The story behind why we now have ‘dwarf planets’

Solar system

Two hundred years ago, there were more than 20 planets in our solar system!

Imagine trying to learn that many in grade school compared to the nine, no wait, eight that we have today.

So, what happened to all of them? It’s not like they could have just left the solar system.

Indeed, all of those planets are still here. They just go by a different name. Most of them are now called asteroids, but the largest of them, Ceres, is part of a relatively new class: dwarf planet.

Although the most famous dwarf planet is Pluto, Ceres has probably suffered the most extreme identity crisis changing names three times before astronomers finally figured out how to define a planet more than 200 years after Ceres was first discovered.

A dwarf planet with an identity crisis

You might have heard the big news that NASA recently sent a spacecraft into orbit around Ceres on Friday, March 6. The event is history-in-the-making because the spacecraft, called Dawn that took the image of Ceres to the right, is the first ever to orbit a dwarf planet.

Ceres was not always a dwarf planet, however.


When it was first discovered in 1801, its size and spherical shape led astronomers to bestow on Ceres the prestigious title of planet. Shortly afterward, astronomers found other, smaller objects floating around Ceres that they also called planets, and before they knew it there were 23 new planets on record.

But these so-called planets were very different from what we recognise as a planet, today. Ceres was by far the largest of this newly-found group, that is now known as the asteroid belt, and it’s only about a quarter of the diameter of Earth’s moon

Ceres comparison

Throughout the first half of the 19th century, Ceres suffered its first identity crisis because astronomers would refer to it as a planet and an asteroid interchangeably. Then, in the latter half of the century, astronomers had settled on the term asteroid for all of the objects located in the asteroid belt between Mars and Jupiter.

And throughout the 20th Century, Ceres was known as an asteroid, and is still considered to be the first asteroid ever discovered.

Pluto loses its planethood

Artist’s concept of Pluto. NASA/JPL-Caltech

While Ceres was enjoying being an asteroid, astronomers looked deeper into our solar system and discovered Pluto in 1930, welcoming it as the ninth and final planet.

But all of that changed in 2003 when American astronomer Michael E. Brown — who now considers himself as the man who killed Pluto — discovered Eris.

Eris is an object beyond the orbit of Neptune and is 25% more massive than Pluto. Well, the discovery gave astronomers a major dilemma because if Pluto was a planet, then surely Eris was one, too.

Surely enough, three years later in 2006, the International Astronomical Union stripped Pluto of its planethood, demoting it to a dwarf planet — a term that was coined in 1990 by American planetary scientist Alan Stern.

Stern proposed the term to refer to the thousands of small objects in the outer solar system, beyond the orbit of Neptune. But the IAU refined Stern’s definition in 2006, which opened the doors for objects like Ceres to qualify and not just object in the outer solar system.

Today there are five objects that the IAU recognises as dwarf planets: Pluto, Eris, Ceres, Haumea, and Makemake.

By definition, a dwarf planet meets the first two of the three criteria that an object must meet to be considered a planet. These three criteria are:

  1. The object is in orbit around the Sun, as opposed to a planet.
  2. The object has enough mass to form a spherical, round shape.
  3. The object is so massive that the space around it is relatively empty because its gravity sucked in all of the small debris around it.
Screen Shot 2015 03 06 at 3.57.31 PM

When Pluto first lost its planethood, many people were distraught. After all, everyone who was old enough to understand what was going on had learned that Pluto was a planet, not this bizarre, second-class, unfamiliar term of “dwarf planet”.

To those people who are still upset, famed astrophysicist and director of Hayden Planetarium at the American Museum of Natural History Neil deGrasse Tyson has some encouraging words, which he told Business Insider in this video:

“Pluto is happier now,” Tyson said. “It’s one of the largest of the dwarf planets.”

NOW WATCH: Neil deGrasse Tyson: Pluto Is Not A Planet So ‘Get Over It’