere could provide important daytime protection against other radiation hazards. Young red dwarfs are very active in the first few billion years of their activity, emitting flares and ultraviolet radiation.

Thick clouds are likely to protect potential life, although hypothetical organisms are more likely to hide deep in planetary waters. In fact, scientists today believe that radiation, for example, in the ultraviolet range, does not prevent the development of organisms. After all, early life on Earth, from which all organisms known to us, including homo sapiens, originated, developed under conditions of strong UV radiation.

This corresponds to the conditions accepted on the nearest Earth-like exoplanet known to us. Astronomers from Cornell University say that life on Earth has experienced stronger radiation than known from Proxima-b.

Proxima-b, located just 4,24 light-years from the solar system and the closest Earth-like rocky planet we know (although we know almost nothing about it), receives 250 times more X-rays than Earth. It can also experience lethal levels of ultraviolet radiation on its surface.

Proxima-b-like conditions are thought to exist for TRAPPIST-1, Ross-128b (nearly eleven light-years from Earth in the constellation Virgo) and LHS-1140 b (forty light-years from Earth in the constellation Cetus). systems.

Other assumptions concern emergence of potential organisms. Since a dark red dwarf would emit much less light, it is hypothesized that if the planet orbiting it contained organisms resembling our plants, they would have to absorb light over a much wider range of wavelengths for photosynthesis, which would mean that “exoplanets” could be almost black in our opinion (see also: ). However, it is worth realizing here that plants with a color other than green are also known on Earth, absorbing light slightly differently.

Recently, researchers have been interested in another category of objects - white dwarfs, similar in size to the Earth, which are not strictly stars, but create a relatively stable environment around them, radiating energy for billions of years, which makes them intriguing targets for exoplanetary research. .

Their small size and, as a result, the large transit signal of a possible exoplanet make it possible to observe potential rocky planetary atmospheres, if any, with new generation telescopes. Astronomers want to use all built and planned observatories, including the James Webb telescope, terrestrial Extremely large telescopeas well as future origin, HabEx i LUVUARif they arise.

There is one problem in this wonderfully expanding field of exoplanet research, research and exploration, insignificant at the moment, but one that may become pressing in time. Well, if, thanks to more and more advanced instruments, we finally manage to discover an exoplanet - the twin of the Earth that meets all the complex requirements, filled with water, air and temperature just right, and this planet will look “free”, then without technology that allows to fly there at some reasonable time, realizing it can be a torment.

But, fortunately, we do not have such a problem yet.