A visual introduction to the dwarf planets of our solar system


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 extremely weak gravitational pull at just 0.07 times the mass of objects in its orbit, which is only a fraction of the Moon’s own force.

At the same time, Pluto’s surface resembles that of terrestrial planets like Mars, Venus or Earth, but its closest neighbors are the Jovian gas planets like Uranus or Neptune. In fact, Pluto’s orbit is so erratic that it led many scientists to initially believe it came from somewhere else in space and that the Sun’s gravity drove it.

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 the many increasingly identified transneptunian objects (objects located beyond the planet Neptune), that the International Astronomical Union (IAU) defined classification criteria for planets.

With Eris and other transneptunian objects sharing similar characteristics with Pluto, the definition of dwarf planets was created and Pluto was declassified in 2006.

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

The history of the dwarf planets

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

  1. Orbiting the Sun.
  2. Have sufficient 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 being moons or satellites, fail to clean up neighborhoods around their orbits. This is the main reason why 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 as many as 200 or more are believed to exist in the Outer Solar System in the aforementioned Kuiper Belt.

Ceres is the oldest 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 sits between Mars and Jupiter in the asteroid belt, and it is the only dwarf planet that orbits the closer to Earth.

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

name Region of
Solar system
Orbital period
(in years)
Mean orbital
speed (km / s)
related to
the moon
Ceres Asteroid belt 4.6 17.9 939 27% 0
Killer whale Kuiper belt (plutino) 247 4.75 910 26% 1
Pluto Kuiper belt (plutino) 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
Gonggong 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 6kg of its vapor mass 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 vapor.

A day on Haumea lasts 3.9 hours

Haumea has a unique appearance due to its rotation, which is so fast that it squeezes the planet into an egg shape. Its speed of rotation 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

Makemake’s discovery near Easter influenced both his name and his nickname. Before being named after the creator of mankind and god of fertility in the myth of the Rapa Nui (the indigenous 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 strong candidate to become the tenth planet, but failed to meet the criteria set by the IAU.

Pluto is a third of ice

The make-up of the planet consists of two-thirds of rock and one-third of ice, mainly a mixture of methane and carbon dioxide. One day on Pluto is 153.6 hours, approximately 6.4 Earth days, making it one of the slowest spinning dwarf planets.

Exploratory missions and new planets on the horizon

With newer technology quickly available to the scientific community and new exploratory missions obtaining more data and information on Transneptunian 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 ticks all the boxes necessary to be classified as a dwarf planet.

So, what holds back Hygiea’s confirmation as a dwarf planet? The criterion to be massive enough to form a spherical shape is up for grabs; it remains unclear whether its roundness is the result of a collision / impact or its mass / gravity.

With Hygiea, other exciting dwarf planets could soon be discovered. Here’s a quick look at some serious contenders:

120347 Salacia

Discovered in 2004, it is a transneptunian object in the Kuiper belt, approximately 850 kilometers in diameter. In 2018, it is located approximately 44.8 astronomical units of the Sun. The status of Salacia is contested because its planetary density is questionable. It is not certain that 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. 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, it has a rotation period of 8.8 hours, a moderately red color (similar to Quaoar) and no apparent planetary geology. Its low albedo has made it difficult to determine whether or not it is a dwarf planet.

174567 Varda

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 the fictional mythology of JRR Tolkien. Varda’s status as a dwarf planet is uncertain as its size and albedo suggest it may not be a fully solid body.

(532037) 2013 financial year27

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 it is a dwarf planet or not.

(208996) 2003 AZ84

It is about 940 kilometers on its longest axis, because it has an elongated shape. This shape is probably due to its rapid rotational speed of 6.71 hours, similar to that of other dwarf planets like Haumea. Like Varda, it is still unclear whether this object compressed into a fully 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 study.

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