Earth is not alone as a planet. Since the beginning of our world, its shared space with its stellar sister and brothers of over 4 billion years, all playing a significant role in the ecosystem of our cosmic home. From mighty Jupiter to nearby Mars, our system is populated with planets as diverse and strange as the wonders of our own world.
Over the centuries of astronomy, new observations have been made. Studies conducted, research done, and new conclusions reached. Some of those conclusions relate to the planets. One, specifically, is how many. Well, you might think! That’s easy! Isn’t it nine? And in fact, that’s no longer correct. No longer correct – but how?
Well, when you hear the name “Pluto” you might imagine a few things. A small, distant, and cold world well beyond our stellar reach comes to mind, one so far it can only make solar orbit every two hundred years or so. Discovered in the 1930’s, it was originally thought to be the system’s ninth planet. As discoveries were made and evaluations conducted, this conclusion was challenged.
In 2005, another small, isolated body of icy-rock was discovered beyond Pluto, named Eris by NASA. Eris, in fact, is larger than Pluto, leading astronomers to redefine how to classify stellar bodies. In short, Pluto and Eris are now referred to as “Dwarf Planets.” Others refer to them as planetoids, but the conclusion is the same: Pluto no longer fit the typical characteristics of a planet. You can imagine this has been met with a variety of criticism – how can you change what it means to be a “planet?”
Dwarf Planets and the Kuiper Belt
The primary reason Pluto is no longer considered a planet is because of its reclassification. The discovery of Eris (and objects like it) gave astronomers a greater understanding of planetoid qualities. Today, both Eris and Pluto are now considered “dwarf planets.”
Aside from dimensions and size, there are reasons for this classification.
- Dwarf planets still orbit the sun
- Still large enough to maintain its own gravitational pull
- However, has not cleared the orbit of other objects
You might wonder what’s meant by “other objects.” Specifically, in relation to Pluto and Eris. These dwarf planets originate from something called the Kuiper Belt, some billions of miles distant from Neptune. Essentially, it’s an ovular shape composed of millions of icy-rocky bodies, likely fragments from the solar system’s creation. Beyond this is the Oort Cloud, even farther from our system. However, the two should not be confused.
Since Pluto and Eris have not cleared the orbit of the Kuiper belt (aka other rock-ice objects) they are defined as Dwarf Planets.
The Planets in the Solar System
With an understanding of why the Solar System is comprised of eight planets versus nine, we can break them down in their position and orbit.
At the center of our system is, of course, the Sun. All planets and stellar bodies native to our system revolve around the sun, and its powerful heat/light is what drives life here on Earth.
The first and smallest planet in our solar system. Because Mercury is so close to the sun, its uninhabitable. It has no atmosphere, no oxygen, and average daily temperatures reach around 400 degrees Celsius.
Although similar in size to planet Earth, Venus is quite different. It’s surrounded by a constant storm an inhospitable terrain, and happens to be the hottest planet in our system. That’s because the thick clouds cloaking it create an infinite greenhouse effect, trapping and cycling heat.
It’s home! It’s the only planet we know (so far) capable of supporting life. Home to millions of incredible species, deep oceans, thick forests, various terrains, and thousands of societies, everything that we ever know and ever occurred as humans happened here.
The pale red dot, Mars has fascinated astronomers for years with characteristics quite similar to earth. Many photos exist of the planet’s surface thanks to machines like the Curiosity rover, which have revealed rocky terrain and frozen bodies of brine (heavily salted) water. Some have speculated, in the right conditions, this fourth world could support life like on Earth.
Fifth and largest, mighty Jupiter is a gas giant consumed by swirling storms of cold ammonia. It’s most known for its “Great Red Spot,” a constant hurricane-like storm. Jupiter also has quite the family of moons, with over 75 lunar objects hanging in its orbit.
The second largest planet, Saturn is the sixth world in our system. Most known for its beautiful rings, which are clusters of millions of icy objects. As a gas giant (like Jupiter), it’s made mostly of helium and hydrogen.
Aside from a name spawning a lifetime of jokes, this planet is distant, dark, and mysterious. It has its own rings – like Saturn – though smaller in comparison. Its atmosphere is made of hydrogen sulfide and it’s an ice giant, mostly comprised of water and ammonia.
The last traditionally defined planet. Far Neptune is the eighth planet in our solar system, it’s another ice giant with one special property: winds. Frozen chunks of ice and methane are flung around the gassy world, estimated to be around 1200MPH/2000KMH.
As you can see, the nature of astronomy and our understanding of astrophysics is always evolving. The discovery of icy bodies from the Kuiper Belt and Oort Cloud changed how astronomers perceive planets and planetoids, shifting the definition of Pluto. As we continue to learn about our solar system, newer facts are always coming to light. Better technology allows us to send rovers to different worlds and satellite imagery demonstrates new visual factoids about things we know.
We hope this quick guide on the solar system rouses your enthusiasm to learn more. We’ve only touched the surface and there are still so many things to know about our solar system and the cosmos as a whole. But, now you know – it’s eight planets, not nine!