Almost a century ago, on March 14, 1930, newspapers around the world echoed the discovery of Pluto with headlines like this: “Discovered the ninth planet on the edge of the solar system: the first found in the last 84 years.”
Its classification as a planet was maintained for decades, but just 15 years ago, on August 24, 2006, it lost that status and was classified as a ‘dwarf planet’, after a heated debate in the XXVI General Assembly of the International Astronomical Union (IAU) held in Prague (Czech Republic) and the subsequent vote: 237 votes in favor of the change, 157 against and 17 abstentions.
Few events have divided the global astronomical community, both hobbyist and professional, as much as the reclassification of Pluto. However, this type of reclassification of objects once their nature has been better understood is not alien to the history of astronomy itself.
There is the case of Ceres. When Giuseppe Piazzi announced his discovery on January 24, 1801, the only known objects in the solar system were the eight planets, some of their moons, and comets. Logically, and after calculating its orbit with reasonable precision, Ceres was initially classified as a planet.
Few events have divided the global astronomical community, both hobbyist and professional, as much as the reclassification of Pluto
Asteroids and minor planets
But the years passed and although books and other educational materials continued to talk about the planet Ceres, the astronomical community continued to find objects with similar properties. Herschel coined the term ‘asteroid’ in 1802 to refer to them, the British Almanac The Nautical Almanac and Astronomical Ephemeris began calling them ‘minor planets’ in 1841, and the US Naval Observatory as ‘small planets’ in 1868.
The words asteroid and minor planet were quickly accepted by the astronomical community and the standard designations (1 Ceres, 2 Pallas, 3 Juno, etc.) for asteroids were adopted in 1851.
No official vote was necessary to reclassify Ceres. Time and the absence of an official guideline made possible a gradual and trauma-free transition.
As new discoveries were made, the astronomical community understood that these were fundamentally different from the known planets and gradually stopped referring to them as planets, beginning to call them asteroids, minor planets or minor bodies, although this last meaning also includes the kites.
In fact, it is rare to find references to Ceres or one of the main belt asteroids as planets in twentieth-century publications. The case of Pluto has some parallels with that of Ceres, although the former has been officially reclassified.
A few years earlier, on August 30, 1992, David C. Jewitt and Jane X. Luu observing from the Mauna Kea observatory in Hawaii (USA) they discovered 15760 Albion, whose provisional designation was 1992 QB1. It was the first time that an object as distant as Pluto had been discovered and the press was quick to refer to it as the tenth planet.
But it did not last long: hundreds of so-called trans-Neptunian objects were discovered between 1992 and 2006, when the IAU decided to reclassify Pluto. Some have orbits similar to yours, others move in much more eccentric and distant trajectories.
Several of the trans-Neptunian objects discovered, specifically Eris (whose study led to Pluto’s ‘banishment’ as a planet), Makemake and Haumea, rival or surpass Pluto in size. Along with Ceres, are the five Tiny planets.
The case of Pluto bears certain parallels with that of Ceres, although in the latter time and the absence of an official guideline made possible a gradual transition without trauma from planet to dwarf planet.
Pluto was no longer a unique, peculiar or special object. Like Ceres, it was now clear to many that it is fundamentally different from the eight classical planets, even though it has also been visited by our space probes. Specifically, by the ship New horizons from NASA, which on July 14, 2015 approached a distance of 12,500 km from Pluto.
However, its mysterious nature had captivated the imagination of both the general public and sections of the astronomical community for decades. A drastic change like the one voted on that August 24, 2006 had to be necessarily controversial and elicit mixed reactions.
Turn the page to a controversy that follows
Unfortunately, the controversy associated with the decision to reclassify Pluto as dwarf planet it still persists fifteen years later. Perhaps it is time to look ahead and put aside unproductive controversies. At the time, Pluto represented the jump of a barrier, that of the 30 astronomical units or AU (1 AU is the average distance between the Earth and the Sun, and 30 is the average distance between Neptune and the Sun).
But the new discoveries have taken us much further, opening previously unexplored horizons, clearing up old unknowns and bringing us countless new questions to solve.
The Pluto controversy has dimmed the recent crossing of a new barrier, the 100 AU barrier that roughly defines the so-called termination shock front and the heliopause or boundary that separates the solar wind from the interstellar medium.
On December 17, 2018, the discovery of 2018 VG18 or FarOut, the first object in the solar system found at a distance beyond 100 AU. It was found by American astronomers Scott S. Sheppard, David J. Tholen and Chadwick Trujillo almost 124 AU from Earth.
The 100 AU window is now open and the astronomical community is ready to start exploring it. Astronomers Malena Rice and Gregory Laughlin of Yale University (USA) announced in December 2020 the discovery, with the TESS space telescope, of eight objects located more than 100 AU, including a candidate located more than 200 AU.
New planets in the solar system?
But this may only be the beginning. The Atacama Cosmology Telescope (ACT) has observed in microwaves multiple candidate objects located at distances between 300 AU and 2000 AU. The collaboration headed by Sigurd Naess believes that some of them could correspond to a celestial body with a mass equal to or greater than the five terrestrial masses; that is, it would be a planet itself, but lower in mass than Uranus and Neptune.
The confirmation and eventual study of objects as remote as these require the largest telescopes available, such as the Great Canary Telescope (GTC) 10.4 meters in diameter, currently the largest fully steerable operational optical telescope.
GTC has been able to observe 2018 VG18 and 2018 AG37 and a poll has begun to try to retrieve the candidates announced by the ACT collaboration and perhaps new ones. The first observations have already been made and the data analysis is ongoing. Preliminary results are expected before the end of the year.
The discovery of Pluto was only the beginning, thanks to having larger telescopes and better technology we can now go much further. New scientific controversies are likely to arise, but the exploration of the more distant solar system, which began with the discovery of Pluto, will undoubtedly bring us great discoveries in the coming years.
The Gran Telescopio Canarias has already observed objects located at more than 100 AU and has begun a survey to try to confirm candidates that includes a possible new planet
* Carlos de la Fuente Marcos He is an astronomer from the Complutense University of Madrid (UCM), a specialist in trans-Neptunian objects.