Why can't there be life in Pluto?

Pluto is just not a planet - finally learn to live with it!

Scientific research should always be open-ended. If it is not and if you orientate yourself too much towards your own expectations, you run the risk of being wrong. Errors of this kind can be found again and again and a very special case has occupied scientists and above all the public for more than 10 years: the "degradation" of the planet Pluto.

In the summer of 2006, astronomers from all over the world met for the General Assembly of the International Astronomical Union (IAU) in Prague. In addition to the scientific lectures customary for such conferences, there was also an event that caught the attention of the media public: An official astronomical definition of the word "planet" was voted on - a definition, the conditions of which the planet Pluto, the furthest from the sun, did not meet. Since then, the small celestial body has only been classified as a "dwarf planet" (actually just another word for "large asteroid"), the solar system only has eight planets.

Ongoing discussions

The decision concerns researchers and the public to this day. Again and again there are initiatives and suggestions for new classifications to give Pluto back its old status. Especially in the USA, where people are apparently particularly angry that the only planet that was discovered by an American is no longer one.

But when Clyde Tombaugh found Pluto at the Lowell Observatory in 1930, he was doing exactly what should be avoided: he was too guided by his expectations. For decades, astronomers had suspected that there must be another planet behind Neptune's orbit. The observed movement of Neptune did not exactly agree with the theoretical predictions and the influence of a still unknown celestial body was blamed for it. The American Percivall Lowell even founded his own observatory in 1894 in order to be able to devote himself intensively to the search for this "Planet X". He did not find it before his death in 1916 - but Clyde Tombaugh discovered Pluto 14 years later.

Unsustainable classification

Everyone assumed that with Pluto they had found what they had been looking for for so long: A large planet in the outer solar system that is responsible for the disturbances in the orbit of Neptune. The fact that Pluto is actually a very small object was discovered in the next few years, but then did not want to change its name as "planet".

However, the more you found out about Pluto, the more untenable this decision became. Pluto was significantly smaller than the rest of the planets. Its orbit was also very different from the others and more like that of an asteroid. At least since the 1990s, the situation was actually clear: From then on, more and more asteroids were found that were in the same region of the solar system as Pluto itself. The "planet" sat in the middle of the Kuiper asteroid belt and many astronomers demanded that one Pluto should finally call itself what it is: A large asteroid that is part of an asteroid belt with many other asteroids.

Late insight

However, it took until 2006 until this decision was officially accepted and the error made almost eight decades earlier could be corrected. A misconception that is by the way not unique: Pluto is not the first planet that the solar system has lost. In 1801 the Italian astronomer Giuseppe Piazzi discovered a previously unknown celestial body. Between the orbits of Mars and Jupiter - exactly where, based on theoretical considerations, a planet has long been suspected.

And in fact, "Ceres", as the new object was christened, was also immediately referred to as "Planet". Just like the celestial bodies "Pallas", "Juno", "Vesta" and "Astraea" discovered in the same area in the following years. More and more "planets" were found and by 1851 the number of new planetary entries grew to 15. Only then did they realize that this was a new class of objects called "asteroids".

Both Ceres and Pluto were guided by false expectations. You were looking for planets - and when you finally found something you couldn't imagine not having found what you were looking for. We now know that the expectations were unfounded. The theoretical considerations that preceded the discovery of Ceres (the so-called "Titius-Bode series", which was believed to explain the distribution of planets in the solar system) turned out to be wrong. And the problems with the orbit of Neptune, which led to the search for "Planet X", disappeared when, thanks to Voyager probes, it was possible to determine the exact mass of Neptune in the 1990s.

Limited definitions

Today nobody is upset that Ceres is called a large asteroid and no longer a planet. You will also get used to the fact that Pluto is not a planet. The history of its origins and its orbital dynamics match an asteroid much better than a planet. And above all, Pluto doesn't get any less interesting just because we no longer call it "planet" (I personally find asteroids much more exciting anyway). It remains the same fascinating celestial body at the edge of the solar system that it always has been.

At some point, astronomers will probably throw the rigid concept of planetary definition overboard anyway. You will have to, because nature did not foresee the limits that we put in these definitions. The transitions from tiny cosmic dust particles to the asteroids to the even larger planets and gas giants like Jupiter are fluid. We will find new ways to classify all of these heavenly bodies and all of the new information we are constantly discovering about them. And whether Pluto is then a "planet" or not, or maybe even called quite differently, then no longer matters. (Florian Freistetter, April 18, 2017)