Tech UPTechnologyWhy did Pluto stop being a planet?

Why did Pluto stop being a planet?

Science, unfortunately, is not usually a popular topic among the general public. Rarely is current affairs defined by a scientific discovery and it almost never stirs passions like politics or sports do. However, from time to time there are advertisements that manage to reach the public opinion and get great attention. One of these announcements was undoubtedly the one made by the International Astronomical Union in August 2006 , when it announced that a redefinition of the term planet had been agreed upon, the main and most discussed consequence of which would be that Pluto would be considered a dwarf planet along with other bodies. of similar characteristics.

That day we went from having nine planets in the solar system to having only eight. After that announcement it was necessary to rewrite the textbooks. But what motivated it? Of course, while there were tears streaming down Earth over Pluto’s reclassification, out there in outer space, nothing happened. Pluto continued its orbit around the Sun, accompanied as always by its large moon Charon and its four other smaller moons. Ultimately, what changed was not Pluto, but the rest of the Kuiper belt, or at least our understanding of it.

Between 2002 and 2005, a team of astronomers led by Michael Brown, from the Palomar Observatory in California, United States, discovered a series of bodies located in the Kuiper belt. Several of these bodies were about 1,000 kilometers in diameter, like Makemake or Gonggong , and a couple were comparable in size to Pluto, like Eris or Haumea. In addition, since 1801 Ceres was known, a body also almost 1000 kilometers in diameter that orbited within the asteroid belt, located beyond Mars. Given these discoveries, the question of how to proceed arose. Should these bodies be included in the list of planets? If so, we would end up with about 15 planets in the solar system, with the largest being more than 100 times as wide as the smallest. But if we decided not to include them, it would mean that we were arbitrarily choosing Pluto as the lower limit of what constituted a planet.

Given the situation, the International Astronomical Union, the world’s highest body in astronomy, met in August 2006 with the aim of finding a solution to this problem. What was decided there was to create a new category , a new type of astronomical object, called a dwarf planet, which would encompass some of the bodies discovered by Michael Brown’s team, as well as Pluto and Ceres.

But the IAU did not make this decision at random, but they did so after modifying the definition of a planet. In science and in any human field, we usually create categories in which to classify different elements in order to make our task (whatever it is) easier. But these categories sometimes do not have a clear delimitation . For example, if we decide that the rainbow has seven colors and only seven, we can use this definition in areas where color is not very important. We will be able to teach children at school the name of those colors, for example. But if we want to analyze the colors present in a real-world photograph, we probably need to resort to new definitions. The green of each leaf is different, the blue of the sky changes according to the time of day, the red and the orange of the fire blend with each other. In the same way , if we use a very basic definition of a planet, we can get confused.

That is why the IAU decided to create a more precise definition of a planet. This definition consisted of three requirements to be met by the different candidate bodies. To be considered a planet, an object in the solar system had to:

  • Orbit directly to the Sun (in this way no satellite could be considered a planet, even if it met the other conditions).
  • Have enough mass to reach a hydrostatic equilibrium , that is, a practically spherical shape (in this way the solitary asteroids or comets that orbit the Sun would not be planets)
  • Having cleaned the neighborhood of its orbit (this was the point that Pluto and the rest of the dwarf planets did not meet).

This third point turned out to be the most controversial, since no planet in the solar system has 100% cleared its orbit of other bodies, not even Jupiter. Clearing the orbit means that there are no other bodies directly orbiting the Sun that share the same orbit as the planet . Jupiter has hundreds of asteroids located at its Lagrange points (the so-called Trojan asteroids) sharing its orbit. Earth also has a couple of them that we know of. However, these cases are not comparable to the situation of Pluto, Eris or Haumea. In fact, Pluto accounts for only 7 percent of the mass of all the objects that share its orbit , while the Earth or the rest of the planets accumulate more than 99% of this mass.

Of course this reclassification did not make Pluto any less interesting , or less worthy of our study. Nine years after it happened, in 2015, Pluto was visited by the New Horizons probe , which gave us an extraordinary amount of information about the dwarf planet.

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