On February 18, 1930, American astronomer Clyde Tombaugh made the major discovery of his life. He discovered Pluto, considered the ninth planet of the Solar System, for many years.
In 2006, the International Astronomical Union (IAU) made the scandalous decision to remove Pluto from the list of planets. But why did astronomers turn against it? And could Pluto regain planet status in the future?
An unusual wanderer on the edge of the Solar System
Already immediately after its discovery, it became clear that Pluto is markedly different from the other planets of the Solar System. If they move in circular orbits lying in the ecliptic plane, Pluto is not like that. Its orbit has a much larger eccentricity (0.2488) and inclination to the ecliptic plane (17.14°). Because of this, Pluto sometimes comes closer to the Sun than Neptune. Also, astronomers have long been unable to see its disk, because of which it was believed that Pluto has a very dark surface, and in mass, it is close to the Earth.
As new observations were made and techniques improved, estimates of Pluto’s diameter and mass were continually revised downward. After the discovery of its largest satellite, Charon, in 1976, astronomers were finally able to determine it accurately. It turned out that Pluto’s mass is 450 times less than Earth’s mass. In comparison, the mass of the Moon is only 81 times less than the mass of our planet. As for its diameter, it is 2375 kilometers. In comparison, the diameter of the same Moon is 3475 km.
However, until now, all this did not affect Pluto’s status, as it was still considered a lonely icy wanderer at the back of the Solar System.
The discovery of the Kuiper belt
Back in the 1930s – 1940s, some scientists cautiously speculated that perhaps Pluto was not a single object but part of something larger – a hypothetical belt of icy objects left over from the formation of the Solar System. However, they had no way to test their speculations at the time.
The situation began to change in the 1990s. The digital revolution and the introduction of new telescopes allowed us to confirm the hypothesis of a belt of icy bodies beyond Neptune’s orbit called the Kuiper belt. Findings followed one after another. Initially, however, astronomers found objects hundreds of kilometers in diameter, which could not shake the dominant status of Pluto. Then American astronomer Mike Brown came on the scene.
Mike Brown has dreamed of finding the tenth planet since childhood – and he almost succeeded in realizing his dream. At the beginning of the 21st century, the team he led began to stamp high-profile discoveries. They found such belt objects as Quaoar (1300 km), Sedna (900 km), Haumea (2100 km), and Makemake (1400 km). It is important to note that the diameter of most of them was estimated to be larger at the time of discovery than in reality. The crowning achievement of Brown’s career was the discovery in 2005 of Eris, whose diameter was originally estimated at 3000 km (we now know that its diameter is 2325 km and it is slightly smaller than Pluto).
Source: NASA, ESA, and M. Brown (California Institute of Technology)
All these findings have put before the astronomical community a legitimate question about their classification. After all, it was previously believed that Pluto may have been very unusual but still a single object, but now it turned out that on the border of the Solar System, there are several bodies comparable in size. It was assumed that the Kuiper belt might hide dozens more “Pluto”, which will be found in the coming years.
In this situation, astronomers faced the need to “clean up the mess”. They had two main options: either recognize Eris (which was then considered larger than Pluto) and possibly other large Kuiper Belt objects as planets. Or they could exclude Pluto from the list of planets and reduce the number of planets to eight.
Meeting in Prague
The fate of Pluto was decided at the 26th session of the International Astronomical Union (IAU), held in Prague from August 14 to 25, 2006. It was held in a rather heated atmosphere. Several astronomers wanted to “save” Pluto and proposed a draft resolution that contained the following definition of the planet:
A planet is a celestial body that (a) has sufficient mass to enter hydrostatic equilibrium (assume a circular shape) and that (b) orbits a star without being a star or a satellite of a planet.
If the document was approved, the Solar System would have 12 planets – the company of the eight classical planets and Pluto should have been Eris, Ceres, and Charon (it was proposed to classify as a planet on the basis that the center of mass of the system Pluto – Charon lies outside Pluto).
However, this proposal did not satisfy a very large number of astronomers, who emphasized that if we proceed from the spherical criterion, there are still a few dozen bodies in the solar system that can claim the status of a planet. The definition was also criticized for the fact that, according to it, brown dwarfs are planets, and ejected planets can no longer be considered them. It was also pointed out that in the distant future, as the Moon moves away, the center of mass of the system will shift away from the Earth – and so it too will become a planet. Funnily enough, Mike Brown himself was among those who criticized the proposed formulation.
As a result, after several discussions and debates, another version of the resolution was prepared and voted on on the last day of the IAU meeting. Astronomers adopted the following definition of a planet:
A planet is a celestial body that (a) revolves around the Sun, (b) has sufficient mass to enter hydrostatic equilibrium (assume a nearly spherical shape), and (c) has “space free of other bodies” near its orbit.
Pluto was thus excluded from the number of planets because there is no space free of other bodies near its orbit. As a result, it was reclassified as a dwarf planet. Ceres, Makemake, Haumea, and Eris also received the same status.
Planet or no planet?
Of course, the decision to exclude Pluto from the number of planets was not without criticism. Its opponents rightly pointed out that if the Earth were in Pluto’s place, it too would not be able to clear the neighborhood of its orbit. So, would it cease to be considered a planet? Another object of criticism was that the vote was held on the very last day of the session, when most astronomers had gone home and was attended by less than 5% of IAU members.
But while these arguments are quite valid, they still fail to take into account the overall context. The IAU may indeed have chosen a far-from-ideal formulation. However, the main purpose of the meeting was to bring order and end the confusion about the number of planets and the misunderstanding of whether Eris is a planet or not.
In this situation, astronomers used a precedent from the 19th century. The fact is that the first few discovered asteroids were initially also considered planets. However, after astronomers continued to discover more and more asteroids, it became clear that we were talking about a completely different group of bodies, which is incorrect to continue to call planets.
In 2006, the same thing happened. Like Ceres, Pluto was initially thought to be a very strange planet orbiting a region where nothing else exists except for it and its satellites. As with Ceres, it was later discovered that this was not the case, and it was the largest member of a whole new, previously unknown population of bodies. Like Ceres, Pluto has since been removed from the list of planets.
At the same time, one cannot help but recognize that if the decision had been postponed for a few years, Pluto would probably still be a planet. Once again, at the time of the vote, Eris was thought to be larger than Pluto. Moreover, it was predicted that new objects of comparable size would be found in the Kuiper belt shortly. However, this prediction has not yet come true. The images of the New Horizons mission, which visited Pluto, showed an amazing and unlike anything else in the world, which exceeded all the expectations of scientists.
Curiously, the astronomer who discovered Eris, Mike Brown, later published a book with the very eloquent title “How I Killed Pluto”. He never gave up on his childhood dream. In 2016, Brown published a hypothesis that there is a yet undiscovered ninth planet in the solar system (it is possible that he even came up with the title for his next book, “How I Found the Real Ninth Planet”). However, the search for this body has so far led nowhere. Thus, 19 years after its demotion, Pluto is still the largest known object in the Solar System beyond the orbit of Neptune.
Can Pluto regain its status as a planet? Some scientists actively advocate revision of its status, but such initiatives have not yet received wide support in the scientific community. As practice shows, astronomers usually decide to make significant changes in the celestial hierarchy only after some high-profile discoveries that require changing the existing classifications.
However, we cannot completely rule out the possibility that Pluto’s status could be changed again in the future. For example, if it turns out that it is radically different from other Kuiper Belt objects. Or if astronomers manage to discover a new ninth planet. In any case, we should not forget that the official classification does not affect the importance of a celestial body. And Pluto has long ago no one and nothing to prove. He has rightfully taken his place in the hearts of all lovers of astronomy.
