Pluto is out!

Our solar system is one planet poorer. For 76 years, Pluto was the ninth planet. Now it has officially been stripped of its planetary title. It has thus been relegated to 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? For if one wants to continue to count Pluto as a planet, then one would also have to grant the same status to the larger Eris and the somewhat smaller Makemake – and possibly to other, as yet undiscovered celestial bodies.

The International Astronomical Union (IAU) in Prague has now reached its verdict: Eris and its colleagues are merely dwarf planets. Their mass, orbit and orbital plane around the Sun clearly distinguish them from their larger colleagues. But Pluto now also no longer falls under the planet definition and must relinquish its title – equal rights for all.

Not everyone is happy about the IAU’s clarification. Opponents of this decision founded the organisation “Pluto’s Friends” and some even went to demonstrate. Their demand: Pluto must become a planet again.

Which is the tenth planet?

The decision to exclude Pluto from the series of planets was preceded by years of searching for the “tenth planet”. At the forefront of the search: American astronomer Mike Brown, who specialises in researching celestial bodies at 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 significantly smaller than Pluto, the list of planets remained untouched.

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

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

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.

Today we know eight planets. To remember their names in the right order, the first letters of the sentence “My father explains our night sky to me every Sunday” help. – or in short: M-V-E-M-J-S-U-N.

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.