Why can't a scientist explore beyond Pluto?

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 shine 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 - compared to the distances in space, that is a stone's throw away. Because it comes so close to the earth and its clouds reflect a lot of light, we can 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 unimaginably many stars, the universe just has to be teeming with planets.


Our solar system is one planet poorer. Pluto was the ninth planet for 76 years. Now his planetary title has been officially revoked. It is descending into the league of "dwarf planets", where it will play a much less important role in the future.

With the discovery of the “planet” Eris, the discussion flared up again: When is a celestial body a planet? Because if you want to continue counting Pluto as a planet, you would have to admit that to the larger Eris and the slightly smaller Makemake - and possibly other, as yet undiscovered celestial bodies.

The International Astronomical Union (IAU) in Prague has now made its judgment: Eris and his colleagues are only dwarf planets. Their mass, orbit and orbital plane around the sun clearly distinguish them from their larger counterparts. But also Pluto no longer falls under the planet definition and has to give up its title - equal rights for all.

Not everyone is happy with the IAU clarification. Opponents of this decision founded the organization “Pluto's Friends” and some even went to demonstrate. Your demand: Pluto must become a planet again.

Which is the tenth planet?

The decision to expel Pluto from the series of planets was preceded by years of searching for the “tenth planet”. Right at the front: the American astronomer Mike Brown, who specializes in the study of celestial bodies on the edge of the solar system.

As early as 2003 he discovered Sedna, which was already considered by some to be the tenth planet. But since Sedna was much smaller than Pluto, the list of planets remained untouched.

On July 29, 2005, the sensation was perfect: Mike Brown announced the discovery of three new celestial bodies - Eris, Makemake and Haumea. With an estimated 2400 kilometers in diameter, Eris is larger than the smallest planet to date, Pluto. However, due to its unusual orbital plane around the sun, it remained undiscovered for a long time.

This discovery made an official definition necessary of what a planet should be. However, this definition did not lead to a tenth planet, but reduced the number of planets to eight.


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 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.

What is a star

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

Stars are simply spheres of gas. But inside it is unimaginably hot, many millions of degrees Celsius. Because of the intense heat, the gas glows and glows - like a lightbulb, 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 kilometers away.

Stars appear to us like tiny points of light - but that's only because of the great distance: in reality, stars are huge. The smallest are about ten times the size of the earth, giant stars can be a hundred thousand times as big!

However, there is one star that is very close to us compared to any other: the sun. After all, it already appears to us as a bright disc in the sky. But even this impression is deceptive: the sun is about a hundred times the size of the earth. We see and feel its power every day, because it gives the earth light and warmth - like a large campfire, by which 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 the fuel supplies are used up at some point, it becomes dark and collapses or explodes. Our sun will end like this one day too. But because stars are so big, the fuel lasts for a long time. Our sun, for example, will shine for another five billion years.

What is the moon

It is the brightest celestial body in the night sky: the moon. It shines so brightly on full moon nights that some people find it difficult to sleep. It appears as big as the sun and the stars look like tiny points of light next to it.

But the impression is deceptive: in reality, the moon (diameter: 3474 km) is only about a quarter the size of the earth (12742 km) - and the sun (1.39 million km) is even four hundred times larger. The moon only appears the same size to us because it is so close to us - the sun (distance to the earth about 150 million km) is also about four hundred times further away than the moon. (384,400 km, an airplane needs 18 days for this distance!)

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Some of this light is then reflected back from the surface of the moon and hits the earth. Just because the moon is so close to us, enough light arrives on earth to light up the night - at least if the moon doesn't just seem to have disappeared without a trace ...

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.