What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky

Why is Pluto no longer considered a regular planet? 
Why did scientists change their minds? 
Has anything like this happened before? 
What is a planet? 

Is Pluto a planet or not? Very shortly after the discovery of Pluto in 1930, some astronomers wondered if Pluto should be called a planet because it was so small. However, at that time, astronomers knew very little about the solar system as far out as Pluto, so they did not have any information to discuss this idea fully. 

But, why did astronomers even question the idea that Pluto is a planet in 1930? Well, let’s go back 129 years to the year 1801 when the first asteroid, Ceres, was discovered. At that time, astronomers decided to call Ceres a planet. But, as more asteroids were discovered during the next 50 years, astronomers realized that there was something different about these planets. For one thing, there were a lot of them orbiting around the Sun in the same area. And, all of them were very small, much smaller that our Moon. So astronomers talked about this and decided to stop calling them planets. They decided to call them Minor Planets or Asteroids because they were nothing like the eight known planets. 

Then, at the end of the 1950s, the astronomer Gerald Kuiper (KYE-per) thought that a belt of icy debris might exist beyond Neptune. He believed that some comets, like Halley’s comet might come from this area. Some time after this, other astronomers started to say that Pluto might be just a large chunk of this icy debris, much like Ceres is a large chunk in the Asteroid Belt. But these were just ideas and there was no evidence that these ideas were true. 

Finally, in the early 1990s, astronomers started to find small icy objects beyond Neptune using new telescopes and cameras. In the following years, hundreds of small objects were discovered orbiting beyond Neptune, but it was not until 2003 that they found the larger ones like Sedna and Eris (Eris had been nicknamed Xena). 

This outer icy belt of objects is now called the Kuiper Belt in honor of Gerald Kuiper. And, the discovery of Sedna and Eris provided good evidence that Pluto was just one of several large round icy objects in this outer icy debris field. 

A Wandering Star
Part of the reason that Pluto was called a planet for 76 years is because the definition for the word “planet” was very old and not specific. It meant “wandering star,” a meaning given by the Greeks over 2,000 years ago, way before telescopes were invented. This vague definition is a reason why the asteroids were called planets when they were first discovered, starting in January of 1801. 

Original Definition of the word planet 
The word planet is a Greek word that means wandering star. This definition is over 2000 years old. And, long ago the Sun and Moon were also considered planets because they wandered or move through the stars in the sky. 

New Definition of the word planet
In the year 2006, there was a meeting of astronomers, from all over the world, in Prague, the capital city of Czechoslovakia. They decided to define a planet as a large and naturally round body that orbits the Sun and does not have other nearby objects that also orbit the sun. Most small objects less than 500 miles in diameter or length are generally not round. Objects about 500 miles and larger in diameter are usually round because they have a greater amount of gravity to shape them. Larger, naturally round objects in the Asteroid and Kuiper Belts are considered dwarf planets. This includes Ceres, Pluto and Eris (“Xena”). So, on August 24, 2006, Pluto was formally demoted from a Planet to a Dwarf Planet.

Science Marches Forward 
Astronomers are scientists who study objects in space. Scientists are people who want to understand how nature works. They collect information by examining objects and performing experiments. 

Often, it takes scientists years to collect information on objects. During this time, they can come to wrong conclusions because they don’t have all the information or they might interpret it incorrectly. But scientists are willing to change their views or ideas with new information or insight. This is how science works. And this is the major reason why Pluto is no longer a planet. New information garnered about our solar system clearly pointed to Pluto being something less than the 8 major planets. 

Largest Kuiper Belt Objects 

Ceres is the largest asteroid between Mars and Jupiter and is listed for comparison. 

Name / Discovered / Approximate Diameter 
• Ceres / 1801 / 957 km or 594 miles •
Pluto / 1930 / 2,306 km or 1,433 miles 
Ixion / 2001 / 500 km or 310 miles 
Quaoar / 2002 / 1,300 km or 807 miles 
Orcus / 2004 / 1,500 km or 930 miles

Sedna and Eris are not technically in the Kuiper Belt, but farther out and might be members of the more distant Oort cloud (left over material farther out than the Kuiper Belt).

Sedna / 2003 / 1,500 km or 930 miles 
Eris (“Xena”) / 2003 / 2,326 km or 1,445 miles

Kuiper Belt Object Facts 

Largest Kuiper Belt Object: As of 2024, the largest Kuiper Belt Object is Pluto. 

Distance from the Sun: The Kuiper Belt extends from 2,800,000,000 miles, which is a little past Neptune, to 4,700,000,000 miles. Pluto averages 3,680,000,000 miles from the Sun. So, about 3 to 5 billion miles. C omposition/Temperature: Frozen ices (including methane, ammonia and water) and rock-type material. The temperatures of objects varies from –390 to less than –430 degree Fahrenheit. 
Revolution period about the Sun: Varies from 165 to over 300 years depending on the distance the Kuiper Belt object is from the Sun. Number of objects: Estimated in the billions. 800 of the largest objects were discovered from 1992 to 2007.