Can planets have liquid atmospheres?
Are there other inhabited planets?
The earth - the planet we live on - is just one of several planets orbiting the sun. And the sun is only one of billions of stars in space. The question arises: Do the other stars also have planets? And is there life on other planets?
What does it look like on our direct neighbor planet? "Martians" have always been a popular subject in stories and films. But at least since the first probes landed there and sent measured values to Earth, we have known that life as we know it is not possible on Mars.
There are very special conditions on earth: it is not too cold, but also not too hot, so that there is liquid water. And the earth is heavy enough to hold an envelope of air. So we can breathe and are protected from radiation and meteorite impacts. Sunlight provides sufficient energy, there are seas, a solid area of land and all the chemical elements and compounds necessary for the construction of living beings.
Earth is the only planet in our solar system on which all of these conditions are met: Mercury and Venus are closer to the sun and therefore too hot. Mars is not heavy enough and therefore has no atmosphere. And the outer planets are too cold because they don't get enough energy from the sun - and the gas planets also lack a solid surface. So in the solar system only the earth remains as an island of life.
So scientists are investigating whether other stars have planets - and whether there could be life there. But that's not that easy, because even the closest stars are so far away that even with the best telescopes you can't see any planets. Only indirect clues reveal the planet - for example, when the star's light darkens for a short time because the planet passes right in front of the star. With this and other tricks several thousand such "exoplanets" have been discovered - but a "second earth" comparable in every respect was not among them.
Even if you find one: the question of whether it is on this planet intelligent Life cannot be answered in this way. That is why scientists started the SETI project to search for radio signals from space. The abbreviation stands for "S.search for E.xtraterrestial I.ntelligence “- search for extraterrestrial intelligence. The idea: If an alien civilization similar to ours develops technology, they may also use radio waves. These could spread into space and maybe reach our antennas. The only question is whether these radio waves reach us at all - and whether we are listening in the right direction at the right moment.
But most scientists are convinced that there are other intelligent life forms somewhere in space. If we were alone in the universe, says SETI founder Carl Sagan, it would be a terrible waste of space.
You have to search for a while in this picture: The sensation is a “pale blue dot”, a tiny light blue dot in the void. It is hard to imagine that this little dot should be our home!
This picture shows the earth. It was picked up by the Voyager 1 probe from the edge of the solar system - 6.4 billion kilometers from Earth. It is part of a unique group photo of our solar system, which is composed of a total of 60 individual photos and contains all planets except Mars and Mercury.
Although the picture has no scientific use, it shows a fascinating and eerie view of our planet: From this distance, the earth is just a tiny grain of sand in space, our island in the middle of an empty, hostile void.
The Voyager 1 probe and its identical sister, Voyager 2, were launched in 1977 to explore the outer solar system. In March 1979 she visited Jupiter, in November 1980 Saturn. It provided impressive close-ups of the moons and rings of both planets. On the further journey of the probe, scientists hope to obtain new, interesting measurement data from the edge of the solar system - and the area beyond it.
Before it finally left the solar system, however, the scientists activated the camera one last time for these recordings.
A record for aliens
As with previous probes, NASA has also equipped Voyager 1 and 2 with a message to aliens. For this purpose, a copper plate was attached to the probe and coated with gold. Instructions for use on how to reproduce the images and sounds on the back are engraved on the front. Like on a record, it contains greetings in 55 languages, animal voices and other noises from nature, music (including by Bach and Mozart) and a personal address by the then US President Jimmy Carter. In addition, photos of life on earth and scientific graphics are stored there.
The idea behind it: These probes will leave our solar system and fly out into the void of space. There is nothing there to damage or decompose the probes. Hence, they could be the man-made objects that have existed the longest - estimated up to 500 million years!
The researchers were drawn to the following idea: What if, in the distant future, far away from the solar system, extraterrestrial astronomers discover, capture and investigate one of the probes? Then they decided to give their extraterrestrial colleagues some information about the builders of the probe, a kind of cosmic message in a bottle.
However, space is unimaginably large and empty. Therefore, it is very unlikely that aliens will actually find the probe. And even if: The earth will then look completely different - and then probably no people will be alive either.
This photo shouldn't actually be possible, but the scientists did it: the first image of a planet outside of our solar system. German planet hunters at the European observatory in Chile now presented a photo of the star "GQ Lupi" - and clearly separated from it the planet "GQ Lupi b" as a small point of light. It is said to be a very large gas planet, about twice the size of Jupiter. It orbits its star more than a hundred times the distance between the earth and the sun. It therefore takes around 1200 years to circumnavigate it. This solar system is about 400 light years away from us.
So far it had never been possible to photograph planets directly. Even high-resolution telescopes usually fail to locate planets next to their much brighter stars. The photo was only possible because the planet moves at a great distance around a relatively young star. "GQ Lupi" was probably only formed 2 million years ago and therefore does not yet shine as brightly as more mature stars. In addition, its young planet is still very hot. The infrared cameras on the “Very Large Telescope” in Chile were therefore able to record its thermal radiation.
"Planet hunt successful for the first time"
Planet hunters had waited a long time for this moment. In the mid-1990s, Professor Michael Mayor's Swiss research team succeeded for the first time in detecting a planet outside our solar system.
The find was a sensation at the time. It has long been assumed that there must be many such “exoplanets”, but they could not be proven. Because the star around which a planet orbits shines so brightly that the planet is as good as invisible to our telescopes.
Nevertheless, the planet "51 Pegasi b" went online for the researchers. Because planets easily cause their star to stagger on their orbit through their own gravity. The researchers were able to observe this movement of the star based on changes in its light spectrum.
The proof of "51 Pegasi b" was the starting signal for a real hunt for exoplanets. Several hundred planets have now been found.
What is a planet
Perhaps one or the other has noticed a particularly bright star in the morning or evening sky: Venus. After the sun and the moon, it is the brightest object in the sky. Because it shines so brightly, it is also called the "morning star" or "evening star" - much to the annoyance of astronomers: Because Venus is not a star, but a planet!
The most important difference: a star shines by itself, a planet does not. Stars have a source of energy inside them, so they glow hot and emit light. A planet, on the other hand, is cold and does not glow by itself. We can only see it when it is illuminated by a star. Then the surface of the planet distributes the star's light in all directions.
Most planets belong to one star. Because planets do not arise alone, but together with a star. They then belong to this star and orbit it - such as Earth and Venus, which orbit the sun.
And why is Venus so easy to see even though it only transmits the light of the sun? This is due to their thick cloud cover, which reflects sunlight particularly well. In addition, after the moon, Venus is the celestial body that comes closest to earth: just 40 million kilometers - that is a stone's throw compared to the distances in space. Because it comes so close to the earth and its clouds reflect a lot of light, we can easily see it in the sky.
Of course, Venus is not the only planet. Like the earth, it is one of the eight planets in our solar system. And the sun is not the only star with planets either. Since there are an unimaginable number of stars, the universe just has to be teeming with planets.
What is our solar system and how did it come about?
The earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. They discovered early on that some stars are moving. These wandering stars were observed and their paths followed. For a long time, however, their movements were not understood - until about five hundred years ago a man by the name of Nicolaus Copernicus solved the riddle: The earth and the "wandering stars" are actually planets, all of which orbit the sun at different distances.
Today we know eight planets. To remember their names in the correct order, the first letters of the sentence "M.a V.ater eclarifies mir jEden S.monday uurens Nachthimmel. “- or in short: M-V-E-M-J-S-U-N.
M.Erkur is the planet that orbits closest to the sun. Then come V.enus, E.rde and M.ars. These four inner planets have a solid surface made of rock and are still relatively close to the sun - only a few hundred million kilometers.
They are circling further out, at a distance of about one to 4.5 billion kilometers from the sun outer planets: Jupiter, S.aturn with his rings, Uranus and all the way outside Neptun. They are made of gas (mostly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the earth, that's why they are also called that Gas giants.
And finally there are asteroids, comets, and clouds of dust that also orbit the sun. The gravitational pull of the sun holds all these heavenly bodies together and forces them to fly in a circle like on a long line. Everything together is called that Solar system. The moons are one of them - but they are held in place by the gravitational pull of the planets.
But why does the sun even have planets? This has to do with how the sun came into being: a cloud of gas and dust contracted by its own gravity and became a star. But not all of the material in this cloud was "built into" the star - around one percent was left over. And when the sun began to shine, the radiation pushed the remaining matter outwards again.
The light gases were pushed far outwards, the heavier dust and rocks remained close to the sun. From these clouds of dust and gas, the planets emerged over time. Therefore there are the gas planets outside in the solar system, further inside the rock planets - including our earth - and in the very center the sun. It contains 99% of the mass of the solar system and holds everything together with its gravity.
How did life come about?
The origin of life on earth has long been puzzled. It is known that simple bacteria developed as early as 3.8 billion years ago. But how was that possible - can life just come into being?
A student named Stanley Miller had an idea in 1953: He wanted to simulate the environmental conditions on earth in an experiment around 3.8 billion years ago. To do this, he filled a glass flask with water and some gases that were probably components of the primordial atmosphere: ammonia, methane and hydrogen. In this gas mixture he ignited electrical discharges in order to simulate the lightning bolts of the thunderstorms of that time. The water should replicate the natural water cycle. There was also a heater where the water evaporated and a cooling coil where it condensed again.
Miller ran this experiment for several days and then examined the water. In it he found a certain kind of chemical compound: amino acids, an important part of the cells of all living things. Miller had shown that the building blocks of life can be created from simple gases.
This is why scientists today assume that the gases in the primordial atmosphere reacted in a similar way to form organic substances. Rain washed them into the sea, and high concentrations could accumulate, especially in shallow waters. Whether through aggressive sunbeams or lightning - the particles must have reacted with one another over and over again. A random combination of molecules then had a special property for the first time: It was able to reproduce itself - the beginning of life.
The effect of sunlight
It's unimaginably hot inside the sun: it's 15 million degrees. After all, it is still 5,600 degrees Celsius on the surface of the sun. This means that the sun is incandescent and appears to our eyes as a white ball.
Without the sun there would be no life on this planet, at least not as we know it today. The sun is a gigantic source of energy that radiates light and warmth into space. Some of their radiation also reaches the earth. This energy warms our atmosphere, the earth and the oceans.
The sun heats up the area around the equator the most, because there its rays hit a relatively small area perpendicularly. The poles, on the other hand, reach the sun's rays at a flatter angle. Here the solar energy is therefore distributed over a larger area; and in these regions it stays cooler. The different levels of solar radiation ensure different climate zones. Seasons and weather are also the result of different levels of solar radiation.
If the earth were to store all of the solar energy, it would be unbearably hot here in no time. This can already be felt on a hot summer day when the temperature climbs to 30 degrees Celsius in a very short time after sunrise. In order for the climate to remain stable for centuries, the earth has to get rid of about the same amount of solar energy.
This happens through the radiation of the earth into space. About a third of the solar energy is immediately reflected back from the atmosphere, land area, bodies of water and ice masses. The earth initially absorbs the rest of the energy in the form of heat. It then slowly releases this heat back into space in all directions.
The greenhouse effect
In a greenhouse, vegetables or flowers can thrive even when it's cold outside. That's because greenhouses are built out of glass. The glass - or a transparent film - allows the short-wave rays of the sun to enter the interior unhindered: the air warms up. On the other hand, the glass is impermeable to long-wave heat radiation, so the heat can no longer get out. That’s why it’s cozy and warm in a greenhouse.
Something similar is happening on a large scale on earth. The greenhouse gases carbon dioxide (CO2) and water vapor are naturally present in the atmosphere. Water vapor enters the air through evaporation, carbon dioxide through exhalation. Volcanic eruptions also contribute to the natural carbon dioxide content of the air. Both gases have the same effect as the glass in a greenhouse: They allow the short-wave rays of the sun to penetrate to the earth. At the same time, like an invisible barrier, they hinder the long-wave thermal radiation on its way back into space. The heat builds up and the atmosphere heats up.
Without this natural greenhouse effect, life on earth would hardly be possible, because it would be far too cold for most living things. Instead of the current average temperature of plus 15 degrees, it would be an icy minus 18 degrees Celsius. The surface of the earth would be frozen!
The problem starts when we increase the amount of greenhouse gases in the atmosphere.This is mainly done by burning oil, natural gas and coal. Heating the apartment, driving a car, burning garbage: all of these processes emit carbon dioxide. This CO2 has the largest share in the man-made greenhouse effect. But the cultivation of rice or cattle farming also increase the effect: Large amounts of methane (CH4) - also a greenhouse gas. In addition, nitrous oxide, ozone and fluorocarbons are greenhouse gases. Because all these gases slow down the earth's heat radiation, the temperatures on our globe continue to rise.
How did the water come to earth?
About two thirds of the earth is covered with water - a unique selling point: the earth is the only planet in the solar system on which there is liquid water. Life originated in water, and water is also vital for us humans. But where does the water actually come from on earth?
Scientists suspect that the water comes from comets. These lumps of ice and dust originally formed on the edge of the solar system. But some also got into the interior of the solar system on orbits and became part of the newly emerging planets.
Initially, the young planets were very hot - so hot that the rock melted and formed a liquid ball. And the ice on the comets not only melted, it even evaporated. Because the water vapor was much lighter than the molten rock, it bubbled up towards the surface. There it escaped into the atmosphere through volcanoes.
As the earth slowly cooled, the steam turned back to liquid water. To put it more clearly: It started to rain. Those first downpours must have been stronger than any thunderstorm we can imagine today. And it must have rained for a very long time - tens of thousands of years. Large parts of the young earth's surface were flooded - in some places up to ten kilometers high. This is how the oceans came into being.
And what happened to the water on the other planets? Why are there no oceans there? Mercury doesn't have enough gravity to hold an atmosphere at all - like all gases, water vapor simply escaped into space. The same thing happened on the moon. On Venus, the solar radiation is so strong that the water has also evaporated into space. On the other hand, it is too cold on Mars, but large ice deposits are suspected there under the surface. And the gas planets have no solid surface on which seas could form. One suspects an ocean of water on Jupiter's moon Europa, but the surface is frozen. So the earth remains the only celestial body in the solar system with seas.
How did our air we breathe come about?
What do people and animals need to live? Food and water, of course, but above all oxygen! We get it from the air we breathe. But that was not always the case: the primordial atmosphere consisted of water vapor and poisonous gases such as carbon dioxide and foul-smelling hydrogen sulfide. We would immediately suffocate in this “air”. But what has changed since then? Why is there oxygen in the atmosphere today? And since when?
If you look back in the history of the earth, you can find traces of living things that must have needed oxygen more than two billion years ago. So there must have been oxygen in the air back then.
Petrified traces of microscopic bacteria, called blue-green algae, are much older. And they have it all: These organisms were the first to use the energy of sunlight for their metabolism. They absorbed water and carbon dioxide from their environment and, with the help of solar energy, converted them into sugar, which they used to store energy. In addition, this chemical reaction produced oxygen - as a waste product, so to speak. However, the bacteria could not do anything with the oxygen and simply released it into the environment.
At that time there was plenty of sunlight and carbon dioxide, and the world's oceans were comparatively warm. These were the best conditions for the blue-green algae to multiply and spread. In doing so, they produced more and more oxygen, which accumulated over millions of years, first in the oceans and later in the atmosphere.
The waste product of these bacteria created the conditions for higher forms of life in water and on land. From the bacteria later emerged the chloroplasts, which to this day capture the solar energy in every plant. The principle of so-called photosynthesis has also remained the same: With the help of sunlight, water and carbon dioxide are converted into sugar and oxygen. The sugar serves as a nutrient for the plant, the oxygen is released into the air and inhaled by humans and animals.
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