Here's how and why we reclassified Pluto

Pluto was discovered almost by accident. In 1929, amateur astronomer Clyde Tombaugh had found a place at the Flagstaff observatory in Arizona to hunt down the so-called Planet X, an alleged planet that would have caused an anomaly in the motion of Uranus. Later it would be discovered that there was no Planet X and that in reality even the anomaly in the motion of Uranus was the result of an incorrect estimate of the mass of Neptune. The hunt for planets at that time required a great deal of patience: Tombaugh had the task of comparing many photographic plates of equal portions of the sky but obtained at different times. If something moved in relation to the background of the sky, of the fixed stars, it could only be an object of the Solar System. And that was exactly how Tombaugh discovered Pluto in February 1930. He thought he had succeeded in the enterprise, that he had found the coveted Planet X. But the emotion quickly died down: Pluto was very small, too small to have the desired gravitational influence on Uranus.

The two photographic plates in which Clyde Tombaugh discovered Pluto, by comparison. The arrow points to the dwarf planet that is moving relative to the background. Credits: Lowell Observatory

A strange world Until 2006 there was no definition of a planet. After all, the urgency had never been felt: there were eight planets, with Pluto they had become nine, but all in all the matter ended there. Yet, while studying Pluto, someone started to turn up their noses. Pluto wasn't just tiny, half the size of the already tiny Mercury in diameter. Pluto had a very flattened and very inclined orbit, and a surface covered with ice. Features that made it look much more like a comet than a planet. But Pluto also has some characteristics that unite it to the planets: it is spheroidal, and it has satellites (Charon, Nix, Hydra, Kerberos and Styx). Its hybrid nature between these bodies has always made it somewhat different from other planets.

The best image we had of Pluto before the arrival of the New Horizons probe. From Hubble Space Telescope. Credits: Nasa / Esa

The arrival of Eris Although leaving someone unsatisfied, the situation remained stable until 2005. That year, a group of astronomers led by Mike Brown of Caltech announced the discovery of a new Trans-Neptunian object - those that lie beyond Neptune's orbit such as Pluto. The procedure was exactly the same that Tombaugh had adopted for Pluto, but done with modern and automated methods. Until then, most of these discovered objects were essentially equivalent to asteroids or comets. But that of Brown and colleagues was different: Eris was spheroidal, had a flattened and inclined orbit. In short, it had the same characteristics that made Pluto problematic. A statement from NASA dated July 2005 entitled "the tenth planet discovered". But it was now clear that Eris's discovery was a point of no return. And it didn't end there: the same team also announced the discovery of Makemake, another team that of Haumea. Both bodies with the same characteristics. In short, the Trans-Neptunian region was suddenly populating with what, if Pluto was a planet, they must have been planets as well.

The images of the discovery of Eris in 2003. Credits: Samuel Oschin Telescope, Palomar Observatory

The resolution of the Iau O accepted a Solar System in which the planets were at least 12, but in all probability destined to increase further, or a definition of planet had to be finally built. In 2006, shortly after the launch of the New Horizons probe which aimed precisely at Pluto, the International Astronomical Union set up a commission to decide what were the characteristics that a celestial object must have to be defined as a planet. The commission then arrived at the historic resolution B5, which defined that a planet, to be defined as such, had to respect three characteristics: to orbit the sun, to have sufficient mass to be more or less spherical, and to have cleaned up the vicinity of its orbit. An object that does not respect this last characteristic is called a dwarf planet. An object that is not even spheroidal is called a minor body, be it an asteroid or a comet. Pluto, Eris, Makemake, Haumea, and even the asteroid Ceres, then became dwarf planets, and Pluto, with resolution B6, recognized the historical role of the progenitor of this class of objects. So we often talk about the downgrading of Pluto, but this is a historical error, because before 2006 a definition of a planet did not exist at all. It would be more correct to say that since the day of the publication of resolution B5, Pluto and all the objects of the Solar System have finally obtained a definition. In the meantime, however, we have discovered many other dwarf planets. In addition to those mentioned, there are Orcus, Quaoar, Gonggong, Salacia, 2002 Ms4 and Sedna, which are on the waiting list to be officially recognized as dwarf planets. And then there are another fifty that could be added to the list in the coming years if further studies confirm their characteristics.

The various candidates or confirmed dwarf planets compared in an illustration. Credits: Nasa

A definition that is not so rigid Still today, however, this resolution of the IAU leaves many dissatisfied, especially because it is so qualitative that it leaves room for a lot of arbitrariness. The boundary between planet and dwarf planet hinges on its ability to have cleaned up the orbit, but a truly clean orbit in the Solar System does not exist. Asteroids, comets, dust, intersect the orbit of every planetary object, including the main planets. It is generally understood that these bodies do not have to be of comparable size, but that statement in the resolution is not written. There have been various proposals over time to mathematically formalize these definitions, but in fact, none have been definitively accepted yet. However, a further criticality is that the current definition does not consider exoplanets as planets, because the resolution explicitly speaks of objects in the Solar System.

Pluto immortalized by New Horizons in 2015. Credits: Nasa

Nasa / New Horizons