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 nothingness. 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 the planets except Mars and Mercury.

While the image has no scientific value, it does show 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.

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 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.

Snapshot of a planet

Actually, this photo should not be possible at all, but the scientists have managed it: the first photo 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.

The rising Eart

Topsy-turvy world – the Earth rises above the lunar horizon. On Christmas Eve 1968, the American astronaut William Anders took this famous photo.

Together with Frank Borman and James Lovell, he circled the moon several times on the Apollo 8 mission. When their space capsule came out from behind the moon again during one of these orbits, they saw the Earth’s sphere emerge from behind the lunar horizon. They were deeply impressed by this sight and took several photos – although Anders jokingly remarked that this was not even foreseen in the mission plan.

However, it is not possible to experience a real “earth rise” – as we see the moon rising while standing on the earth – on the lunar surface. This is because the Moon always faces the Earth on the same side. When you are on this side, you see the Earth all the time – and always at the same place in the sky. And from the back of the moon, the Earth can never be seen.

The second face

The moon shows itself from a completely new side: On 7 October 1959, the Russian probe “Luna 3” took the first photo of the far side of the moon. However, the world still had to wait eleven days for this historic photo: only when the probe flew back towards Earth was the radio connection good enough to send the picture.

At first glance, the image does not look very spectacular. The resolution is poor, and since the sun shines almost vertically on the lunar surface, no shadows of mountains and craters can be seen.

But there was one surprise: the moon has far fewer dark spots on the back than on the front. Astronomers are still puzzling over the reason!

Until this photo was taken, mankind had no idea what it looked like there. Because from Earth, you only ever see the same side of the moon.

The blue planet

Seen from space, the earth’s sphere appears a strong blue. This is because almost three quarters of the earth is covered with water. Although water is transparent in small quantities, from a certain depth it takes on an increasingly strong blue shimmer. Because we see the mighty oceans as blue, the Earth is also called “the blue planet”. The term is particularly true south of the equator. This is because the southern hemisphere is almost completely covered by sea, because a large part of the continents have migrated northwards due to plate movement.

The vast oceans contain almost all the water on earth. A lot of salt is dissolved in seawater, which is why it is not suitable as drinking water. What little fresh water there is on earth is mainly frozen in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, lakes and rivers or in the air.

But the view from the outside is deceptive: the Earth’s surface is largely covered by water, but measured against the diameter of the Earth, the oceans are only a wafer-thin layer. Therefore, the water makes up only a fraction of the Earth’s mass. By comparison, if the Earth were the size of a basketball, all the Earth’s water would fit into a ping-pong ball. And the drinking water would be proportionally even smaller than a single popcorn.

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.

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.