We all know our solar system has 8 planets (or 9, before Pluto’s demotion).
But few people know that there are actually many others! Technically, they’re dwarf planets, and are still being discovered and debated as we speak.
These are the dwarf planets in our solar system
The International Astronomical Union (IAU) came up with the “dwarf planet” classification in 2006. Since then, we have established that Pluto, Haumea, Makemake, Eris, and Ceres are dwarf planets. However, astronomers estimate that there are around 200 dwarf planets in our solar system alone. Some of the more obscure candidates include Orcus, Quaoar, Gonggong, Sedna, Varuna, Ixion, and Charon. But unlike the first five, the IAU hasn’t yet confirmed them.
The definitions of planets and dwarf planets are quite open to interpretation. Let’s take a closer look at what these definitions are, and whether these new dwarf planet candidates make the cut!
What Makes A Dwarf Planet Different From A Planet?
The term “dwarf planet” was coined by Alan Stern, a famous planetary scientist, when he introduced the three possible ways of categorizing planets: classical planets, such as Earth and Mars, dwarf planets like Pluto, and satellite planets, such as the Moon.
The term has been used to describe planets that are too small to be listed as full-fledged planets but too large to be considered asteroids or any other smaller celestial objects. In 2006, the term “dwarf planets” was adopted by the International Astronomical Union – IAU, and here is how.
In January 2005, a new space object was discovered – Eris. At first, it was considered more massive than Pluto, and for that reason, it was listed as the tenth planet in our solar system. A colossal controversy soon started, as not all scientists agreed with this new addition to the planet’s list.
The debate that followed was intense, and it came to an end during the IAU General Assembly in August 2006, when Resolution 5A: “Definition of ‘planet'” was introduced.
According to this resolution, all celestial bodies can be divided into three categories, as follows:
- Dwarf planets
- Small solar system bodies (a catch-all term for whatever isn’t a planet, dwarf planet, satellite, or moon)
As reported by the International Astronomical Union, a planet is any celestial object orbiting around the Sun with enough gravity to overcome rigid body forces to pull its mass into a round shape.
This state is known as hydrostatic equilibrium, and the celestial object can clear its orbital path so that no other smaller bodies can be found near it.
The last criterion is the one that sets the borderline between planets and dwarf planets as the gravity of a dwarf planet is not strong enough to push away the smaller celestial objects that converge its orbit.
As a result of this new standard, Pluto lost its status as the ninth planet of our solar system and was listed as a dwarf planet, along with Eris, the planet which started the debate in the first place. However, many astronomers still believe that Pluto should be considered a planet and not a dwarf planet.
The Current Dwarf Planets In Our Solar System
After the 2006 General Assembly of IAU and their acceptance of this new category, three celestial bodies have been listed as dwarf planets: Pluto, Ceres, and Eris.
Later in 2008, Haumea and Makemake were also announced as dwarf planets in an IAU press release, even though it couldn’t be proved at the time.
Today, these are the only five dwarf planets accepted by the International Astronomical Union. Let’s see some details regarding them.
Location: Kuiper belt
Pluto was first discovered on February 18, 1930, by Clyde W. Tombaugh and announced a month later. It is named after the Roman god of the underworld. Pluto was considered a planet for the next 76 years until it was reclassified as a dwarf planet on August 24, 2006.
Pluto is the largest dwarf planet, with a diameter of 2,372 km / 1,473 mi, and the second most massive after Eris, with a mass of 1.31 × 10^22 kg.
It is found at about 5.9 billion km (3.7 billion miles) from the Sun, and because of this incredible distance, it takes five hours for the Sun’s light to reach it. For that reason, it has extremely low temperatures, around -229°C, and most of the water present on Pluto, which constitutes one-third of its total mass, is found in the form of ice.
On July 14, 2015, Pluto was visited by the New Horizons spacecraft, which took a series of images and took some measurements. It discovered a series of craters, which means that the surface of the dwarf planet is relatively young.
Its orbital rotation around the Sun takes about 248 years to be completed. Pluto has a gravity strong enough to attract five known moons: Hydra, Nix, Charon, Kerberos, Styx, and the less imaginatively named S/2012 (134340) 1.
Location: Asteroid belt
It was first discovered on January 1, 1801, by the Italian astronomer Giuseppe Piazzi, and it was considered a planet for the next half a century before being classified as an asteroid. Its status changed again in 2006 when it was listed as a dwarf planet.
Ceres is the closest dwarf planet to the Sun, being located between Mars and Jupiter. It is the smallest dwarf planet with a diameter of only 950 km / 590 mi, and it is the only dwarf planet that has no moon of its own.
Ceres is usually described by scientists as an “embryonic planet” as it is believed that Jupiter’s gravity prevented Ceres from becoming a fully developed planet billions of years ago.
Location: Kuiper belt
Eris was discovered on January 5, 2005, and announced as the tenth planet on July 29, but since Resolution 5A had been adopted in 2006, it is listed as a dwarf planet.
Eris is the most massive dwarf planet, with a mass of 1.66 × 10^22 kg, exceeding Pluto’s mass by 28%. It is so far from the Sun that its atmosphere collapses and freezes on the surface.
As a result, it reflects the Sun’s light the same way the snow does, and due to its relative brightness, it was considered the largest dwarf planet at first, but later it was established that it is only the second largest, after Pluto.
Eris is orbited only by one moon, Dysnomia. It is named after a Greek goddess of discord, and rightfully so since its discovery was the first step in Pluto’s reclassification and a lot of controversies ever since.
Location: Kuiper Belt
It was discovered on March 31, 2005, by Michael E. Brown, Chad Trujillo, and David Rabinowitz and announced on July 29. Three years later, on July 11, 2008, it was listed as a dwarf planet.
It is the third-largest dwarf planet, with an equatorial diameter of 1,434 km / 891 mi, and a polar diameter of 1,422 km / 883 mi, and it is the second furthest dwarf planet from the Sun.
Until April 2016, it was believed that Makemake is the only one not to have a moon, but it was soon discovered that it is orbited by MK 2, which was spotted about 20,000 km from Makemake by the Hubble Space Telescope.
Location: Kuiper Belt
Haumea was discovered on December 28, 2004, by Michael E. Brown and his team of astronomers, and announced on July 27, 2005. It was named only later on July 27, 2005, when it was also listed as a dwarf planet.
Its uniqueness lies in its odd shape that makes it the least spherical of the dwarf planets. It’s quick spinning led to an elongated shape, turning itself into an ellipsoid with an equatorial diameter of 1,960 km / 1,217 mi and a polar diameter of only 996 km / 618 mi.
It is the fastest spinning dwarf planet out there, with a day on Haumea lasting only 3.9 hours. Haumea is also the only currently known dwarf planet to have a ring system. Haumea is known to be orbited by two moons Hi’iaka & Namaka.
Possible Dwarf Planets In Our Solar System
It is believed that in the solar system, including the Kuiper Belt, there can be up to 200 dwarf planets, if not even more. Many celestial objects can be classified as dwarf planets, and some astronomers pledged for their recognition.
For example, in 2008, Tancredi proposed to the IAU to list Orcus, Sedna, and Quaoar as dwarf planets, but they did not respond to this proposal until this day.
As the “dwarf planet” category is relatively new, it is hard to make a specific distinction between small solar system bodies and dwarf planets.
For that reason, any possible candidate for this title has to go through a thorough investigation, but most importantly, they must meet the four criteria of a dwarf planet:
- It must orbit the Sun.
- Its gravity must overcome rigid body forces to pull its mass into a round shape, a state known as hydrostatic equilibrium.
- It has to be able to clear its orbital path so that no other smaller bodies can be found near it.
- It cannot be a satellite/moon
To help this process, Alan Sterna and Harold F. Levison introduced a new parameter named lambda – Λ. The value of this newly introduced parameter is inversely proportional to the period and proportional to the mass’s square.
Lambda can be used to tell if a celestial object is capable of clearing its neighborhood or if it doesn’t hold this capacity. If lambda is bigger than one, the celestial object will clear its orbit in time.
Using this parameter, astronomers could tell if a mass can become a dwarf planet or not, but there are still plenty of debates on this matter.
We have to keep in mind that the number of dwarf planets in the solar system is unknown, so the possibilities are infinite.
As of writing several celestial bodies that have already been discovered and will most likely become official dwarf planets, but are yet confirm. These are: Orcus, Quaoar, Gonggong, Sedna, Varuna, Ixion, and Charon.
As remote as the heavenly bodies are, we learn (and decide) more about them every day. We won’t “lose” another planet besides Pluto’s reclassification, but who’s to say what other surprises might lay ahead?