Buzz
It’s a tiny world, after all. This was the conclusion drawn during an August 2006 meeting of the International Astronomical Union. Even though only 424 out of 9,000 eligible voting members were involved, it led to a planetary eviction notice for Pluto.
But the meeting did more than just shrink the solar system from nine planets to eight. It also established a completely new category for celestial bodies. Let’s dive into the world of our solar system's five small wonders: the dwarf planets.
10. Ceres: The Dwarf Planet Twice Classified
Though Ceres is the smallest of the five dwarf planets, it holds the title of the first one to be discovered. Positioned in the Asteroid Belt between Jupiter and Mars, it is the closest. So, even though it’s the smallest, Sicilian astronomer Giuseppe Piazzi first observed it in 1801 – well over a century before Pluto made its planetary debut.
Referring to Ceres as a planet – even a dwarf one – seems a bit of a stretch. With a diameter of just 950 km, its mass is only 0.015 percent of Earth’s and 14 times smaller than Pluto. In fact, Ceres is so tiny that it’s also classified as an asteroid. However, in Ceres' defense, it holds the title of being the largest asteroid in the solar system.
Ceres stands out like a boulder among relatively small pebbles: it makes up a quarter of the total mass of the Asteroid Belt and, unlike its neighbors, boasts a nearly spherical shape and likely contains water ice beneath its surface. In 2014, the European Space Agency’s Herschel Space Observatory observed water vapor erupting from two distinct regions of Ceres – possibly from ice volcanoes, which is quite remarkable.
Ceres stands apart enough from its surroundings to have earned the title of dwarf planet in 2006. Its name is derived from the Roman goddess of harvests – the same goddess after whom breakfast cereal was named.
9. Mission to Ceres: A Journey Through Time
Ceres remains an intact relic from the earliest days of the solar system, dating back 4.6 billion years. Studying it reveals valuable insights into the fundamental building blocks of the solar system. In 2007, NASA launched an ambitious mission to thoroughly map Ceres (and Vesta, the next-largest object in the Asteroid Belt), sending a spacecraft into orbit around it. The mission, named the Dawn Mission, reached Ceres in March 2015, reflecting its goal to uncover ancient cosmic origins.
The data collected suggests that Ceres likely originated farther from the sun and gradually moved into the inner solar system. This hypothesis is based on the fact that Ceres' surface contains significant amounts of condensed ammonia, which only forms in the colder regions of the outer solar system. The mission also uncovered abundant water, including a rocky mantle enriched with water-rich minerals like clays, as well as an icy outer shell.
Scientists were particularly fascinated by Ceres’ organics – the essential building blocks of life. In one of Ceres’ deepest craters, a specific carbon-hydrogen chain of organic compounds was detected, indicating that the material likely formed in its once-immense ocean.
Adding to the excitement is the evidence that Ceres was geologically active relatively recently. This is supported by its stunning array of more than 300 bright features, known as faculae, which shine brightly against the dark surface. The brightest feature harbors the largest deposit of carbonate minerals ever observed outside Earth. This suggests that briny water rose to Ceres' surface in the recent past, leaving behind salts – a sign of a vibrant, “living” planetoid.
8. Eris: A Misunderstood Nail in Pluto’s Coffin
One of the reasons Pluto was demoted in 2006 was the discovery of Eris the previous year (to this day, the two celestial bodies are not on speaking terms). When Eris was first discovered in January 2005, it was hailed as the largest dwarf planet known, with a diameter estimated between 2,300-2,400 km, making it 27% more massive than Pluto. This prompted the International Astronomical Union (IAU) to reevaluate its definition of a planet – the first blow to Pluto's hold on full planethood.
The issue is… well, they were just mistaken. Further observations revealed that Eris is actually slightly smaller than Pluto. Unfortunately, this realization didn’t come until 2010 – four years after Pluto was demoted. Oops. Fortunately, Pluto’s downgrade wasn’t solely based on its size – more on that later.
Back to Eris, which certainly has a rebellious side. For one, its orbit is incredibly erratic, crossing Pluto’s and nearly intersecting Neptune’s in an unusual oval-shaped path. This eccentric orbit takes Eris and its moon, Dysnomia, 557 years to complete a full revolution around the sun – more than twice as long as Pluto’s journey. As it orbits, Eris moves in and out of the Kuiper Belt, a region beyond Neptune filled with mostly asteroid-sized bodies, home to all the dwarf planets except Ceres.
Eris' surface likely consists of nitrogen and methane spread out in a super-thin layer, just one millimeter thick. Some scientists speculate that what we observe as the surface of Eris might actually be its condensed atmosphere, which could turn into gas when the planetoid gets closer to the sun.
7. The Tenth Planet?
The previous entry explored how the discovery of Eris triggered a reevaluation of planetary classifications, ultimately leading to Pluto's yearlong demotion. But this outcome wasn’t inevitable; one alternative was to welcome Eris as the solar system’s tenth full-fledged planet. Unfortunately for Pluto, the debate culminated in the International Astronomical Union (IAU) rewriting the definition of a true planet, with one key criterion – the ability to clear its orbital path through gravitational influence – being met by neither Eris nor Pluto.
As a result, on August 26, 2006, Pluto was reclassified, and a new category, “dwarf planet,” was established. Along with Pluto, Eris and Ceres also received this new designation, with many referring to Eris as “The First Dwarf” due to its pivotal role in the creation of this category.
In fact, Eris' disruptive nature is reflected in its very name. When it was first discovered in 2005, it was informally called Xena. However, due to its controversial impact on both the IAU and elementary school science classes, the planetoid was renamed to honor the Greek goddess of discord.
“[Eris] stirs up jealousy and envy to cause fighting and anger among men,” explains Mike Brown, a professor of planetary astronomy at the California Institute of Technology. According to legend, after being snubbed for a wedding invitation, Eris vengefully sparked a quarrel among the goddesses that eventually led to the Trojan War.
“She’s quite a fun goddess, really,” Brown adds. Pluto might use a different word to describe her.
6. The Potato-Shaped Dwarf
Another resident of the Kuiper Belt is Haumea, notable for its distinct, flattened shape. This odd form is due to Haumea’s rapid rotation, which is one of the fastest among large solar system objects. The centrifugal force generated by this spin stretches the planetoid into an oblong, potato-like shape, more resembling an American football than the European variety.
Days on Haumea are remarkably short, as it completes one full rotation every four hours. This incredibly fast spin has led scientists to speculate that a colossal impact billions of years ago could have triggered Haumea’s swift rotation and possibly created its two moons, Namaka and Hi’iaka. (Haumea is named after the Hawaiian goddess of fertility, while her moons are named after her mythological daughters.)
Haumea stands out even more with its unique feature: rings. In 2017, scientists confirmed their existence after observing Haumea pass in front of a star, making it the first Kuiper Belt object discovered to have rings.
Aside from that, however, Haumea remains somewhat enigmatic. NASA’s website acknowledges that little is known about the dwarf planet’s atmosphere and surface. It’s roughly the same width as Pluto and takes slightly longer (285 years) to orbit the sun. Still, the known details about Haumea already set it apart from its dwarf planet peers.
5. Makemake Finishes What Eris Started
Another notable member of the Kuiper Belt is Makemake, which also played a significant role in Pluto’s removal from the planet lineup. This was largely due to timing; like Eris, Makemake was discovered in early 2005, prompting the IAU to reconsider what qualifies as a planet.
As mentioned earlier, scientists initially – and mistakenly – thought Eris was larger than Pluto, which led to its brief consideration as a full planet. However, two months after Eris’s discovery, Makemake was found. Unlike the size debate surrounding Eris and Pluto, Makemake was clearly smaller than both, putting an end to any similar size-related arguments.
With Makemake, scientists had an object that was too small to be classified as a planet. As it became apparent that Pluto resembled Makemake more than it did Mars, it became clear: three similar-sized objects in the Kuiper Belt were acting in much the same way. This realization led to the 2006 rule that only objects that can “clear their neighborhood” through gravitational dominance are allowed to be considered full planets.
None of the three met this crucial criterion. Eris was denied full planet status, and Pluto was completely demoted. Thanks a lot, Makemake.
4. Make Way for Makemake
Makemake is more than just a rival for wannabe planets. It actually outshines its larger counterpart Eris, standing as the second-brightest object in the Kuiper Belt, with only Pluto shining brighter. Despite taking 305 Earth years to orbit the sun, a single day on Makemake lasts about 22 1/2 hours, making its days very similar to those on Earth and Mars.
From the vast distance that separates it from us (Makemake lies about 6,847,000,000 km from the sun, which is a mere 150 million km from Earth), astronomers believe it has a reddish-brown hue, much like Pluto. Its surface appears to be rich in ethane and large frozen methane pellets.
Makemake is also home to a tiny moon. Officially known as S/2015 (136472) 1 and affectionately called MK 2, this moon has an estimated radius of only 80 km. (For comparison, the smallest moon in the solar system, Deimos, one of Mars’ two moons, has a diameter of just 11 km.) MK 2 is about 1,300 times dimmer than Makemake, so difficult to spot that NASA still refers to it as a “provisional” moon.
Makemake’s name came about in a rather peculiar way. Initially, its discoverer called the object “Easterbunny.” But when it came time to give it an official name – following the tradition of naming celestial bodies after gods – they chose Makemake, the creator god from the Rapa Nui people of Easter Island.
3. A Distant Dwarf?
In 2003, three years before Pluto was relegated to the ranks of the planetary minors, a trio of researchers discovered the coldest and most distant known object in the solar system at that time. In honor of its chilling nature – with temperatures believed never to rise above -400° Fahrenheit – they named their discovery Sedna, after the Inuit sea goddess who, according to legend, resides in the depths of the Arctic Ocean.
Located far beyond the Kuiper Belt, where four of the five dwarf planets reside, Sedna is so distant it’s nearly outside of the solar system – about 130 billion kilometers away. (For perspective, the sun is only 146 million kilometers from Earth.) Sedna follows an unusual elliptical orbit, taking around 10,500 Earth years to complete one orbit around the sun. It’s an unfathomably long time.
Sedna’s size, however, is less remarkable, measuring between 1300-1770 km in diameter – around 75% of Pluto’s size. Although that’s small compared to many other celestial bodies, it’s still not tiny by dwarf planet standards. The reason Sedna hasn’t been classified as the sixth dwarf planet is due to its inability to achieve hydrostatic equilibrium – a balance of forces that stabilizes a planet's shape. Sedna is too far from Earth for current technology to measure this, so it remains a “minor planet,” which seems unfair given the limitations of our technology. It's just another example of Earthly arrogance in science.
2. Still, Pluto is the Least Dwarfy Dwarf
Even though it doesn’t measure up to the likes of Neptune, Uranus, or Mercury – or even some of the moons of full planets, including our own – Pluto still manages to stand out in its class.
To begin with, Pluto holds the title of the largest dwarf planet – a status briefly lost to Eris, before astronomers discovered that Eris wasn’t as massive as originally believed. Pluto is also the brightest of its kind, outshining the runner-up, Makemake. Additionally, it follows an eccentric orbit with a notable tilt, which means that, during its 248-Earth-year journey around the sun, Pluto sometimes comes closer to the sun than Neptune, the farthest full planet.
We also know far more about Pluto compared to the other dwarf planets. This is thanks to NASA’s New Horizons spacecraft, which, in January 2006, set off to explore the distant world just seven months before Pluto was officially demoted from full planet status. The spacecraft reached Pluto in July 2015 and uncovered fascinating features, such as mountains made of ice that may float on a layer of nitrogen ice.
New Horizons also focused on Pluto’s largest moon, Charon, where it spotted a north pole covered in reddish material. This is likely debris that escaped Pluto’s on-and-off atmosphere, which forms and dissipates depending on Pluto’s distance from the sun.
1. Pluto Was Just Waiting to Be Picked Off
Finally, we arrive at Pluto, the little planet that couldn’t… Despite spending three-quarters of a century as a full-fledged planet, Pluto was unceremoniously ousted from the planetary league by the combined efforts of Eris and Makemake, as detailed earlier.
It all began so promisingly for the little planet. Pluto was discovered in February 1930 by American astronomer Clyde William Tombaugh, marking the peak of years of predictions by fellow scientists that a ninth planet might exist. Pluto became the only planet to be named by an 11-year-old girl when Venetia Burney from Oxford, England, suggested it should be named after the Roman god of the underworld. Later, Pluto’s five moons were named after figures connected to the underworld, with its largest moon, Charon, named after the ferryman of the River Styx.
But here’s the catch: scientists were already searching for a ninth planet, and when they discovered Pluto, they were eager to label it as such. This despite Pluto being a mere 2,250 km in diameter – roughly half the width of the United States. Things became even more complicated when Charon was discovered in 1978. It quickly became clear that Charon was nearly half the mass of Pluto itself. The two bodies’ equal gravitational pull caused them to orbit each other like a double-planet system. This wasn’t a moon, it was more like a companion.
So in the end, Pluto was just waiting for its time to be removed from the planet list. Had Eris been discovered first, it could have easily been the ninth planet for 75 years.
