It was necessary to rewrite textbooks. People were angry. Our understanding of the solar system was completely changed on August 24, 2006 when the IAU voted to reclassify Pluto from a planet to a dwarf planet.

Difficulties in the definition of "planet" are exposed today. A planet is defined by the IAU as a body with a nearly spherical appearance and that has cleared debris from its neighborhood. This set of metrics is not universally accepted.

Despite their large size, Earth and Jupiter have not been able to clear asteroids from their regions. There are small worlds that are not considered planets because they are not in the same league as the big ones.

There are places on Earth that look like they're from a different planet.

Elizabeth Howell headshot

Larger issues about how to define any object in the solar system or even in space are raised by the demotion of Pluto. Sometimes science can't fit objects into easy categories. It's not clear how to assess the many non-circular objects that circle our sun if the definition of a planet expands again. The big band of small objects between Mars and Jupiter is referred to as the asteroid belt. If a planet is broken up, what will happen?

Almost two decades later, many still don't understand why Pluto was knocked from its planetary position. The ability to change seemingly steadfast definitions in light of new evidence is one of the greatest strengths of science.

What is a planet, anyway?

The word planet is derived from the Greek word "planetes," which means wanderer star. The five classical planets are visible to the naked eye and can be seen shifting in strange pathways across the sky compared with the more distant stars.

Astronomers discovered two new planets, which are too faint to see with the naked eye, after using telescopes. During antiquity, the naked-eye planets were observed around the world.

It was initially categorized as a planet by the scientific community of the day. As further measurement showed it was smaller than other planets, that started to change. We now know that hundreds of thousands of these asteroids are in the asteroid belt.

The IAU was formed in 1919, but the discovery of a tiny dot in the sky was made in 1930. The solar system's newest candidate was seen as an odd one. For the next 20 years, it will get closer to the sun than Neptune will ever be. The plane of the other solar system is called the ecliptic.

The first Kuiper Belt object, 1992 QB1, was discovered in 1992. The objects were found to be similar to the asteroid belt between Mars and Jupiter. In July 2005, the distant body of Eris was discovered, which at first was thought to be larger than the dwarf planet.

earth on the right, the moon in the upper left and pluto to the lower left.

Size of Pluto (lower left) in comparison to Moon and Earth. (Image credit: Tom.Reding, CC BY-SA)
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A planetary conundrum

It was necessary for researchers to ask themselves if Eris was a planet as well. What about the littler objects in the asteroid belt. Where was the cutoff line for defining a body as a planet? A word that had appeared simple was suddenly shown to be very slippery.

There was a lot of new ideas for the definition of a planet.

In 2005, Brian Marsden, a member of the IAU Executive Committee in charge of coming up with a new meaning for the word planet, told that "every time we think some of us are reaching a consensus, then someone says something to show very clearly that we're not

The dilemma hung like a dark cloud over the IAU General Assembly meeting in 2006 when astronomer were no closer to a solution. Four different proposals were offered at the conference. It was proposed that the total number of planets in the solar system be increased to 12.

astronomer Mike Brown of Caltech said the suggestion was a complete mess.

The astronomer voted to create three new categories for objects in the solar system. Only Mercury through Neptune would be considered a planet. dwarf planets are round objects that share the neighborhood of their orbital path with other entities Small solar system bodies are the objects that are around the sun.

The New Horizons mission and the planet debate

Professionals took the decision very seriously. "I'm embarrassed for astronomy because less than 5 percent of the world's 10,000 astronomer participated in the vote," said Alan Stern, a leader of NASA's New Horizons mission.

The flyby by Pluto showed a world that was far more dynamic than anyone had thought. Large mountains, battered craters and signs of liquid flowing on its surface all point to a world that underwent massive geological change since it was formed. Since it is a dynamic place and not static, it should be considered a planet.

There is a red cap on the pole of Charon that appears to change appearance with the slow seasonal change in the solar system. Two planets, Mercury and Venus, do not have many moons. The definition of a planet is even more complicated due to the fact that many asteroids and dwarf planets have moons.

Many people in the public share similar views. The definition of a planet was debated by experts at the CfA before the flyby. A science historian who chaired the IAU's planet-definition committee said that "planet is a culturally defined word that changes over time." The audience watching the debate chose a different definition of what a return to the planetary fold would look like.

There are alternative classification schemes popping up all the time. A planet is a round object in space that is smaller than a star. This would bring the total number of officially recognized planets in the solar system to more than 100. The IAU's definition was "hastily drawn" and "flawed" and should be reconsidered, according to an opinion article written by two scientists.

Will Pluto ever become a planet again?

It seems unlikely that the IAU will revisit the controversy in the near future. The simple fact is that when it was first discovered, it was never on the same footing as the other eight worlds.

Mike Brown joined the conversation. Pluto is not a planet at this time. Never was. For 50 years, we thought it was something else. We now know what we're talking about. Nostalgia for Pluto isn't a very good argument, but it is what it is. Brown has embraced his role in the redefinition with the handle "plutokiller."

Why does it matter?

Calculating what makes a planet and what doesn't can be difficult, and we may have to look beyond our solar system in considering what makes a planet and what doesn't.

There are more than 5,000 planets outside our solar system that have been discovered to date. The types of planetary environments to consider are changing rapidly.

The proliferation of planet-forming options shows us that each solar system has its own environment. Stars can form planets from collapsing gas and dust in their environment, but the individual dynamics that control planet formation are far more complex. Is it possible that there are more than one star involved? What amount of dust is out there? Is there a black hole that is blowing dust and gas away from planets?

If planets are lucky enough to grow up, how they are connected with other planets is not well understood. As worlds interact with each other, their mutual gravities seem to shift planets closer and closer to their parent star or in some cases, to fling worlds out of the solar system.

Our definition of a planet may need to become more situational to account for the many scenarios in which a world may form. It is possible that planets are tied to particular formations or zones. As we collect data, planethood and the debate that has arisen will continue to be debated for quite some time.

Additional resources

NASA has a website where you can learn more about the Kuiper Belt. You can see images from the flyby ofPluto.


There is an international astronomy union. The theme is "Pluto and the Developing Landscape of Our Solar System"

The Applied Research Laboratory is part of the JohnsHopkins There will be a new year in2022. There is a new tab on the website, called "New Horizons."

There is a space agency called NASA. The year ends on December 19th. "What is a planet?" is a question on the solar system's website.

The solar system is being explored by NASA science. The "Kuiper Belt" can be found on the solar system.

Siegel is the name of the person The year ended on May 8th. You won't like the consequences of making a planet again You won't like the consequences of making pluto a planet again.