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Pluto and the introduction of dwarf planets

Since its discovery in 1930, Pluto has been a bit of a puzzle.

For starters, Pluto is not only smaller than any other planet in the solar system, but also smaller than Earth’s moon. It also has extremely low gravity at just 0.07 times the mass of the objects in its orbit, which is only a fraction of the moon’s own power.

At the same time, Pluto’s surface resembles that of terrestrial planets such as Mars, Venus or Earth, but its closest neighbors are the gaseous Jupiter planets such as Uranus or Neptune. In fact, Pluto’s orbit is so erratic that it led many scientists to initially believe that it originated elsewhere in space and that the sun’s gravity pulled it in.

These qualities have challenged the scientific view of Pluto’s status as a planet for years. It wasn’t until the 2005 discovery of Eris, one of several increasingly identified trans-Neptunian objects (objects beyond the planet Neptune), that the International Astronomical Union (IAU) defined criteria for classifying planets.

With Eris and other trans-Neptunian objects sharing similar characteristics to Pluto, the definition for dwarf planets was created and Pluto was downgraded in 2006.

So what are dwarf planets, how do they differ from “real” planets, and what are their characteristics?

The History of Dwarf Planets

A dwarf planet is a celestial body that almost meets the definition of a “true” planet. According to the IAU, which defines planetary science, a planet must:

  1. Orbit around the sun.
  2. Provide enough mass to achieve hydrostatic equilibrium and assume an almost round shape.
  3. Dominate his orbit and don’t share it with other objects.

Dwarf planets, along with the fact that they are neither moons nor satellites, fail to clear the neighborhoods around their orbits. This is the main reason Pluto lost its status: because it shares part of its orbit with the Kuiper Belt, a dense region of icy space bodies.

Based on this definition, the IAU has recognized five dwarf planets: Pluto, Eris, Makemake, Haumea, and Ceres. There are four other planetary objects*, Orcus, Sedna, Gonggong, and Quaoar, which are recognized as dwarf planets by the majority of the scientific community.

Six more can be recognized in the coming years, and as many as 200 or more are believed to exist in the outer solar system in the aforementioned Kuiper Belt.

Ceres is the earliest known and smallest of the current category of dwarf planets. Previously classified as an asteroid in 1801, it was confirmed to be a dwarf planet in 2006. Ceres lies between Mars and Jupiter in the asteroid belt, and it is the only dwarf planet orbiting closest to Earth.

Here’s a brief introduction to the most well-known dwarf planets:

Name Region of the
Solar system
Turnaround time
(in years)
mean orbital
speed (km/s)
relative to
the moon
Orcus Kuiper calls (plutino) 247 4.75 910 26% 1
Ceres asteroid belt 4.6 17.9 940 27% 0
Pluto Kuiper calls (plutino) 248 4.74 2377 68% 5
Haumea Cooper calls (12:7) 285 4.53 1560 45% 2
Quaoar Kuiper belt (cubewano) 289 4.51 1110 32% 1
Would like Kuiper belt (cubewano) 306 4.41 1430 41% 1
gong gong Scattered Disc (10:3) 554 3.63 1230 35% 1
There is Scattered Disk 558 3.62 2326 67% 1
Sednan detached ~11,400 ~1.3 995 29% N/A

Interesting Facts About Dwarf Planets

Here are a few interesting facts about the dwarf planets discovered in our solar system:

Ceres loses 6 kg of its mass in steam every second

The Herschel Space Telescope observed plumes of water vapor rising from the surface of Ceres; this was the first definitive observation of water vapor in the asteroid belt. This happens when parts of Ceres’ icy surface heat up and turn into steam.

A day on Haumea lasts 3.9 hours

Haumea has a unique appearance because of its rotation, which is so fast that it compresses the planet into an egg-like shape. The rotational speed and origin of the collision also make Haumea one of the densest dwarf planets discovered to date.

Makemake was named three years after its discovery in 2005

Makemake’s discovery close to Easter influenced both the name and the nickname. Before Makemake was named after the creator of mankind and god of fertility in the mythos of the Rapa Nui (the native people of Easter Island), Makemake was nicknamed “Easter Bunny” by its discoverer Mike Brown.

Eris was once considered for the position of the 10th planet

Eris is the most massive dwarf planet in the solar system, exceeding Pluto’s mass by 28%. As such, it was a serious contender to become the tenth planet, but failed to meet the IAU’s criteria.

Pluto is a third ice

Two-thirds of the planet’s composition is rock and one-third ice, mostly a mixture of methane and carbon dioxide. A day on Pluto is 153.6 hours, approximately 6.4 Earth Days, making it one of the slowest rotating dwarf planets.

Exploratory missions and new planets on the horizon

With newer technology soon available to the scientific community and new reconnaissance missions gaining more data and information about trans-Neptunian objects, our understanding of dwarf planets will increase.

Nestled in the asteroid belt between Mars and Jupiter, the asteroid Hygiea remains a controversy. Hygiea is the fourth largest object in the asteroid belt behind Ceres, Vesta and Pallas and meets all the requirements to be classified as a dwarf planet.

So what’s stopping Hygiea’s confirmation as a dwarf planet? The criterion of being massive enough to form a spherical shape conflicts; it remains unclear whether its roundness is due to collision/impact disturbance or its mass/gravity.

Along with Hygiea, other exciting dwarf planets could soon be discovered. Here’s a quick rundown of some serious contenders:

120347 Salacia

Discovered in 2004, is a trans-Neptunian object in the Kuiper Belt, about 850 kilometers in diameter. As of 2018, it is approximately 44.8 astronomical units of the sun. Salacia’s status is in question as its planetary density is debatable. It is uncertain whether it can exist in hydrostatic equilibrium.

(307261) 2002 MS4

With an estimated diameter of 934±47 kilometers, 2002 MS4 is similar in size to Ceres. Researchers need more data to determine whether 2002 MS4 is a dwarf planet or not.

(55565) 2002 AW197

Discovered at the Palomar Observatory in 2002, has a rotation period of 8.8 hours, a moderate red color (similar to Quaoar) and no apparent planetary geology. Its low albedo makes it difficult to determine whether it is a dwarf planet or not.

174567 Name:

Varda takes its name from the Queen of the Valar, creator of the stars, one of the most powerful servants of the almighty Eru Iluvatar in JRR Tolkien’s fictional mythology. Varda’s status as a dwarf planet is uncertain because its size and albedo suggest it may not be a completely solid body.

(532037) 2013 FY27

This space object has a surface diameter of about 740 kilometers. It revolves around the sun once at a time 449 years. Researchers need more data on the planet’s mass and density to determine whether it is a dwarf planet or not.

(208996) 2003 AZ84

It is about 940 kilometers about its longest axis, because it has an elongated shape. This shape is presumably due to the fast rotational speed of 6.71 hours, similar to those of other dwarf planets such as Haumea. Like Varda, it remains unknown whether this object has been compressed into a completely solid body and thus remains controversial among astronomers about its planetary status.

*Note: The IAU officially recognizes five dwarf planets. We include four additional dwarf planets widely recognized by members of the scientific community, especially among leading planetary researchers such as Gonzalo Tancredi, Michael Brown and William Grundy. There are many more potential dwarf planets not listed here that are still under investigation.

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