Pluto Is Not The First Dwarf Planet We Discovered

Most people know Pluto as the first dwarf planet we discovered. In fact, many people also assume it is the largest dwarf planet, but things get a little messy there too. Pluto was not the first dwarf planet we discovered. That title belongs to Ceres. And while Eris is more massive than Pluto, Pluto is slightly larger in diameter.

Ceres was discovered in 1801 by the Italian astronomer Giuseppe Piazzi, and the Dawn spacecraft, launched in 2007, entered orbit around Ceres in March 2015 after an eight-year journey. So why do we ignore Ceres while we followed New Horizons for days and launched campaigns for Pluto? Is it because Pluto was first accepted as a planet and then stripped of that status? What exactly does Pluto owe its fame to?

Ceres - 2015

I said Ceres was discovered in 1801. After its discovery, it was accepted as a planet for a long time. Surprised? Back then, Neptune had not even been discovered yet, so Ceres had a place in the early planetary lists of our solar system. Roughly 150 years before Pluto, Ceres went through something very similar. But later, when many more objects similar to Ceres were discovered in the region between Mars and Jupiter, it was removed from the list of planets and began to be treated as part of the asteroid population.

A Pluto-Like Body Few People Talk About: Orcus

People who are even a little interested in astronomy usually know Ceres, but what about Orcus?

Orcus, officially known as 90482 Orcus, is a trans-Neptunian dwarf planet candidate. Orcus looks so much like Pluto that it almost feels like Pluto’s lost twin. Let’s briefly look at the similarities.

Orcus

Orcus is at almost the same distance from Earth as Pluto, sitting roughly 6 billion kilometers from the Sun.

Orcus also has a relatively large moon, just like Pluto has Charon. That moon is called Vanth. Orcus completes one orbit around the Sun in 243 years, while Pluto does it in nearly 248 years.

Despite all these similarities, the idea of Orcus being treated as a planet was never seriously on the table. Let’s add a little note here: Orcus is the name of the god of the underworld in Roman mythology, and that name was given according to the International Astronomical Union’s naming convention for Pluto-like bodies of similar size and comparable orbits, which are often named after underworld deities.

How Pluto Was Found

Anyway, I got sidetracked. We were talking about Pluto. Pluto was discovered during the search for the ninth planet of the solar system. Speaking of which, Neptune, the eighth planet, had itself been found because of strange irregularities in Uranus’s orbit. The logic was simple: there must be another planet causing those disturbances. But for astronomers who were not fully satisfied even after Neptune’s discovery, the search continued.

Percival Lowell was one of those astronomers. He devoted his life to finding Pluto, yet never found it. After Lowell’s death, the search for Planet Nine entered a period of stagnation. Later, a young astronomer named Clyde Tombaugh continued the hunt.

Clyde Tombaugh

Tombaugh spent a long time photographing the night sky over and over again, searching for objects that could be planets. In 1930, he found the long-sought planet. Or rather, he thought he did. He was looking for an object moving among stars that otherwise stayed fixed in place. Pluto’s discovery.

Pluto Is Smaller Than Most People Imagine

Pluto is only a little larger than half of Earth’s Moon in diameter, and its mass is far less than half of the Moon’s. Its surface gravity is 0.658 m/s², which means that someone who weighs 80 kilograms on Earth would feel the equivalent of about 5 kilograms and 365 grams on Pluto. So really, none of us are overweight. Earth’s gravitational pull is just a little too enthusiastic.

As Pluto moves along its elliptical orbit, it gets as close as 4.4 billion kilometers to the Sun. At its farthest, it is about 7.4 billion kilometers away. The temperature ranges roughly from -215 degrees Celsius to -234 degrees Celsius. In other words, summers on Pluto are very cold and cool, while winters are freezing and somehow even cooler.

Charon Changed Everything

For many years, Pluto was known as a lonely planet far away from us all. Then, in 1978, James Christy noticed Charon. But Charon was an unusually large moon compared to Pluto. It soon became clear that these two were not a typical planet-moon pair. Instead, they formed a very unusual dual structure orbiting a shared center of mass.

Pluto and Charon constantly show the same face to each other and complete this mutual dance in about 6.4 days. So no, Charon does not know Pluto any better than we do.

Besides Charon, Pluto also has four other moons named Hydra, Nix, Styx, and Kerberos. They are basically oddly shaped chunks of rock, and Hydra in particular looks like a full-on potato. As far as we currently know, that is the complete list. New Horizons searched carefully for possible additional objects during its mission, but today Pluto’s known moon count remains five.

What We Know For Now

A hazy layer around Pluto was detected. Pluto is not surrounded by empty stillness the way many people once imagined.

Pluto is not smaller than Eris in diameter. In fact, it may be slightly larger, although Eris is more massive. But this still does not remove the main obstacle standing between Pluto and full planetary status, because the real issue is not size. The real issue is whether it has cleared its orbital neighborhood.

Pluto

There are mountains about 3,500 meters high south of the bright heart-shaped region. These geologically young structures suggest that Pluto’s surface may still be geologically active.

Because the surface temperature hovers around -230 degrees Celsius, some of the glaciers there may be made not of water, but of nitrogen. The heart Pluto sent us is now known as Tombaugh Regio, in honor of Clyde Tombaugh.Pluto, much like Mars, is gradually losing its atmosphere.

One reason Pluto does not appear to have giant craters like Mars or Venus is that its surface may be reshaped over time by nitrogen ice and other volatile materials.

Pluto

In 2006, Pluto was removed from the category of planets by a decision of the International Astronomical Union. Then Pluto sent us that heart, and suddenly the cries of “Make Pluto a planet again” started rising everywhere. But there is one thing people forget: if Pluto is accepted as a planet again, then many other trans-Neptunian dwarf planets and similar bodies would have to be pulled into the same debate.

New Horizons transmitted the data it gathered back to Earth at an incredibly slow speed of roughly 1 to 2 kilobits per second, which is why the full data transfer was expected to continue for about 16 months.

The name Pluto itself was one of the three names proposed after the dwarf planet’s discovery, alongside Minerva and Cronus. The suggestion of Pluto came from Venetia Burney, an 11-year-old schoolgirl from Oxford.

Since Pluto was discovered on February 18, 1930, it has not yet returned to the same position once again. It will complete its full orbit on March 23, 2178. And because it is in a 3:2 orbital resonance with Neptune, contrary to what many people imagine, a collision between them is not expected.