In the history of world space science, August 24, 2006 is an unforgettable day. Since this day, mankind’s previous understanding of the Solar System has been changed, the information in the textbooks has been rewritten, and the public has been very dissatisfied.

That was the day scientists at the International Astronomical Union (IAU) voted to remove Pluto from the list of planets in the Solar System and classify it as a dwarf planet. This was a major change that looked like a sort of Pluto downgrade and is still talked about to this day.

The IAU has defined a planet as an object orbiting the Sun, having a spherical appearance, and having absorbed all the debris in the vicinity of its orbit. Although this definition is not widely agreed.

Neither Earth nor Jupiter will remove all asteroids from their orbits, even though these two planets are larger than those asteroids. In addition, the dwarf planet Ceres is also spherical and orbits the Sun but is not considered a true planet.

The downgrade of Pluto to a dwarf planet has raised questions about how to identify any entity that exists in the Solar System in particular and in the universe in general. It shows the confusion of science when it is not possible to arrange entities into clear categories.

Many people still do not really understand why Pluto has been downgraded like that, the Solar System has been reduced from 9 planets to only 8 planets. And the scientific community has given a lot of evidence to convince the public of this change.

What is a planet?

The word planet in English means Planet – is a word from ancient times. It comes from the Greek word “planetes” which means wandering star. All 5 earliest discovered planets, Mercury, Venus, Mars, Saturn, Jupiter are visible to the naked eye and can be seen moving in the sky relative to the position of other stars. .

Since the appearance of the telescope, astronomers have discovered two new planets, Uranus and Neptune. These two planets are difficult to see with the naked eye because they are so dim.

When astronomers discovered the dwarf planet Ceres, they initially classified it as a planet, but later calculations showed it to be smaller than the planets known at the time due to that Ceres is ranked in a group of rocky bodies, called asteroids.

Pluto was found and classified as a planet in 1930, it should be noted that the IAU was born in 1919. Astronomer Clyde Tombaugh of the Lowell Observatory in Arizona discovered the star. Pluto, after comparing photos of the sky over many different days, noticed a small dot appearing between the stars in the sky.

However, at that time Pluto was considered an odd planet because its orbit was so eccentric, that in its 248-year long orbit around the Sun, it had 20 years to come close to the Sun. Heaven than Neptune. In addition, it also has a very large tilt of up to 122.53°.

Size comparison between Earth, Earth’s Moon and Pluto (bottom left).

In 1992, astronomers discovered the first Kuiper Belt object, named 1992 QB1. It is a small celestial body that orbits Pluto’s vicinity and is completely outside the orbit of Neptune. Many such objects were later discovered showing humanity a belt of celestial bodies similar to the asteroid belt between Mars and Jupiter.

In 2005, scientists also discovered an object called Eris that is believed to be even larger than Pluto.

Hard questions about the planet

At that time, scientists had to ask themselves a skepticism: If Pluto is a planet, then Eris is also a planet. What about all the icy bodies other than the Kuiper belt or the smaller objects in the asteroid belt? Where is the boundary to classify a planet?

Much debate has taken place, with many new proposals on the definition of a planet being put forward by the parties. And to find consensus is not easy because opinions are always challenged in some way.

Until 1 year later, scientists did not come to a final conclusion, and this dilemma engulfed the IAU General as‌sembly meeting in Prague 2016. Here, scientists experienced through 8 days of debate with 4 different proposals. There is even an idea that the number of planets in the Solar System should be 12, including the largest asteroid Ceres and Pluto’s moon Charon.

Pluto’s Moon Charon.

Caltech astronomer Mike Brown, who discovered the celestial body Eris, says the idea doesn’t fit at all.

Near the end of that year’s conference, 424 astronomers voted to create three new types of entities in the Solar System. Accordingly, there are only 8 planets from Mercury to Neptune, and Pluto and other bodies that share their orbital neighborhoods and other entities are called dwarf planets. All other objects orbiting the Sun would be called asteroids in the Solar System.

New Horizons spacecraft mission and planetary debate

This vote left many unhappy, including Alan Stern, head of NASA’s New Horizons spacecraft mission, who said that less than 5% of the 10,000 astronomers worldwide. participate in the vote. Yet the outcome of the vote was highly variable.

The New Horizons spacecraft, which passed by Pluto in 2015, took many pictures of its surface, creating an important turning point in the debate. On the surface of Pluto there are large mountains, craters and even signs of liquid flowing on the surface. All that evidence suggests that Pluto has undergone major geological changes since its formation.

New Horizons spacecraft images of Pluto’s surface show that complex geological activities are more powerful than previously thought.

Based on that, thinkers like Alan Stern argue that Pluto should be classified as a planet because it is a place of internal movement, not static as one thinks.

Pluto’s moon Charon is also a notable entity, it has a red area located on the poles and it seems to change with the seasons, only the seasons on it last much longer than on Earth. long. On the other hand, Pluto has several moons orbiting while the other 2 recognized planets Mercury and Venus don’t even have any moons. However, many asteroids and dwarf planets also have moons, which complicates the definition of a planet.

Such views are quite widely supported in the community, but there are frequent debates among experts. An opinion released in 2017 defined a planet as a circular object in space less than a star in size. With this proposal, Pluto will be reclassified as a proper planet. But if so, then the Earth’s Moon and many other moons in the Solar System are also suddenly recognized as planets. And the total number of planets in the Solar System will reach 110.

Will Pluto be classified as a planet of the future?

Astrophysicist Ethan Siegel once stated that, in fact, Pluto has been misclassified since its discovery, it cannot be a true planet like the other eight planets in the Solar System. .

Astronomer Mike Brown also thinks that Pluto is not a planet yet, just that people have misunderstood it for 50 years. We now have a deeper understanding and should accept reality.

Even so, the IAU can still review the controversy at any time to make changes if deemed necessary.

Why has this become important?

To answer the next generation about whether Pluto is a planet or not, perhaps scientists need time to study more deeply about the universe, beyond the Solar System to search and consider. What is a planet and what is not a planet.

To date, humanity has found more than 5000 exoplanets – distant planets located outside the Solar System that reflect an infinitely large universe. Many super-Earths have been discovered (planets that are larger than Earth but smaller than Uranus, or planets like Jupiter but have very hot temperatures, and there are a whole bunch of planets the size of Uranus). difference.

These show us that each solar system has its own characteristics. Planet formation is also very different and is influenced by many factors such as stars, black holes or supernovas. And when planets are formed, how the connection between them and other planets will be, how the parent star’s impact will still be a mystery.

Therefore, the definition of a true planet is likely to change in the future to better match what science has discovered that we do not know today. Maybe it will be tied to specific circumstances and regions.

For now, the only thing certain is that the planetary and Pluto debates will continue for quite some time.