Since its discovery in 1930, Pluto has been a bit of a puzzle.
To begin with, Pluto is not only smaller than any other planet in the solar system, but it is also smaller than the Earth’s moon. It also has only an extremely low gravitational pull 0.07 multiply the mass of objects in its orbit, which is only a fraction of the Moon’s own strength.
At the same time, Pluto’s surface is similar to the surface of land planets like Mars, Venus or Earth, but its closest neighbors are the gaseous Jovian planets like Uranus or Neptune. In fact, Pluto’s orbit is so irregular that it led many scientists to first believe that it originated elsewhere in space and that the Sun’s gravity drew 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.
When Eris and other trans-Neptunian objects shared similar properties 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 “true” planets, and what are their properties?
The history of the dwarf planet
A dwarf planet is a celestial body that almost meets the definition of a “true” planet. According to the IAU, which sets definitions for planetary science, a planet must:
- Orbits around the Sun.
- Has enough mass to achieve hydrostatic equilibrium and assume an almost round shape.
- Dominate its path and do not share it with other objects.
Dwarf planets, along with not being moons or satellites, fail to clear the neighborhoods around their orbits. This is the primary reason Pluto lost its status: Because it shares part of its orbit with the Kuiper Belt, a dense area of icy space bodies.
Based on this definition, the IAU has recognized five dwarf planets: Pluto, Eris, Makemake, Haumea and Ceres. There are four more 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 coming years, and as many as 200 or more are believed to exist in the outer solar system of the aforementioned Kuiper Belt.
Ceres is the earliest known and smallest of the current category of dwarf planets. Formerly classified as an asteroid in 1801, it was confirmed to be a dwarf planet in 2006. Ceres is located between Mars and Jupiter in the asteroid belt, and it is the only dwarf planet that orbits closest to Earth.
Here is a brief introduction to the most recognized dwarf planets:
speed (km / s)
|Orcus||Kuiper calls (plutino)||247||4.75||910||26%||1|
|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 to have||Kuiper belt (cubewano)||306||4.41||1430||41%||1|
|Gonggong||Scattered disk (10: 3)||554||3.63||1230||35%||1|
|Sedna||Detached||~ 11,400||~ 1.3||995||29%||Not applicable|
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 steam every second
The Herschel Space Telescope observed water tubes shooting up from Ceres’ surface; 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 to steam.
A day at 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 collision origin 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 near Easter affected both its name and nickname. Before Makemake was named after the creator of mankind and the god of fertility in the myths of Rapa Nui (the natives of Easter Island), he was nicknamed the “Easter Bunny” by its discoverer Mike Brown.
Eris was once considered the position on the 10th planet
Eris is the most massive dwarf planet in the solar system, surpassing Pluto’s mass 28%. As such, it was a serious challenger to become the tenth planet, but failed to meet the criteria set by the IAU.
Pluto is a third ice
The composition of the planet makes up two-thirds of rock and one-third of ice, mostly a mixture of methane and carbon dioxide. A day on Pluto is 153.6 hours, around the rain 6.4 Earth Days, making it one of the slowest rotating dwarf planets.
Exploration missions and new planets on the horizon
With newer technology rapidly available to the scientific community and new exploratory missions gaining more data and information on trans-Neptunian objects, our understanding of dwarf planets will increase.
Located 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 marks all the boxes needed to be classified as a dwarf planet.
So what is holding Hygiea’s confirmation back as a dwarf planet? The criterion of being massive enough to form a spherical shape is in conflict; it is still 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 is a quick overview of some serious candidates:
Discovered in 2004, it is a trans-Neptunian object in the Kuiper Belt, approx. 850 kilometers in diameter. From 2018, it is located approx. 44.8 astronomical units from the sun. Salacia’s status is in conflict because its planetary density can be argued. It is uncertain whether it can exist in hydrostatic equilibrium.
(307261) 2002 MS4
With an estimated diameter of 934 ± 47 kilometers, 2002 MS4 is comparable in size to Ceres. Scientists need more data to determine if the 2002 MS4 is a dwarf planet or not.
(55565) 2002 AW197
Discovered at the Palomar Observatory in 2002, and has a rotation period of 8.8 hours, a moderate red color (similar to Quaoar) and no apparent planetary geology. Its low albedo has made it difficult to determine whether it is a dwarf planet or not.
Varda takes its name from the queen of Valar, the 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 that it may not be a completely solid body.
(532037) 2013 FY27
This space object has a surface diameter of approx. 740 kilometers. It orbits the Sun once each 449 years. Scientists need more data on the planet’s mass and density to determine if it is a dwarf planet or not.
(208996) 2003 AZ84
It’s approx 940 kilometers across its longest axis as it has an elongated shape. This shape is probably due to its fast rotational speed 6.71 hourssimilar to other dwarf planets like Haumea. Like Varda, it is still unknown whether this object has 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, particularly among leading planetary scientists such as Gonzalo Tancredi, Michael Brown, and William Grundy. There are many more potential dwarf planets not listed here that are still under investigation.