In August 2006, Pluto lost its status as the ninth planet of the solar system. Not on a whim: due to three specific criteria adopted by astronomers gathered in Prague. Here are the criteria and why Pluto no longer meets them.
In August 2006, Pluto lost its status as the ninth planet of the solar system. Not on a whim: due to three specific criteria adopted by astronomers gathered in Prague. Here are the criteria and why Pluto no longer meets them.
Pluto was discovered on February 18, 1930 by American astronomer Clyde William Tombaugh (1906-1997). For more than seventy-five years, it held the position of the ninth and last planet in the solar system, the smallest and farthest from the Sun.
At that time, the order of the planets was memorized in school, from closest to farthest from our star: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and at the very end, Pluto. This small icy body brought up the rear, at an average distance of 5.9 billion kilometers from the Sun.
Interestingly, 1930 is also the year when the International Astronomical Union published its official list of 88 constellations dividing the sky. In other words, astronomy was then in the midst of organizing the heavens... and it was this same institution that, much later, would reshuffle the cards to Pluto's detriment.
If Pluto is no longer considered a planet, it is due to a decision made in Prague in August 2006 during the General Assembly of the International Astronomical Union (IAU). Astronomers gathered there to reassess the number and, above all, the "quality" of the planets in the solar system.
The result was unequivocal: Pluto was downgraded from its status as a planet to that of a dwarf planet. The IAU even went so far as to name this new category after the fallen goddess, as this type of object is also referred to as a "pluton."
This was not the only possible outcome. Another proposal had circulated: to expand the list to twelve planets by adding Ceres, Charon, and the distant 2003 UB 313 alongside the nine already recognized bodies. Ultimately, astronomers preferred to tighten the definition rather than indefinitely extend the list.
The definition adopted by the International Astronomical Union in 2006 is based on cumulative and verifiable conditions. To deserve the title of planet, a celestial body must be in orbit around a star, not be a star itself, have a diameter greater than 800 km, and possess sufficient mass for its own gravitational forces to give it roughly a spherical shape.
This last point deserves attention. A body that is too light retains the irregular shape of a giant pebble: this is the case for most asteroids. Beyond a certain mass, however, gravity compresses the material towards the center and rounds the object. Roundness thus becomes a measurable indicator of the body's actual mass.
Nota Bene: the "dwarf planet" is not a failed planet; it is a category in its own right. It includes spherical bodies in orbit around the Sun that do not meet all the requirements set for the class of true planets.
Pluto no longer meets the required conditions because the criteria voted on in 2006 do not merely require a round celestial body orbiting the Sun: they impose a true "quality" of planet that this small body does not possess. With an equatorial diameter of 2,400 km and a mass representing only 0.17 times that of Earth, Pluto was already the smallest of the nine traditional planets.
Its orbit further distinguishes it from the other eight. While the planets describe relatively regular trajectories, Pluto follows a very elongated orbit: during its revolution of 247.7 years, it approaches the Sun to within 4.4 billion kilometers, then moves away to as far as 7.4 billion. A planet that changes its distance by three billion kilometers stands out in the landscape.
The last element, and perhaps the most decisive: Pluto is not an isolated case. Astronomers have identified other comparable spherical bodies, such as Ceres or 2003 UB 313. Keeping Pluto classified as a planet would have required including a whole family of objects of the same nature, hence the creation of a specific category.
Charon is the large satellite of Pluto, and its size is so unusual that some astronomers have long referred to it as a double planet rather than a satellite. Its diameter reaches 1,270 km, which is half that of Pluto, whereas in the solar system, a satellite is usually considerably smaller than the planet it accompanies.
In addition, there is a staggering proximity: only about 17,000 km separate the two bodies. To measure what this represents, remember that Mercury, the planet closest to the Sun, is 58 million kilometers away. Charon orbits Pluto in 6.4 days, exactly the rotation period of Pluto, making its orbit geostationary: the satellite remains permanently above the same point.
This duo has fueled all sorts of hypotheses: a simple moon, half of a double planet, or even the tenth planet of the solar system for the more daring. The situation, which remained unclear for years, was resolved during the same session in August 2006 that reexamined Pluto's status, with Charon being classified into the new category created on that occasion.
Pluto is not alone in its category: Ceres and 2003 UB 313 share with it the status of dwarf planet, alongside Charon. What brings them together is a nearly spherical shape and an orbit around the Sun, but a size too modest to join the club of eight planets.
Ceres has been known since 1801 and is located in the asteroid belt, between Mars and Jupiter. Its diameter is about 950 km, its mass is 9.445 x 10 to the power of 20 kilograms, and it completes its orbit around the Sun in 4.6 Earth years. It is precisely its roundness that argued in its favor during the 2006 re-examination.
2003 UB 313, discovered in 2005 by American astronomer Michael Brown (born in 1965), is the most distant body in the solar system. Made of ice and methane, it is located 97 astronomical units from the Sun and takes about 557 years to complete its orbit. Its discoverers proposed naming it Xena, a name submitted for approval by the International Astronomical Union.
Nota Bene: the astronomical unit (au) is the standard for distances in the solar system. One au is approximately 150 million kilometers, which is roughly the distance between the Earth and the Sun.
The physical characteristics of Pluto make it an extreme and perfectly inhospitable world: the surface temperature drops to -230 °C and the dwarf planet is devoid of atmosphere. Observed through a telescope, it presents a recognizable yellowish color.
Its composition would explain this hue: Pluto is thought to be made up of a rocky core surrounded by a layer of methane frozen by the cold. Its density of 2.1 (with water being 1) confirms this mixture of rock and ice, much lighter than the iron and nickel that make up the cores of planets close to the Sun.
Here are its main numerical data:
Equatorial diameter: 2,400 kmMass: 0.17 times that of EarthGravity: 0.38 times that of EarthAverage distance from the Sun: 5,900,000,000 kmRotation period: 153 hours, or 6.4 daysRevolution period: 247.7 yearsOrbital speed: 4.7 km per secondEscape velocity: 5 km per secondSatellites: 3
Above all, note the eccentricity of its orbit: unlike other planets, Pluto oscillates between 4.4 and 7.4 billion kilometers from the Sun over its nearly 248-year revolution. It is this atypical behavior, as much as its small size, that has long made it difficult to classify.
Yes, Pluto's status continues to spark discussion, and it's not illogical: the decision made in August 2006 resolved a situation that astronomers themselves described as vague. Before Prague, several positions coexisted, with some scientists viewing the Pluto-Charon pair as a double planet, while others even called for Charon to be classified as the tenth planet.
The alternative scenario of a solar system with twelve planets, including Ceres, Charon, and 2003 UB 313, clearly shows that the classification was more about a choice of definition than a physical certainty. Shifting the threshold of diameter or mass would have been enough to produce a very different list. The same ambiguity is reflected in the names themselves: 2003 UB 313 is still waiting to find out if it will officially be called Xena.
One thing is certain: the classification of the sky evolves with instruments. Increasingly powerful telescopes reveal new bodies every year, allowing for the discovery of over two hundred new comets in a single year (a record reached in 2005). There is nothing to prevent a future discovery from necessitating a reevaluation of the boundaries between planets and dwarf planets.
In summary, remember three things to never get it wrong again: Pluto, discovered in 1930, was reclassified as a dwarf planet in Prague in August 2006; the criteria require a celestial body orbiting a non-stellar star, over 800 km in diameter, and sufficiently massive to be spherical; and Pluto now shares its category with Ceres, Charon, and 2003 UB 313. The next time you recite the planets of the solar system, stop at Neptune... all while knowing why.
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