Snapshot of a planet

Actually, this photo should not be possible at all, but scientists have managed it: the first photograph of a planet outside our solar system. German planet hunters at the European Observatory in Chile have 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 at more than a hundred times the distance between the Earth and the Sun. It therefore takes about 1200 years to orbit it. This solar system is about 400 light years away from us.

Until now, it has never been possible to photograph planets directly. Even high-resolution telescopes do not normally manage to locate planets next to their much brighter stars. The photo was only possible because the planet is moving at a great distance around a star that is still quite young. “GQ Lupi” was probably formed only 2 million years ago and therefore does not yet shine as brightly as more mature stars. In addition, its planet, which is also young, is still very hot. This is why the infrared cameras at the Very Large Telescope in Chile were able to record its thermal radiation.

Planet hunt successful for the first time”

Planet hunters had been waiting for this moment for a long time. In the mid-1990s, the Swiss research team led by Professor Michael Mayor succeeded for the first time in detecting a planet outside our solar system.

The discovery was a sensation at the time. It had long been suspected that there must be many such “exoplanets”, but they could not be detected. This is because the star around which a planet orbits shines so brightly that the planets are virtually invisible to our telescopes.

Nevertheless, the planet “51 Pegasi b” caught the researchers’ eye. This is because planets cause their star to lurch slightly in their orbit due to their own gravity. The researchers were able to observe this movement of the star by means of changes in its light spectrum.

The detection of “51 Pegasi b” was the starting signal for a real hunt for exoplanets. In the meantime, several hundred planets have been found.

Are there other inhabited planets?

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. So the question is obvious: do the other stars also have planets? And is there life on other planets?

What is it like on our immediate neighbouring planet? “Martians” have always been a popular topic in stories and films. But at the latest since the first probes landed there and sent measurements back to Earth, we know that life as we know it is not possible on Mars.

On Earth, there are very special conditions: 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 enough energy, there are oceans, a solid land surface and all the necessary chemical elements and compounds to build living things.

Earth is the only planet in our solar system where all 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 – in addition, the gas planets lack a solid surface. This leaves only the Earth as an island of life in the solar system.

So scientists are investigating whether other stars have planets – and whether life could exist there. But this is not so easy, because even the nearest stars are so far away that planets cannot be detected even with the best telescopes. Only indirect clues reveal the planet – for example, when the star’s light dims briefly because the planet passes right in front of the star. With this and other tricks, several thousand such “exoplanets” have been discovered in the meantime – but a “second Earth” comparable in every respect was not yet among them.

Even if one were to be found: However, the question of whether there is intelligent life on these planets cannot be answered in this way. That is why scientists have started the SETI project to search for radio signals from space. The abbreviation stands for “Search for Extraterrestial Intelligence”. The idea: if an extraterrestrial civilisation develops technology similar to ours, they might also use radio waves. These could propagate into space and perhaps reach our antennae. The only question is whether these radio waves will 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.

What is a planet?

Perhaps some of you have 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 glows by itself, a planet does not. Stars have an energy source inside them, so they glow hot and emit light. A planet, on the other hand, is cold and does not glow on its own. We can only see it when it is illuminated by a star. Then the surface of the planet spreads the light of the star in all directions.

Most planets belong to a star. This is because planets do not form alone, but together with a star. They then belong to this star and orbit it – like Earth and Venus, for example, which orbit the sun.

And why is Venus so easy to see, even though it only transmits the light of the sun? It’s because of its dense cloud cover, which reflects the sunlight particularly well. In addition, Venus is the closest celestial body to the Earth after the Moon: just 40 million kilometres – that’s 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 see it well in the sky.

Venus is not the only planet, of course. Like 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 must be teeming with planets.

What is a star?

When it is particularly dark at night and the sky is clear, we see thousands of stars twinkling as tiny points of light above our heads. But why do the stars shine? What are stars anyway?

Stars are simply balls of gas. But inside them it is unimaginably hot, many millions of degrees Celsius. Because of the intense heat, the gas glows and shines – like a light bulb, only much brighter. The light from the stars is so strong that we can see it from Earth, even though the stars are many trillions of kilometres away.

Stars appear to us like tiny points of light – but that is only because of the great distance: in reality, stars are huge. The smallest ones have about ten times the diameter of the Earth, giant stars can be a hundred thousand times larger!

However, there is one star that is very close to us compared to all the others: the sun. It already appears to us as a bright disc in the sky. But even this impression is deceptive: the diameter of the sun is about one hundred times that of the earth. We see and feel its power every day, because it gives the earth light and warmth – like a big campfire where we sit in the cold universe.

However, a star does not burn wood. It consists mainly of hydrogen gas and draws its energy from the hydrogen atomic nuclei. So a star slowly burns itself, so to speak. When at some point the fuel supplies are exhausted, it goes dark and collapses or explodes. Our sun will also end up like that one day. But because stars are so big, the fuel lasts for a long time. Our sun, for example, will shine for about five billion years.

How do stars form?

The stars shine in the sky – that’s obvious. But where do the stars actually come from?

To answer this question, scientists are interested in certain areas in the universe. These look misty and blurred in the telescope – and are therefore also called “nebulae”. One example is the Orion Nebula in our own galaxy. In it, stars have been discovered that are still very young – just 30,000 years old. So they are still “baby stars”, because a star can become several billion years old. The nebula is the cradle of stars, so to speak.

And what do they come from? To find out, the nebula has been studied more closely: In it, there are dense clouds of dust and, above all, the gas hydrogen – the substance that stars are made of. The gas clouds must therefore be the origin of the stars.

But how does a gas cloud become a star? It is a process that takes many millions of years. The driving force is the gravity of the gas particles in the cloud. It causes the particles to attract each other. So the cloud becomes smaller and denser over time – it becomes a massive ball of gas.

The gas particles inside the cloud are thus under ever higher pressure, they are literally squeezed together. As the pressure increases, so does the temperature. At some point, it is so hot on the inside that the hydrogen atoms begin to fuse with each other. This reaction releases a lot of energy, so that the star begins to shine.

This “heating” from inside the star creates additional heat and a back pressure. Thus, the freshly ignited star does not contract further, but forms itself into a stable, glowing ball of gas.

In most cases, several stars are formed from a cloud at once. They attract each other and orbit each other. This is called a double, triple or multiple star. However, only one star was formed from “our” cloud – the sun.

What is our solar system and how was it formed?

Earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. In the process, they discovered early on that some stars move. These wandering stars were observed and their paths followed. But for a long time, their movements were not understood – until about five hundred years ago, when a man named Nicolaus Copernicus solved the mystery: the Earth and the “wandering stars” are actually planets, all orbiting the Sun at different distances.

Mercury is the planet that orbits closest to the sun. Then come Venus, Earth and Mars. These four inner planets have a solid surface of rock and are still relatively close to the sun – only a few hundred million kilometres.

Further out, at a distance of about one to 4.5 billion kilometres from the Sun, orbit the outer planets: Jupiter, Saturn with its rings, Uranus and, on the very outside, Neptune. They consist of gas (mainly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the Earth, which is why they are also called the gas giants.

Finally, there are asteroids, comets and dust clouds that also orbit the sun. The Sun’s gravitational pull holds all these celestial bodies together, forcing them to fly in circles as if on a long leash. All of this together is called the solar system. The moons are also part of it – but they are held in place by the gravitational pull of the planets.

But why does the sun have planets at all? It has to do with how the sun was formed: a cloud of gas and dust pulled together by its own gravity and became a star. But not all the material in this cloud was “built up” in the star – about one percent remained. And when the sun then began to shine, the radiation pushed the remaining matter back out.

The light gases were pushed far outwards, while the heavier dust and rock fragments remained close to the Sun. In the course of time, the planets were formed from these dust and gas clouds. This is why the solar system has the gas planets on the outside, the rocky planets further in – including our Earth – and the Sun right in the middle. It contains 99% of the mass of the solar system and holds everything together with its gravity.

What is the Milky Way?

If you look up at the sky on a clear night, you can see a bright band stretching across the sky. Because it looks so milky-bright, it is called the Milky Way. The ancient Greeks thought the gods had spilled milk here and called it the “Galaxy” – after the Greek word for milk.

Of course, the Milky Way is not really made of milk. If you look at it with a telescope, you can see that it consists of many stars. To the naked eye, their light blurs into a bright band. But why do so many stars gather in this narrow strip of sky?

To solve this puzzle, astronomers had to look much further out into space. There they discovered bright spots they called “nebulae”. With a powerful telescope, they realised that these nebulae were a collection of many billions of stars – and that most of the nebulae had the shape of a large, flat disc. So it was clear: the sun itself is a star in such a disc. And because we live in the middle of this disc, it looks to us like a strip that stretches across the sky around us.

This disc, our galaxy, consists of several hundred billion stars, their planets and lots of dust and gas. This matter attracts each other with its gravity, so the galaxy keeps its shape: A flat disc where the stars are arranged in spiral arms and orbit around the centre of the galaxy.

Our sun is the centre of the solar system, but is itself only a small star in an outer spiral arm of the Milky Way. So it also moves and needs a little more than 200 million years for one round.

Even our galaxy is by no means special, but only one medium-sized galaxy among many billions in the universe. The nearest galaxy is the Andromeda Nebula, about 2 million light years away from us. So a simple glance at the sky leads quite quickly out into unimaginable expanses.

A small blue dot

You have to look for a while in this picture: The sensation is a “pale blue dot”, a tiny light blue dot in the void. It’s hard to imagine that this little dot should be our home!

This picture shows the Earth. It was taken by the Voyager 1 probe from the edge of the solar system – 6.4 billion kilometres 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.

While the image has no scientific value, it shows a fascinating and eerie view of our planet: From this distance, Earth is just a tiny grain of sand in space, our island in the middle of an empty, hostile nothingness.

The Voyager 1 probe and its identical sister, Voyager 2, were launched in 1977 to explore the outer solar system. It visited Jupiter in March 1979 and Saturn in November 1980. It provided impressive close-up images of the moons and rings of both planets. As the probe continues its journey, scientists hope to obtain new and interesting measurement data from the edge of the solar system – and the region beyond.

Before it left the solar system for good, however, the scientists activated the camera one last time to take these pictures.

A record for aliens

As with previous probes, NASA has also equipped Voyager 1 and 2 with a message to extraterrestrials. For this purpose, a copper plate was attached to the probe and covered with gold. On the front is engraved instructions on how to play the images and sounds on the back. This contains greetings in 55 languages, animal voices and other sounds from nature, music (including Bach and Mozart) and a personal address by the then US President Jimmy Carter, just like on a record. 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. Therefore, they could be those man-made objects that exist the longest ever – estimated up to 500 million years!

The researchers were attracted by the following idea: what if in the distant future, far away from the solar system, extraterrestrial astronomers discover, capture and study one of the probes? They then decided to give their extraterrestrial colleagues some information about the probe’s builders, a kind of cosmic message in a bottle.

However, space is unimaginably vast and empty. Therefore, it is very unlikely that aliens will actually find the probe. And even if they do: Earth will look very different then – and probably no humans will be alive then either.