What planet do we live on
The world we live in
The earth is the only planet in our solar system on which higher life has formed. (We do not know how many planets there are outside of our solar system in the universe; with 100 billion stars in 100 billion galaxies, the number is unpredictably large - but we have only discovered about 2,000 of them. We know nothing about life there; the> > The search for extraterrestrial life has long since begun.) Why could higher life develop on earth in particular?
A prerequisite for life on earth: our earth lies in the Life zone the sun - an area in which the solar radiation is neither too strong nor too weak for the development of life. Own illustration.
The right position in the Milky Way
The earth is not too close to the center of the Milky Way (it is hostile to life there because of hot dust and gas clouds), but also not too far out: there are not enough heavy elements there. Most of the elements that make up the earth and living beings did not arise during the Big Bang, but only in the course of the earth's history when entire generations of stars passed in front of our sun (>> background information: origin of the universe); Without these elements there would be neither rocks nor living things: the iron in hemoglobin, the red blood pigment that transports oxygen from the lungs to the body; the oxygen itself; the potassium in our bones - they are the products of glowing stars. So we are all children of the stars.
We are all children of the stars ...
Particularly large stars pass as a supernova (>> The Structure of the Universe), thereby creating the radioactive heavy elements, which essentially generate the heat in the Earth's interior, which in turn drives the >> plate tectonics: Without these, there would be no water and no carbon dioxide in the earth's atmosphere more (see the page on the carbon cycle, >> carbon exchange with the rock); And also life on land - including the formation of humans - would never have existed: without a story of volcanism, first plate cores, subsequent collisions and the unfolding of mountains, the earth would be flat and covered with a 2.5-kilometer-high layer of water everywhere.
The right position in the solar system
The earth is just far enough from the sun to get the "right" amount of solar energy. The Solar energy is by far >> the most important source of energy for the earth. It arises because the sun with its mass develops such a force of attraction that the repulsive forces of the nuclei of the hydrogen atoms (which make up three quarters of the solar mass) are overcome and the nuclei fuse; However, the atoms that are created are not stable and give off energy, making them stable helium atoms. In this way, the sun “burns” 637 million tons of hydrogen per second, producing 632 million tons of helium and 385 billion billion megawatts. The position of the earth - about 150 million kilometers from the sun - means that the incoming solar radiation is energetic enough to drive reactions necessary for life, but not so strong that all water would evaporate. The basic principle of the earth's radiation budget is shown in the following figure:
The Earth's Radiation Budget: From the incoming solar radiation will
a part reflects, the remaining part heats water, land and
the atmosphere and is given off again as heat radiation.
Figure translated, original NASA (source: http://visibleearth.nasa.gov/).
This area, in which the solar radiation is neither too strong nor too weak, is the "Life zone”Of a star (see figure >> at the top of the page). The location of the living zone is not only dependent on the distance from the sun, but is also influenced by whether a planet has an atmosphere or not - greenhouse gases in the atmosphere can increase the temperature of the planet and thus shift it outwards (see> > below). When researching our solar system, it was also found that Jupiter's moons Io and Europa are warmer than can be explained by solar radiation: They are also heated by tidal forces, so that even liquid water occurs on Europe. This means that moons in the vicinity of large gas planets can also be in the life zone of stars. (On earth, however, the contribution of the tides to the energy balance is negligibly small.) The life zone also moves during the life of a star: since it gets hotter with increasing age, the life zone moves away from its star over time.
The life zone of a star migrates outward over time, as a star >> becomes hotter and hotter in the course of its life. The earth was presumably in the life zone of the sun from the beginning. Own illustration according to Fig. 5.1 from >> Tim Lenton & Andrew Watson 2011.
(How long the earth will remain in the sun's life zone can only be estimated approximately - the estimates available give us between 0.5 and 1.5 billion years. Smaller stars that burn their nuclear fuel >> less quickly are therefore possible more life-friendly than the earth - in any case they have a life zone that is stable for longer.)
The role of the atmosphere
The earth itself shouldn't be much smaller, however, because planets need a certain size to hold an atmosphere of volatile gases with their gravity. This not only needs life on land, it also plays a central role in temperature regulation. Central are the >> greenhouse gases, they allow more (short-wave) solar radiation to reach the earth's surface than (long-wave) heat radiation:
The greenhouse effect of the earth's atmosphere: Part of the heat radiation emitted
is held back by the atmosphere. Modified from Wikipedia, article
“Radiation budget of the earth”, viewed on December 22, 2005.
The strength of the Greenhouse effect depends on the composition of the atmosphere (for the current state see the page on >> Earth's climate); this has changed in the course of the earth's history. The result borders on the unbelievable: the sun was created around the same time as the earth and initially only gave off around 70 percent of its current radiation. As shown above, the life zone was much closer to the sun in the early days of the solar system. And yet geological studies and also the history of life show that for over four billion years the temperature of the earth has always been high enough that there was liquid water - the prerequisite for life. The fact that the earth was warm enough for liquid water even when the sun was weak suggests that there was some kind of thermostat (this is one of the key findings that led to the acceptance of the model of the Earth ecosystem have led, see also the >> Introduction). Corresponding >> control loops were actually found; The carbon cycle in particular plays this role: In the early atmosphere there was a high concentration of the greenhouse gas carbon dioxide, which was then bound by the rock (see also >> Earth's climate history).
In the entire history of life, the temperature of the earth has fluctuated only a few degrees Celsius.
The result: in the entire history of life, the temperature of the earth has only fluctuated by a few degrees. (The fact that the greenhouse effect is intensified because human activities actually release greenhouse gases that have long been separated from the earth's atmosphere is the most important cause of human-made >> climate change.)
The earth as a system
What could the "thermostat" of early Earth have looked like? (Thermostat in quotation marks, since there is no set point that someone has set, but control loops that have kept the earth temperature in a life-friendly range.) This requires control loops that lower the temperature when it rises, and others that do it increase as it cools. Such "system stabilizing" control loops are also called negative feedback (negative does not come from bad, but means that they counteract changes - with a rise in temperature, for example, lead to cooling). There are also positive feedbacks that intensify the effects of changes and therefore determine the inner and outer limits of the life zone. There are both on Earth, and the most important ones that could have influenced the temperature on early Earth are shown in the following figure:
Feedback that may have contributed to temperature regulation on early Earth. Black arrows with "+" mean a change in the same direction, gray arrows with "-" mean an opposite change. Reading examples: With increasing temperature, the silicate weathering increases (in the same direction, therefore "+"), but the ice cover of the earth decreases (opposite, therefore "-"). How these feedbacks influence the temperature of the earth is explained in the following text. Own illustration according to Fig. 7.2 from >> Tim Lenton & Andrew Watson 2011.
The control circuit that determines the inner limit of the life zone is the Steam control circuit: Rising temperatures (when the planet approaches the sun or when the sun gets hotter) leads to greater evaporation, and this to a higher water vapor concentration in the atmosphere. Since water vapor is a greenhouse gas, this causes the temperature to rise further, which in turn increases the water vapor concentration. So there is positive feedback here. However, this is slowed down by an effect on which the functional principle of pressure cookers is based - with increasing water vapor concentration ("vapor pressure"), additional evaporation is made more difficult. But at some point - when the sun is hot enough - the evaporation can "go through", and in addition the water vapor content in the upper atmosphere rises, where UV radiation can split water molecules and the light hydrogen is no longer gravity in the atmosphere is held and is lost in space. This is probably how Venus lost its water. The area in which the temperature is not yet high for this is the inner limit of the living zone.
The outer limit of the life zone is from Ice albedo control loop certainly. As the planet moves further away from the Sun and cools, ice caps form on the poles and expand as the temperature continues to cool. However, ice reflects the sunlight very strongly, so that the reflectivity (albedo, from lat. albus = white) the earth increases - so less solar radiation is absorbed by the planet, so that it cools down further. This, too, is a positive feedback, and beyond the outer limit of the life zone, this control circuit leads to the fact that all water on the planet freezes and there is no more liquid water.
Negative control loops stabilize the living zone by making the planet less susceptible to changes in solar radiation, for example. One example is that Silicate weathering control circuit , which - viewed in geological time - determines the concentration of the greenhouse gas carbon dioxide in the earth's atmosphere. The carbon bound in the rock is released in hot springs in the sea and in the event of volcanic eruptions in the form of carbon dioxide, and when silicate rock is weathered it is bound again in the rock (see: >> Carbon cycle - carbon exchange with the rock). The weathering of silicate rock increases with increasing temperature. If the temperature increases, the increasing silicate weathering removes a larger amount of the greenhouse gas carbon dioxide and thus slows down the warming (negative feedback). Conversely, when the temperature falls, the silicate weathering decreases, more carbon dioxide remains in the atmosphere and leads to increasing warming, thus slowing down the cooling. Result: this control loop ensures that the inner and outer limits of the life zone cannot be reached so easily, so it stabilizes the planet in the life zone. Its limits are that it has a very long-term effect, but the positive feedback described above is much faster.
On earth, however, it should not stay with the inanimate control loops. After all, these have already made the earth into a system - in a system the elements interact with one another in such a way that they can be viewed as a unit. But with the >> emergence of life, life began to interact with the control loops of the earth: the earth system became the earth ecosystem.
The size of the earth not only holds the atmosphere in place, but also has a second effect: the earth hardly cools down inside; this maintains the drive of plate tectonics. For the heat balance of the earth, the energy in the earth's interior is of little importance compared to solar radiation; But it protects life in a different, twofold way: First, the melting of rock in the earth's interior ensures that the water and carbon bound in the rock are released again and again; without this recycling there would probably be no water and no more carbon in the earth for a long time the atmosphere, and no life on earth either. Second, convection movements in the liquid outer part of the earth's core are probably the cause of the creation of the earth's magnetic field; this Earth's magnetic field protects the earth from the solar winds. Solar winds consist for the most part of hydrogen nuclei, which are electrically charged and are therefore mostly deflected around the earth by the magnetic field. In the far north you can sometimes see these solar winds: A small part of the particles, following the field lines, reaches the polar region and creates the spectacular polar lights there through interaction with air particles.
The earth's magnetic field protects the earth from solar winds. Fig .: >> NASA
But the solar winds also have their good: They weaken the high-energy cosmic radiation, so that their influence on the earth is relatively small. The atmosphere also contributes to this: at a height of around 80 kilometers there is a layer (the ionosphere) in which particles, for example from burning meteorites, are ionized and thereby intercept the deadly cosmic radiation. High-energy UV radiation from the sun, which penetrates the ionosphere, is largely filtered by an >> ozone layer in the earth's atmosphere. Without these layers, life on land would hardly be possible.
The help of Jupiter and the moon
And finally, Jupiter and the moon also help: The gravity of the Jupiter helped in the formation of Mars and Earth and banished smaller stone and ice bodies to an asteroid belt - thus protecting the earth from even more frequent impacts by celestial bodies. We humans owe our existence to one of these impacts: In the collision, in which the moon was created (more >> here), part of the earth's crust was thrown into space - only then did the earth's crust become so thin that plate tectonics could arise (the importance of which for life on earth has already been shown above). Even today’s biological diversity would presumably not exist without this collision - the inclination of the earth's axis, which provides for the seasons, goes back to it. However, the seasons played just as important a role in the emergence of the diversity of living beings that we find on earth today as the diverse habitats that emerged as a result of plate tectonics - which is why the moon has already been called the “architect of evolution”. And finally the moon stabilizes the axis of rotation of the earth with its gravity like a cantilever - without this stabilization the earth would temporarily reach an inclination of 85 degrees; the ice caps of the poles would be pointed directly at the sun and would melt, while the tropics would sink into ice and snow. The resulting climatic fluctuations might have destroyed all life at an early stage, but in any case made it look different. (Since the moon moves almost four centimeters away from the earth every year, its gravity will no longer be sufficient in a billion years to stabilize the earth's axis of rotation - should there still be intelligent life on earth, climate change is likely to return to Become a topic).
The origin (?, See >> The origin of life) and further development of life on earth depends on many factors (see also >> The earth as an ecosystem). The better this is understood, the more earth scientists and biologists are interested in astronomy again. Finding more planets where the conditions for life are met would be the first step in discovering extraterrestrial life. And perhaps brings us closer to an answer to the question of whether the existence of life on earth is only due to a happy coincidence or is laid out in the laws of nature - then it should actually be on many planets on which life is possible Give life.
Is there anybody? - the search for extraterrestrial life
Is there extraterrestrial life in the universe? Even the ancient Greeks speculated about this question, and even today nobody knows exactly - but our technical capabilities are now sufficient to search for answers directly in space.
Galaxy NGC 240: There is life on earth, but what does it look like elsewhere? (Photo: NASA, ESA, the Hubble Heritage (STScI / AURA) -ESA / Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville / NRAO / Stony Brook University; public domain)
The oldest approach is the use of radio astronomy: intelligent civilizations, like us on earth, could use radio waves to communicate, and these have been sought with giant radio telescopes since 1960. So far without success. The main limitation of the method is that it can only discover technical civilizations using radio waves. There has been life on earth for 3.5 billion years; with the methods of radio astronomy it would only have been discovered about 100 years ago (see also >> Are there other intelligent civilizations in space?)
In 1995 Michel Mayor and Didier Queloz from the University of Geneva discovered the first planet outside our solar system (51 Pegasi b); and now the astronomers are looking Planets on which the conditions for life are met. Finding planets is not easy: >> Tim Appenzeller compared the idea of capturing the light of a planet the size of the earth next to a star that is a billion times brighter with the attempt to see a glow worm next to the headlight of a lighthouse, 3000 km away and on a foggy night. As unbelievable as it sounds, this can be done; so far there were 11 planets seen directly (There is of course a trick here: the light from the star is covered so that it does not cover the light emitted by the planets. Techniques such as the Interferometry (in which the "disruptive" starlight is extinguished by phase shifting light waves - based on the functional principle of headphones, which reduce ambient noise) or the starlight is blocked by masks; other researchers try to track down planets through the differences in the color of light from planets and stars). Most of the planets have been so far, however indirectly discovered through their influence on "their" star, three different methods are used:
- Doppler method and astrometric measurements Both are based on the fact that the planet's gravity attracts its star and causes it to tumble slightly as a result of the planet's orbit. On the one hand, this leads to regular fluctuations in the wavelength of the light emanating from the star: it becomes longer-wave when the star moves away from the earth (“redshift”) and shorter-wave when it moves towards the earth. With the Doppler method, this “Doppler effect” is measured; the astrometric measurements, on the other hand, measure the star's lateral movements against other stars in the background.
- The third method uses the Light shadingformed when planets cross the surface of the star when viewed from Earth.
With these methods, astronomers are currently primarily looking for “Jupiters”: A planet like Jupiter in our solar system helps during planet formation through the influence of its gravity in the formation of planets the size of Mars and Earth, and it banishes smaller stone and ice bodies into an asteroid belt . However, Jupiter causes a “star tumbling” in decades, depending on its orbit. The first series of measurements will soon be long enough to discover such planets using the Doppler method. In April 2007, the first planet was found with a surface temperature of 0 to 40 degrees Celsius, so water would be liquid - it orbits the red dwarf star Gliese 581. In total, astronomers today know more than 2,000 “exoplanets”, such as the planets outside of our solar system.
Planets the size of Earth are far better discovered and studied in space missionsthat do not interfere with the earth's atmosphere. Most of the planets were discovered by NASA's Kepler mission, which began in March 2009 and is supposed to examine 100,000 stars for light shadowing. From the findings so far, some astronomers conclude that around a fifth of all suns could have planets in the "life zone". In any case, the ancient Greeks believed in other worlds: anything else would be like a field with only one ear of wheat. Today we can only assume that this field exists: planets on which the conditions are right for life. But we do not know how likely it is that life will come about and what it could look like. Because >> life as we know it depends on liquid water, the astronomers look for liquid water on the celestial bodies they have discovered: it is believed to be deep in Mars and has been found on Jupiter's moons Europa and Enceladus; and they also look for atmospheres that - like the >> earth's atmosphere - are not in chemical equilibrium: this, too - like the >> oxygen content on earth - could be a sign of life.
None of this has anything to do with intelligence: we know even less than the likelihood of life arising, how likely it would develop into intelligent life. But to discover "non-intelligent" extraterrestrial life: That would be a second Copernican revolution. 500 years ago Copernicus discovered that the earth is not the center of our solar system; and then it would no longer be the only planet that harbors life - but part of a web of life in the universe ... One can hardly imagine how such a discovery would change our self-image. Whether it will ever be necessary remains to be seen.
Further information on the Internet:
>> PlanetQuest: NASA Jet Propulsion Laboratory website to search for exoplanets (in English)
>> NASA Kepler Mission: Scientific background, description of the mission and its current status, teaching materials (in English).
>> Are there other intelligent civilizations in space?
How it went on ...
That the prerequisites for life on earth are given is proven by the existence of life on earth. But how life came to earth or originated on it is still a mystery. The next section deals with this question and with the development of life on earth: Life.
© Jürgen Paeger 2006 - 2019
The number of planets discovered - stated here as around 2,000, as of June 2015 - growing rapidly. So by the time you read it, the number may already be out of date.
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