Comparison of objects in our solar system by rotation, size, etc.

Pluto and the introduction of dwarf planets

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

For starters, not only is Pluto smaller than any other planet in the solar system, it’s also smaller than Earth’s moon. It also has an extremely weak gravitational pull at only 0.07 times the mass of objects in its orbit, which is only a fraction of the strength of the Moon.

At the same time, the surface of Pluto resembles that of terrestrial planets such as Mars, Venus or Earth, but its closest neighbors are the gaseous planets Jovian planets like 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 was not until the discovery of Eris in 2005, one of many 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 with Pluto, the definition of 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 “real” planet. According to the IAU, which establishes definitions for planetary science, a planet must:

  1. In orbit around the Sun.
  2. Have enough mass to achieve hydrostatic equilibrium and assume an almost round shape.
  3. Dominate its orbit and do not share it with other objects.

Dwarf planets, in addition to not being moons or satellites, fail to clear the quarters 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 recognized five dwarf planets: Pluto, Eris, Makemake, Haumea and Ceres. There are four other planetary objects*, namely Orcus, Sedna, Gonggong and Quaoar, which the majority of the scientific community recognizes as dwarf planets.

Six more could be recognized in the years to come, and up to 200 or more are believed to exist in the outer solar system in the Kuiper Belt.

Ceres is the oldest known and the 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 is the only dwarf planet that orbits the closer to Earth.

Here is a brief introduction to the most recognized dwarf planets:

Last name Region of
Solar system
Orbital period
(in years)
Mean Orbital
speed (km/s)
related to
the moon
orcus Kuiper belt (plutin) 247 4.75 910 26% 1
Ceres asteroid belt 4.6 17.9 940 27% 0
Pluto Kuiper belt (plutin) 248 4.74 2377 68% 5
haumea Kuiper Belt (12:7) 285 4.53 1560 ≈ 45% 2
Quaoar Kuiper belt (cubewano) 289 4.51 1110 32% 1
makemake Kuiper belt (cubewano) 306 4.41 1430 41% 1
gong gong Scattered disc (10:3) 554 3.63 1230 35% 1
Eris Scattered Disc 558 3.62 2326 67% 1
Sedna Detached ~11,400 ~1.3 995 29% N / A

Interesting facts about dwarf planets

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

Ceres loses 6 kg of its mass in vapor every second

The Herschel Space Telescope observed plumes of water vapour springing from the surface of Ceres; it 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 vapor.

A day on Haumea lasts 3.9 hours

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

Makemake was named three years after its discovery in 2005

The discovery of Makemake near Easter influenced both its name and nickname. Before being named after the creator of humanity and god of fertility in the myth of the Rapa Nui (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, surpassing the mass of Pluto by 28%. As such, it was a strong contender to become the tenth planet but failed to meet the criteria set by the IAU.

Pluto is one-third ice

The planet’s composition is two-thirds rock and one-third ice, mostly a mixture of methane and carbon dioxide. One day on Pluto is 153.6 hoursabout 6.4 Earth daysmaking it one of the slowest-rotating dwarf planets.

Exploratory missions and new planets on the horizon

With newer technology rapidly available to the scientific community and new exploratory missions obtaining more data and information on trans-Neptunian objects, our understanding of dwarf planets will increase.

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

So what’s holding back Hygiea’s confirmation as a dwarf planet? The criterion of being massive enough to form a spherical shape is in contention; it is unclear if its roundness is a result of collision/impact disturbance or its mass/gravity.

Along with Hygiea, other exciting dwarf planets may soon be discovered. Here is a brief overview of some serious contenders:

120347 Salacia

Discovered in 2004, it is a trans-Neptunian object in the Kuiper belt, approximately 850 kilometers in diameter. In 2018, it is located approximately 44.8 astronomical units of the sun. Salacia’s status is in contention because her planetary density is questionable. It is not certain that it can exist in hydrostatic equilibrium.

(307261) 2002MS4

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

(55565) 2002AW197

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

174567 Varda

Varda is named after 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 as its size and albedo suggest it may not be an entirely solid body.

(532037) Financial year 201327

This space object has a surface diameter of approximately 740 kilometers. It orbits the Sun once a 449 years. Researchers need more data on the mass and density of the planet to determine whether or not it is a dwarf planet.

(208996) 2003AZ84

It’s about 940 kilometers on its longest axis, because it has an elongated shape. This shape is likely due to its rapid rotation rate of 6.71 hours, similar to that of other dwarf planets like Haumea. Like Varda, it is still unclear whether this object compressed into an entirely solid body and therefore remains controversial among astronomers regarding 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 like Gonzalo Tancredi, Michael Brown, and William Grundy. There are many other potential dwarf planets not listed here that remain under investigation.

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