The term habitat means that almost all conditions for life are met, we just don't see it.
Habitability is determined by the following factors: the presence of water in liquid form, a sufficiently dense atmosphere, chemical diversity (simple and complex molecules based on H, C, N, O, S and P) and the presence of a star that brings the required amount of energy.
History of study: terrestrial planets
From the point of view of astrophysics, there were several incentives for the emergence of the concept of a habitable zone.
Consider our solar system and four terrestrial planets: Mercury, Venus, Earth and Mars.
Mercury has no atmosphere and is too close to the Sun to be of much interest to us. This is a planet with a sad fate, because even if it had an atmosphere, it would be carried away by the solar wind, that is, a stream of plasma continuously flowing from the corona of a star.
Consider the remaining terrestrial planets in the solar system - these are Venus, Earth and Mars. They arose almost in the same place and under the same conditions ~ 4,5 billion years ago.
And therefore, from the point of view of astrophysics, their evolution should be quite similar. Now, at the beginning of the space age, when we have advanced in the study of these planets with the help of spacecraft, the results obtained have shown extremely different conditions on these planets.
We now know that Venus has very high pressure and is very hot on the surface. 460 –480 ° C are the temperatures at which many substances even melt. And from the first panoramic images of the surface, we saw that it is completely inanimate and practically not adapted to life.
The entire surface is one continent.
Image: Terrestrial planets - Mercury, Venus, Earth, Mars.
On the other hand, Mars It's a cold world. Mars has lost its atmosphere.
This is again a desert surface, although there are mountains and volcanoes. The carbon dioxide atmosphere is very rarefied; if there was water there, it was all frozen.
Mars has a polar cap, and the latest results from a mission to Mars suggest that there is ice under the sand cover - regolith. And Earth. Very favorable temperature, the water does not freeze (at least not everywhere). And it was on Earth that life arose - both primitive and multicellular, intelligent life.
It would seem that we see a small part of the solar system in which three planets, called terrestrial planets, formed, but their evolution is completely different. And on these first ideas about the possible paths of evolution of the planets themselves, the idea of \u200b\u200bthe habitable zone arose.
Habitable zone boundaries
Astrophysicists observe and explore the world around us, the outer space surrounding us, that is, our solar system and planetary systems around other stars.
And in order to somehow systematize where to look, what objects to be interested in, you need to understand how to determine the habitable zone.
We always assumed that other stars must have planets, but the instrumental capabilities allowed us to discover the first exoplanets - planets located outside the solar system - just 20 years ago. How are the inner and outer boundaries of the habitable zone determined?
In our solar system, the habitable zone is believed to be at a distance from 0,95 before 1,37 astronomical units from the sun. We know that the earth is 1 astronomical unit (AU) from the Sun, Venus - 0,7 a. e., Mars 1,5 a. e. If we know the luminosity of a star, then it is very easy to calculate the center of the habitable zone - you just need to take the square root of the ratio of the luminosity of this star and relate it to the luminosity of the Sun, that is:
R ae \u003d (L star / L sun) 1/2.
Here Rae is the average radius of the habitable zone in astronomical units, and L star and L sun - bolometric indicators of the luminosity of the desired star and the Sun, respectively.
The boundaries of the habitable zone are established based on the requirement that the planets in it have water in a liquid state, since it is a necessary solvent in many biomechanical reactions.
Beyond the outer edge of the habitable zone, the planet does not receive enough solar radiation to compensate for radiation losses, and its temperature will drop below the freezing point of water. A planet closer to the sun than the inner edge of the habitable zone would be overheated by its radiation, causing the water to evaporate.
More strictly, the inner boundary is determined both by the distance of the planet from the star, and by the composition of its atmosphere, and in particular by the presence of so-called greenhouse gases: water vapor, carbon dioxide, methane, ammonia, and others. As is known, greenhouse gases cause the atmosphere to heat up, which in the case of a catastrophically growing greenhouse effect (for example, early Venus) leads to the evaporation of water from the surface of the planet and loss from the atmosphere.
The outer border is another side of the issue.
As soon as the amount of energy becomes insufficient, greenhouse gases (water vapor, methane, and so on) condense from the atmosphere, fall out as rain or snow, and so on. And actually greenhouse gases have accumulated under the polar cap on Mars.
It is very important to say one word about the habitable zone for stars outside our solar system: potential - the zone of potential habitability, that is, the conditions necessary, but not sufficient for the formation of life, are met in it. Here we need to talk about the viability of the planet, when a number of geophysical and biochemical phenomena and processes come into play, such as the presence of a magnetic field on the planet, plate tectonics, the duration of the planetary day, and so on.
These phenomena and processes are now being actively studied in a new direction of astronomical research - astrobiology.
Search for planets in the habitable zone
Astrophysicists simply look for planets and then determine if they are in the habitable zone.
From astronomical observations, you can see where this planet is located, where its orbit is located.
If in the habitable zone, then immediately interest in this planet increases. Next, you need to study this planet in other aspects: the atmosphere, chemical diversity, the presence of water and the source of heat.
This already slightly takes us out of the brackets of the concept "potential". But the main problem is that all these stars are very far away.
It is one thing to see a planet around a star like the Sun. There are a number of exoplanets similar to our Earth - the so-called sub- and super-Earths, that is, planets with radii close to or slightly larger than the radius of the Earth.
Astrophysicists study them by examining the atmosphere, we do not see the surface - only in isolated cases, the so-called direct imaging, when we see only a very distant point. Therefore, we must study whether this planet has an atmosphere, and if so, what is its composition, what gases are there, and so on.
Image: Exoplanet (red dot on left) and brown dwarf 2 M1207 b (middle). The first image taken with direct imaging technology in 2004 year. (ESO/ VL T)
In a broad sense, the search for life outside the solar system, and in the solar system as well, is the search for so-called biomarkers.
It is believed that biomarkers are chemical compounds of biological origin.
We know that the main biomarker on Earth, for example, is the presence of oxygen in the atmosphere. We know that there was very little oxygen on the early Earth.
The simplest, primitive life arose early, multicellular life arose quite late, not to mention intelligent. But then, due to photosynthesis, oxygen began to form, the atmosphere changed.
And this is one of the possible biomarkers. Now, from other theories, we know that there are a number of planets with oxygen atmospheres, but the formation of molecular oxygen there is caused not by biological, but by ordinary physical processes, say, the decomposition of water vapor under the influence of stellar ultraviolet radiation.
Therefore, all the enthusiasm that, as soon as we see molecular oxygen, it will already be a biomarker, it is not entirely justified.
Mission "Kepler"
Space Telescope (CT) "Kepler"- one of the most successful astronomical missions (after the Hubble Space Telescope, of course).
It is aimed at finding planets.
Thanks to CT "Kepler" we have made a qualitative leap in the study of exoplanets. CT "Kepler" was focused on one method of discovery - the so-called transits, when the photometer - the only instrument on board the satellite - tracked the change in the brightness of the star at the time the planet passed between it and the telescope.
This gave information about the planet's orbit, its mass, and temperature conditions. And this made it possible to determine on the first part of this mission the order 4500 potential planetary candidates.
Image: Kepler Space Telescope (NASA)
In astrophysics, astronomy, and, probably, in all natural sciences, it is customary to confirm discoveries.
The photometer detects that the brightness of the star is changing, but what can this mean?
Maybe some internal processes in the star lead to changes; planets pass - it darkens. Therefore, it is necessary to look at the frequency of changes.
But in order to say for sure that there are planets there, you need to confirm this in some other way - for example, by changing the radial velocity of the star. That is, now about 3600 planets are planets confirmed by several methods of observation.
And potential candidates are almost 5000 .
Proxima Centauri
In August 2016 2009, confirmation was received of the presence of a planet, called Proxima b, near the star Proxima Centauri.
Why is everyone so interested?
For a very simple reason: it is the closest star to our Sun at a distance 4,2 light years (that is, light covers this distance in 4,2 of the year). This is the closest exoplanet to us and possibly the closest celestial body to the solar system on which life can exist.
The first measurements were taken in 2012 year, but since this star is a cold red dwarf, a very long series of measurements had to be taken. And a number of scientific teams at the European Southern Observatory (ESO) have been observing the star for several years. They made a website, it's called Pale Red D ot (palereddot.org - ed.), i.e. "pale red dot", and observations were posted there.
Astronomers attracted different observers, and it was possible to track the results of observations in the public domain. So, it was possible to follow the very process of the discovery of this planet almost online.
And the name of the observation program and website goes back to the term Pale Red. D ot, proposed by the famous American scientist Carl Sagan for images of the planet Earth transmitted by spacecraft from the depths of the solar system. When we try to find an Earth-like planet in other star systems, we can try to imagine what our planet looks like from the depths of space.
This project was called Pale Blue D ot( "Pale Blue Dot"), because from space, due to the luminosity of the atmosphere, our planet is visible as a blue dot. The planet Proxima b ended up in the habitable zone of its star and relatively close to Earth.
If we, planet Earth, are on 1 astronomical unit from its star, then this new planet is on 0,05 , that is, in 200 times closer. But the star shines weaker, it is colder, and already at such distances it falls into the so-called tidal capture zone.
As the Earth captured the Moon and they rotate together, the situation is the same here. But at the same time, one side of the planet is heated, and the other is cold.
Image: Estimated landscape of Proxima Centauri b as depicted by an artist (ESO/ M. Kornmesser
There are such climatic conditions, a system of winds that exchange heat between the heated part and the dark part, and on the borders of these hemispheres there can be quite favorable conditions for life.
But the problem with the planet Proxima Centauri b is that the parent star is a red dwarf.
Red dwarfs live quite a long time, but they have one specific property: they are very active. There are stellar flares, coronal mass ejections, and so on.
Quite a few scientific articles on this system have already been published, where, for example, they say that, unlike the Earth, there in 20 –30 times higher than the level of ultraviolet radiation. That is, in order to have favorable conditions on the surface, the atmosphere must be dense enough to protect against radiation.
But it is the only exoplanet closest to us that can be studied in detail with the next generation of astronomical instruments. Observe its atmosphere, see what is happening there, whether there are greenhouse gases, what kind of climate it is, whether there are biomarkers there.
Astrophysicists will study the planet Proxima b, this is a hot object for research.
prospects
We are waiting for several new ground and space telescopes, new instruments to be launched.
In Russia it will be a space telescope "Spectrum-UV". The Institute of Astronomy of the Russian Academy of Sciences is actively working on this project.
AT 2018 The American Space Telescope will be launched this year. James Webb is the next generation compared to CT. Hubble. Its resolution will be much higher, and we will be able to observe the composition of the atmosphere of those exoplanets that we know about, somehow resolve their structure, the climate system.
But you need to understand that this is a general astronomical instrument - naturally, there will be very big competition, as well as on CT. Hubble: someone wants to watch the galaxy, someone wants to watch the stars, someone else wants to see something else. Several dedicated exoplanet exploration missions are planned, such as NASA's TESS (Transiting Exoplanet Survey Satellite). Actually, in the coming 10 years, we can expect a significant advance in our knowledge of exoplanets in general and potentially habitable exoplanets like Earth in particular.
Take a look at the scattering of stars in the black night sky - they all contain amazing worlds like our solar system. According to the most conservative estimates, the Milky Way galaxy contains more than a hundred billion planets, some of which may be similar to the Earth.
New information about "alien" planets - exoplanets- opened the Kepler space telescope, exploring the constellations in anticipation of the moment when a distant planet will be in front of its luminary.
The orbital observatory was launched in May 2009 specifically to search for exoplanets, but failed four years later. After many attempts to return the telescope to work, NASA was forced to decommission the observatory from its "space fleet" in August 2013. Nevertheless, over the years of observations, Kepler has received so much unique data that it will take several more years to study them. NASA is already gearing up to launch Kepler's successor, the TESS telescope, in 2017.
Super-Earths in the Goldilocks Belt
Today, astronomers have identified almost 600 new worlds out of 3,500 candidates for the title of "exoplanet". It is believed that among these celestial bodies, at least 90% may turn out to be "true planets", and the rest - double stars, "brown dwarfs" that have not grown to stellar sizes and clusters of large asteroids.
Most of the new planet candidates are gas giants like Jupiter or Saturn, as well as super-Earths - rocky planets several times larger than ours.
Naturally, far from all planets fall into the field of view of Kepler and other telescopes. Their number is estimated at only 1-10%.
To definitely identify an exoplanet, it must be repeatedly fixed on the disk of its star. It is clear that most often it turns out to be located close to its sun, because then its year will last only a few Earth days or weeks, so astronomers will be able to repeat observations many times over.
Such planets in the form of hot balls of gas often turn out to be "hot Jupiters", and one in six is like a flaming super-Earth covered with seas of lava.
Of course, in such conditions, the protein life of our type cannot exist, but among hundreds of inhospitable bodies there are pleasant exceptions. So far, more than a hundred terrestrial planets have been identified, located in the so-called habitable zone, or goldilocks belt.
This fairy-tale character was guided by the principle "no more, no less." Similarly, the rare planets included in the "zone of life", the temperature should be within the limits of the existence of liquid water. Moreover, 24 planets out of this number have a radius less than two radii of the Earth.
However, so far only one of these planets has the main features of the Earth's twin: it is located in the Goldilocks zone, is close to Earth's size and is part of a yellow dwarf system similar to the Sun.
In the world of red dwarfs
However, astrobiologists, persistently looking for extraterrestrial life, do not lose heart. Most of the stars in our galaxy are small cool and dim red dwarfs. According to modern data, red dwarfs, being about half the size and colder than the Sun, make up at least three-quarters of the "stellar population" of the Milky Way.
Around these "solar cousins" miniature systems the size of the orbit of Mercury revolve, and they also have their own Goldilocks belts.
Astrophysicists at the University of California at Berkeley even compiled a special TERRA computer program, with the help of which a dozen terrestrial twins were identified. All of them are close to their life zones near small red luminaries. All this greatly increases the chances of the presence of extraterrestrial centers of life in our galaxy.
Red dwarfs, in the vicinity of which Earth-like planets have been found, were previously thought to be very quiet stars, and flares accompanied by plasma ejections rarely occur on their surfaces.
As it turned out, in fact, such luminaries are even more active than the Sun.
Powerful cataclysms constantly occur on their surface, generating hurricane gusts of "stellar wind" that can overcome even the powerful magnetic shield of the Earth.
However, for proximity to their star, many twins of the Earth can pay a very high price. Fluxes of radiation from frequent flashes on the surface of red dwarfs can literally “lick off” part of the atmosphere of planets, making these worlds uninhabitable. At the same time, the danger of coronal ejections is enhanced by the fact that a weakened atmosphere will poorly protect the surface from charged particles of hard ultraviolet and x-rays of the "stellar wind".
In addition, there is a danger that the magnetospheres of potentially habitable planets will be suppressed by the strongest magnetic field of red dwarfs.
Broken Magnetic Shield
Astronomers have long suspected that many red dwarfs have a powerful magnetic field that can easily break through the magnetic shield surrounding potentially habitable planets. To prove this, a virtual world was built in which our planet rotates around a similar star in a very close orbit in the "life zone".
It turned out that very often the magnetic field of a dwarf not only strongly deforms the Earth's magnetosphere, but even drives it under the surface of the planet. In such a scenario, in just a few million years, we would have no air or water left, and the entire surface would be scorched by cosmic radiation.
Two interesting conclusions follow from this. The search for life in red dwarf systems may turn out to be completely hopeless, and this is another explanation for the "great silence of the cosmos."
However, perhaps we cannot detect extraterrestrial intelligence in any way because our planet was born too early ...
Who can live on distant exoplanets? Maybe such creatures?
The sad fate of the firstborn
Analyzing data obtained with the help of the Kepler and Hubble telescopes, astronomers found that the process of star formation in the Milky Way has slowed down significantly. This is due to the growing shortage of building materials in the form of dust and gas clouds.
Nevertheless, there is still a lot of material left in our galaxy for the birth of stars and planetary systems. Moreover, in a few billion years, our stellar island will collide with the giant galaxy of the Andromeda Nebula, which will cause a colossal burst of star formation.
Against this background of future galactic evolution, the sensational news was recently heard that four billion years ago, at the time of the emergence of the solar system, only a tenth of the potentially habitable planets existed.
Considering that it took several hundred million years for the birth of the simplest microorganisms on our planet, and several billion years more developed forms of life were formed, it is highly likely that intelligent aliens will appear only after the extinction of the Sun.
Perhaps here lies the solution to the intriguing Fermi paradox, which was once formulated by an outstanding physicist: and where are these aliens? Or does it make sense to look for answers on our planet?
Extremophiles on Earth and in space
The more we become convinced of the uniqueness of our place in the Universe, the more often the question arises: can life exist and develop in worlds completely different from ours?
The answer to this question is given by the existence on our planet of amazing organisms - extremophiles. They got their name for their ability to survive in extreme temperatures, poisonous environments and even airless space. Marine biologists have found similar creatures in underground geysers - "sea smokers".
There they thrive under colossal pressure in the absence of oxygen at the very edge of hot volcanic vents. Their "colleagues" are found in salty mountain lakes, hot deserts and subglacial reservoirs of Antarctica. There are even "tardigrade" microorganisms that endure the vacuum of space. It turns out that even in the radiation environment near red dwarfs, some "extreme microbes" can arise.
Acid lake located in Yellowstone. Red plaque - acidophilus bacteria
Tardigrades can exist in the vacuum of space
Academic evolutionary biology believes that life on Earth originated from chemical reactions in a "warm shallow pool" penetrated by ultraviolet and ozone streams from raging "lightning storms." On the other hand, astrobiologists know that the chemical building blocks of life are found on other worlds as well. For example, they were noticed in gas and dust nebulae and satellite systems of our gas giants. This, of course, is far from a “full life”, but the first step towards it.
The “standard” theory of the origin of life on Earth has recently received a strong blow from…. geologists. It turns out that the first organisms are much older than previously thought, and formed in a completely unfavorable environment of a methane atmosphere and boiling magma pouring out of thousands of volcanoes.
This makes many biologists think about the old hypothesis of panspermia. According to it, the first microorganisms originated somewhere else, say, on Mars, and came to Earth in the core of meteorites. Perhaps the ancient bacteria had to travel a longer distance in cometary nuclei from other star systems.
But if this is so, then the paths of "cosmic evolution" can lead us to "brothers in origin" who drew the "seeds of life" from the same source as we...
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According to a Yale University (USA) researcher, in the search for habitable worlds, it is necessary to make room for the second “Goldilocks” condition.
For many decades, it was thought that a key factor in determining whether a planet could support life was its distance from its sun. In our solar system, for example, Venus is too close to the Sun, Mars is too far away, and Earth is just right. Scientists call this distance the “habitable zone” or “Goldilocks zone”.
It was also believed that the planets were able to independently regulate their internal temperature with the help of mantle convection and underground displacement of rocks caused by internal heating and cooling. The planet may initially be too cold or too hot, but will eventually come to the right temperature.
New study published in the journal Science Advances August 19, 2016, shows that just being in the habitable zone is not enough to sustain life. The planet must initially have the required internal temperature.
A new study has shown that for the origin and maintenance of life, the planet must have a certain temperature. Credit: Michael S. Helfenbein/Yale University
“If you collect all kinds of scientific data about how the Earth has evolved in the last few billion years and try to make sense of it, you eventually realize that convection in the mantle is pretty indifferent to internal temperature,” said Jun Korenaga, author of the study and professor of geology and geophysics at Yale University. Korenaga presented a general theoretical framework that explains the degree of self-regulation expected for convection in the mantle. The scientist suggested that self-regulation is hardly a characteristic of terrestrial planets.
“The absence of a self-regulating mechanism is of great importance for planetary habitability. Research in planetary formation suggests that terrestrial planets are formed by powerful impacts, and the outcome of this highly random process is known to be highly variable,” writes Korenaga.
A variety of sizes and internal temperatures would not hinder planetary evolution if the mantle self-regulated. What we take for granted on our planet, including oceans and continents, would not exist if the Earth's internal temperature was not in a certain range, which means that the beginning of Earth's history was not too hot or too cold.
NASA's Institute of Astrobiology supported the study. Korenaga is a co-researcher on NASA's Alternative Earths project team. The team is busy asking how the Earth maintains a permanent biosphere throughout most of its history, how the biosphere manifests itself in planetary-scale "biosignatures", and the search for life inside and outside the solar system.
An example of a system for finding the habitable zone depending on the type of stars.
in astronomy, habitable zone, habitable zone, life zone (habitable zone, HZ) is a conditional area in space, determined on the basis that the conditions on the surface of those in it will be close to the conditions on and will ensure the existence of water in the liquid phase. Accordingly, such planets (or their) will be favorable for the emergence of life similar to the earth. The likelihood of life occurring is greatest in the habitable zone in the vicinity ( circumstellar habitable zone, CHZ ) located in the habitable zone ( galactic habitable zone, GHZ), although research on the latter is still in its infancy.
It should be noted that the presence of a planet in the habitable zone and its favorable for life are not necessarily related: the first characteristic describes the conditions in the planetary system as a whole, and the second - directly on the surface of a celestial body.
In English-language literature, the habitable zone is also called goldilocks zone (Goldilocks Zone). This name is a reference to the English fairy tale Goldilocks and the Three Bears, in Russian known as "Three Bears". In the fairy tale, Goldilocks tries to use several sets of three homogeneous objects, in each of which one of the objects turns out to be too large (hard, hot, etc.), the other is too small (soft, cold ...), and the third, intermediate between them , the item turns out to be “just right”. Similarly, in order to be in the habitable zone, the planet must be neither too far from the star nor too close to it, but at the "right" distance.
Habitable zone of a star
The boundaries of the habitable zone are established based on the requirement that the planets in it have water in a liquid state, since it is a necessary solvent in many biochemical reactions.
Beyond the outer edge of the habitable zone, the planet does not receive enough solar radiation to compensate for radiation losses, and its temperature will drop below the freezing point of water. A planet closer to the sun than the inner edge of the habitable zone would be overheated by its radiation, causing the water to evaporate.
The distance from the star where this phenomenon is possible is calculated from the size and luminosity of the star. The center of the habitable zone for a particular star is described by the equation:
(\displaystyle d_(AU)=(\sqrt (L_(star)/L_(sun)))), where: - average habitable zone radius in , - bolometric index (luminosity) of the star, - bolometric index (luminosity) .Habitable zone in the solar system
There are various estimates of where the habitable zone extends in:
| Inner boundary, a.e. | Outer border a. e. | Source | Notes |
|---|---|---|---|
| 0,725 | 1,24 | Dole 1964 | Estimation under the assumption of optically transparent and fixed albedo. |
| 0,95 | 1,01 | Hart et al. 1978, 1979 | K0 stars and beyond cannot have a habitable zone |
| 0,95 | 3,0 | Fogg 1992 | Valuation using carbon cycles |
| 0,95 | 1,37 | Casting et al. 1993 | |
| - | 1-2% further... | Budyko 1969, Sellers 1969, North 1975 | … leads to global glaciation. |
| 4-7% closer... | - | Rasool & DeBurgh 1970 | …and the oceans won't condense. |
| - | - | Schneider and Thompson 1980 | Criticism of Hart. |
| - | - | 1991 | |
| - | - | 1988 | Water clouds can narrow the habitable zone as they increase the albedo and thus counteract the greenhouse effect. |
| - | - | Ramanathan and Collins 1991 | The greenhouse effect for infrared radiation has a stronger effect than the increased albedo due to clouds, and Venus should have been dry. |
| - | - | Lovelock 1991 | |
| - | - | Whitemire et al. 1991 |
Galactic habitable zone
Considerations about the fact that the location of the planetary system, located within the galaxy, should have an impact on the possibility of the development of life, led to the concept of the so-called. "galactic habitable zone" ( GHZ, galactic habitable zone ). Concept developed in 1995 Guillermo Gonzalez despite being challenged.
The galactic habitable zone is, according to currently available ideas, a ring-shaped region located in the plane of the galactic disk. The habitable zone is estimated to be located in a region 7 to 9 kpc from the center of the galaxy, expanding with time and containing stars 4 to 8 billion years old. Of these stars, 75% are older than the Sun.
In 2008, a group of scientists published extensive computer simulations that, at least in galaxies like the Milky Way, stars like the Sun can migrate long distances. This goes against the concept that some areas of the galaxy are more suitable for life than others.
Search for planets in the habitable zone
Planets in habitable zones are of great interest to scientists who are looking for both extraterrestrial life and future homes for humanity.
The Drake equation, which attempts to determine the likelihood of extraterrestrial intelligent life, includes a variable ( ne) as the number of habitable planets in star systems with planets. Finding Goldilocks helps refine the values for this variable. Extremely low values may support the unique-Earth hypothesis, which states that a series of extremely unlikely occurrences and events led to the origin of life on . High values can reinforce Copernican's principle of mediocrity in position: a large number of Goldilocks planets means the Earth is not unique.
Searching for Earth-sized planets in the habitable zones of stars is a key part of the mission, which uses (launched March 7, 2009, UTC) to survey and collect characteristics of planets in the habitable zones. As of April 2011, 1235 possible planets have been discovered, of which 54 are located in habitable zones.
The first confirmed exoplanet in the habitable zone, Kepler-22 b, was discovered in 2011. As of February 3, 2012, four reliably confirmed planets are known to be in the habitable zones of their stars.
