How many planets are in our solar system? Is it eightโฆ or nine?
For a long time, children around the world were taught that our solar system had nine planets, including a small, distant one named Pluto. But in 2006, Pluto was reclassified. It was no longer considered a โrealโ planet, and the official number of planets dropped to just eight.
Why did this happen? Why was Pluto removed from the list of planets? And what exactly is a planet anyway?
In todayโs episode of Thinking in English, weโll look at what a planet actually is, why Pluto used to be a planet but isnโt anymore, and how science changes over time. Weโll also talk about the history of planet discoveries, like Neptune and Uranus, and how our understanding of the solar system has developed.
This episode is also a great chance to learn scientific vocabulary and practice thinking critically in English. Weโll explain important terms like orbit, gravity, dwarf planet, and clearing its orbit. And weโll discuss some fascinating facts about space, science, and the nature of definitions themselves.
Soโฆ is Pluto really a planet? Or not? Letโs find out.
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Vocabulary
- Planet (Noun): A large, round object in space that orbits a star, like the Sun.
- Earth is the only planet in our solar system known to support life.
- Solar System (Noun): The Sun and all the objects that orbit around it, including planets, moons, and asteroids.
- The solar system includes eight planets, dozens of moons, and countless asteroids.
- Orbit (Verb): To move around something in a circular or elliptical path.
- The Moon orbits the Earth once every 27.3 days.
- Astronomer (Noun): A scientist who studies stars, planets, and other objects in space.
- The astronomer used a powerful telescope to observe a distant galaxy.
- Object (Noun): Anything that has a distinct shape and is considered a thing or item, particularly in space.
- Pluto is a small icy object located in the Kuiper Belt.
- Telescope (Noun): An instrument used to observe distant objects in space.
- Galileo built one of the first telescopes and used it to study the Moon.
- Prediction (Noun): A statement about what will happen in the future based on evidence or reasoning.
- The scientist made a prediction that a new planet might exist beyond Neptune.
What Is the Solar System?
Before we can talk about Pluto, and whether or not itโs a planet, we need to understand the solar system.
The solar system is the name we give to the Sun and everything that orbits around it. To orbit means to travel in a circular or oval path around something. When we say that Earth orbits the Sun, we mean that Earth moves around the Sun in a regular path, taking one year to complete one full orbit.
The solar system includes:
- Planets
- Moons (which orbit planets)
- Asteroids (small rocky objects)
- Comets (icy objects with long tails)
- And other things like dust and gas.
Today, scientists say there are eight official planets in the solar system. They are, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
These planets can be divided into groups. Mercury, Venus, Earth, and Mars are small, rocky, and have solid surfaces. Jupiter and Saturn are gas giants, and Uranus and Neptune are ice giants. They are much bigger, made mostly of gas and ice, and donโt have a solid surface like Earth.
But the solar system doesnโt end with Neptune. Far beyond Neptune is a region called the Kuiper Belt. This area is filled with icy objects, small bodies, and dwarf planets. Pluto is one of those icy objects, and for a long time, it was considered the ninth planet in the solar system.
The solar system is much bigger and more complex than many people realise. Itโs not just eight planets. There are hundreds of thousands of objects moving through space, and our knowledge of the solar system beyond Neptune is very limited.
Discovering New Planets: From Uranus to Neptune to Pluto
When people in the past looked at the night sky, they could see five planets with just their eyes: Mercury, Venus, Mars, Jupiter, and Saturn. These planets have been known since ancient times and by cultures across the world.
But the next planet, Uranus, is much harder to see without a telescope. It wasnโt discovered until 1781 by a British astronomer named William Herschel. This was a big moment in science as it showed that there could be more planets out there that we couldnโt see with our eyes.
Later, scientists noticed that Uranus wasnโt moving exactly as expected. Something seemed to be โpullingโ on it with gravity. A prediction was made that another large planet must be out there beyond Uranus.
Incredibly, Neptune (the 8th planet) was discovered through mathematics rather than observation. The French astronomer Urbain Le Verrier studied the movement of Uranus, and based on his findings calculated the location and size of Neptune without every seeing it. He made a prediction where it would be in the sky, and based on his predictions Neptune was finally discovered in 1846.
After Neptune, scientists thought there might be another planet even farther away. In 1930, an American astronomer named Clyde Tombaugh discovered a small, icy object in the sky. It was later named Pluto, and many people celebrated the discovery of the โninth planet.โ
Pluto was very different, though. It was much smaller than the other planets. Pluto is even smaller than Earthโs Moon. Its orbit was also strange. While most planets orbit in a neat, flat path, Plutoโs orbit is tilted and stretched out.
Still, Pluto was considered a planet for over 70 years. But as technology improved, scientists started discovering more and more Pluto-like objects in the same region of space, called the Kuiper Belt.
In 2005, astronomers found an object called Eris that was even larger than Pluto. This created a problem: should Eris become the tenth planet? And what about the other similar objects?
This debate led scientists to rethink the definition of what a planet actually is.
The Definition of a Planet
Soโฆ what is a planet?
For a long time, there was no official scientific definition. People just used the word “planet” to describe big objects that orbit the Sun. But after Plutoโs discovery, and especially after the discovery of Eris and other similar objects, astronomers realised they needed a clear and formal explanation.
In 2006, the International Astronomical Union (or IAU) made an official definition. According to the IAU, for something to be called a planet, it must meet three conditions:
- It must orbit the Sun.
โ This means the object moves around the Sun, like Earth or Mars. - It must be big enough that its gravity pulls it into a round (or nearly round) shape.
โ Gravity is the force that pulls things together. If an object has enough mass, gravity makes it form a sphere. - It must have โcleared its orbit.โ
โ This means the planet has become the dominant object in its path. It has cleared away other rocks, ice, and small objects near its orbit using its gravity.
Pluto meets the first two rules. It orbits the Sun, and itโs round. But it doesnโt meet the third rule: Pluto hasnโt cleared its orbit. It shares its part of space with many other icy objects.
Because of this, Pluto was reclassified as a โdwarf planet.โ A dwarf planet is a space object that is round and orbits the Sun but hasnโt cleared its orbit.
This decision caused a lot of debate. Some scientists and many members of the public were upset. People had grown up learning that Pluto was the ninth planet, and didnโt want to see it โdemoted.โ
Why Pluto Still Matters
Even though Pluto is no longer officially a planet, it is still very important in our understanding of space.
Pluto helped astronomers realise that the solar system is much bigger than we once thought. After Pluto was discovered in 1930, scientists later found many similar icy objects in the same region, now called the Kuiper Belt.
In 2015, NASA sent a spacecraft called New Horizons to visit Pluto. It took more than nine years to reach it!
When New Horizons arrived, it sent back amazing pictures and data. Before that mission, we had only seen blurry images of Pluto. But New Horizons showed us a world with mountains made of ice, smooth plains, and possibly even an ocean under the surface.
It also has an atmosphere and five moons.
Scientists had thought Pluto would be cold, dead, and boring, but it turned out to be complex and active.
Pluto is also important because it reminds us that science is always changing. As we discover new information, our definitions, categories, and ideas sometimes need to be updated.
What Should Count as a Planet?
Ever since Pluto was reclassified in 2006, people have been asking: Should Pluto become a planet again?
Some scientists argue that the current definition of a planet is too strict and even a little confusing. For example, the rule that a planet must โclear its orbitโ is difficult to measure. Earth hasnโt cleared 100% of objects from its orbit either, so why does Pluto get disqualified?
Others say that the current definition helps us stay organized and logical. If we call Pluto a planet again, we might also need to include many other icy bodies out there. This could mean we suddenly have 50 or 100 planets.
Also, some scientists argue that the word โplanetโ should mean something special. It should be a large, dominant object in the solar system, not just anything round that orbits the Sun.
This debate is not just about Pluto. Itโs also related to larger questions in science.
Science changes over time. Our ideas about the universe will grow as we build better telescopes, explore space, and understand more.
The discussion over what is a planet is important because in the next century or so, we may have the ability to find even more planet-like objects (maybe even a full planet) at the edge of the solar system where it is currently too dark to see!
Final Thought
Pluto may no longer be called a planet, but its discovery changed how we understand our solar system.
Today, I have tried to introduce lots of scientific vocabulary while talking about the discovery and definition of Pluto.
I find space fascinating and a little overwhelming. It is so vast and dark. We know so little about the edges of our own solar system.
But it is also exciting, because as we build better telescopes and technology, we might discover some incredible things that are out there in the night sky.
What do you think? Do you still consider Pluto a planet?
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Topic: Planets
Exercise 1
Fill in the blanks using the following expressions.
1. spacecraft
2. solid surface
3. dwarf planet
4. discovery
5. orbit
1. Earth and other planets __________ the Sun.
2. Pluto is now called a __________, not a planet.
3. Earth has a __________.
4. Plutoโs __________ was made in 1930.
5. NASA sent a __________ to study Pluto.
Answer key
1. orbit
2. dwarf planet
3. solid surface
4. discovery
5. spacecraft
The exercise is adapted from Tom Wilkinsonโs Episode 339: Pluto: What is a Planet? (Thinking In English)
Muhammad Hanif, an IELTS tutor
Topic: Planets
Exercise 2
Fill in the blanks using the following expressions.
1. complex world
2. blurry images
3. prediction
4. official status
5. modern telescopes
1. Before 2015, we only had __________ of Pluto.
2. Pluto lost its __________ as the ninth planet.
3. We use __________ to look deeper into space.
4. Pluto is not simple; itโs a __________ with moons.
5. The scientist made a _____ that a new planet might exist beyond Neptune.
Answer key
1. blurry images
2. official status
3. modern telescopes
4. complex world
5. prediction
The exercise is adapted from Tom Wilkinsonโs Episode 339: Pluto: What is a Planet? (Thinking In English)
Muhammad Hanif, an IELTS tutor