What Is a Dwarf Planet?

This site contains affiliate links to products. We may receive a commission for purchases made through these links.

Most people first heard the term “dwarf planet” around the time when Pluto lost its planetary status. Although it’s been well over a decade since the former ninth planet officially became a dwarf planet, the term might still be unclear to most of the public.

And indeed, this term isn’t as simple as it might seem. That’s why, in this article, we’ll explain what a dwarf planet is and what it isn’t, following the official definition of the term. Furthermore, we’ll delve into the interesting history of dwarf planet exploration.

What Is a Dwarf Planet?

To answer this question, we should first step back and ask: What is a planet?

The answer might seem straightforward—a planet should be something like Earth or Mars, i.e., a space object that’s firm, round, and large. However, this answer would be found lacking upon more thorough inspection.

First, planets don’t have to be firm, as shown by the massive gas giants in the outer sections of the solar system. Size is likewise an issue since there are two moons—one orbiting Jupiter, the other Saturn – which are larger than the planet of Mercury.

The only accurate part of the intuitive, common sense explanation of what a planet is would be the one referring to the shape.

Luckily, we don’t have to bust our heads coming up with a definition of a planet—the International Astronomical Union (IAU) has already done that job. But you might be surprised to learn that the official definition of a planet didn’t come around until 2006.

According to the IAU, a planet must directly orbit the sun, surpass 2,000 km in diameter, and has sufficient mass and gravity to sustain a stable shape. But there’s one more condition—a planet must be the dominant body in the immediate neighborhood.

This last part of the definition was what cost Pluto its planetary status.

Pluto is located within the Kuiper belt, a massive asteroid belt with numerous other bodies that orbit the sun beyond Neptune. Since Pluto didn’t “clear its neighborhood,” as is the official scientific expression, and continues to share the orbit with other bodies, it was disqualified per the new definition of being a planet.

At the same time, the new category of dwarf planet was conceived to describe all celestial bodies similar to Pluto. By IAU definition, a dwarf planet was supposed to orbit the sun and have enough mass and gravity to reach a hydrostatic equilibrium, i.e., a nearly round, stable shape. This part of the definition is the same as for planets.

The point where the two definitions diverge is in the neighborhood. Unlike a regular planet, a dwarf planet will share its orbit with celestial neighbors. Finally, a satellite of another planet can’t be classified as a dwarf planet.

This answers the question of what a dwarf planet in the strictest sense is. However, the classification has plenty of history, which might be worth exploring here.

How Did the Dwarf Planet Classification Come to Be?

The first troubles with unusual planets in our solar system came long before Pluto was discovered.

In fact, the long history of dwarf planets as we know them started in 1801 and the discovery of Ceres within the asteroid belt between Jupiter and Mars. However, while Ceres was initially deemed a new planet, a crucial problem appeared the following year.

In 1802, another body was found in practically the same orbit as Ceres, which was thought impossible. This was Pallas, and it wouldn’t be the last of such an object in the main asteroid belt.

By the decade’s end, two more bodies, Juno and Vesta, were discovered, all sharing nearly identical orbits to Ceres and Pallas. Then, as the 19th century entered its second half, the number of discovered asteroids surpassed a hundred.

At that point, it was clear that, whatever the original four objects were, they couldn’t be planets in the way we understood them. So the scientific community started referring to Ceres, Pallas, Juno, and Vesta as “minor planets,” grouping them with other asteroids.

It’s worth mentioning that the term “asteroid” didn’t mean the same thing in the 1800s as today. Then, it had a broader definition that encompassed most small celestial bodies.

When Pluto was discovered in 1930, its definition underwent pretty much the same journey as Ceres. At first, Pluto was considered unique in its neighborhood since no other objects past Neptune could be viewed. However, the newly discovered planet already raised some eyebrows as soon as it appeared.

Pluto didn’t share the traits of its nearest planets, Uranus and Neptune. Both of those had large masses, circular orbits, and were moving in the vicinity of the ecliptic. Pluto, on the other hand, had a mass six times less than our Moon and an orbit that would occasionally intersect Neptune’s. In addition, Pluto was a firm, tiny body covered in ice in a region of space entirely dominated by gas giants.

Still, the planet was left alone for about six decades. Then, in the early 1990s, advancements in astronomy allowed the discovery of other trans-Neptunian objects. And several of them shared Pluto’s general characteristics.

dwarf planets

This was the discovery of the Kuiper belt, analogous to how the main asteroid belt was found.

Since Pluto existed in very similar circumstances as Ceres, it was suggested that it, too, should be counted among minor planets. However, the scientific community was far from reaching a consensus on that matter.

Further exploration of the Kuiper belt raised even more issues for Pluto’s planetary status. Similar bodies like Sedna and Quaoar were discovered. Finally, Eris, an object larger than Pluto in the same orbit, was discovered in 2005.

Initially, it was suggested that Eris be counted as the tenth planet. However, by that point, it was clear that the real issue was in Pluto’s classification.

At long last, the following year after the discovery of Eris, the IAU definition of planets and dwarf planets was determined by vote. Besides Eris and Pluto, Sedna, Quaoar, Makemake, and Haumea also received the status of dwarf planets.

The astronomical community resolved a crucial question by accepting these definitions. But unfortunately, with the introduction of dwarf planets, it became impossible for the number of planets to keep growing. If the situation had stayed the same, we wouldn’t have eight planets today but tens of them.

Scientific Disagreement

Although it might sound as if the entire scientific community has agreed on defining what a dwarf planet is, the actual situation is far from idyllic.

Like a considerable portion of the public, several researchers have voiced their disagreement with the reclassification. And as far as the experts were concerned, the issue wasn’t only in Pluto losing its status—it was with the definitions themselves.

For instance, there’s an argument that any object with enough mass to achieve hydrostatic equilibrium should be considered a planet. In that case, our solar system should have about 95 planets, not counting 19 moons that also meet this criterion.

Another argument comes from the point of geology. This argument states that all objects with a similar formation cycle as the recognized planets should be placed in the same category.

The issue with the planet’s location in the galaxy and the universe was also brought up. The IAU defined planets as only those bodies orbiting the sun. Simply put, by that definition, no other body orbiting a star can be classified as a planet, regardless of whether or not it meets other requirements.

In his study “A Quantitative Criterion for Defining Planets,” Jean-Luc Magot proposed another way to define a planet. According to the proposed definition, a planet would need to orbit its parent star, have the greatest mass and optimal orbital distance in its orbit, and be able to clear its neighborhood and remain in a stable orbit within the parent star’s lifetime.

However, even with a changed definition of a planet, our solar system wouldn’t get any new members in its planetary family.

The truth is that the definition of a dwarf planet might be very much on-point. After all, the celestial bodies that we consider planets have similar traits. But, perhaps most importantly, each planet is the dominant influence in its part of the solar system.

The same can’t be said about dwarf planets. These exist alongside their mass and gravitational equals, making them vastly different from planets and planet-like satellites.

It would seem that, even if the definition of a planet changed drastically, we wouldn’t be getting a ninth planet again. At least not without dozens more to follow.

Understanding the Unusual Solar System Inhabitants

Dwarf planets might look similar to planets at a glance. However, upon closer inspection, these two types of celestial objects show many differences.

If the definition of planets is set, their number in the solar system will never grow past eight. But the same doesn’t apply to dwarf planets. So these celestial bodies still have the potential for new discoveries and surprises within our neighborhood.

Leave a Comment

Your email address will not be published.

Special offer for our visitors

Get your Space Exploration And Astronomy Free Guide

We will never send you spam. By signing up for this you agree with our privacy policy and to receive regular updates via email in regards to industry news and promotions