The decision to reclassify Pluto as a ‘Dwarf Planet’ back in 2006 was highly controversial. At the time it seemed as though everybody, whether they knew anything about planetary science or not, had something to say about the matter (I was one such person). Most thought that the new classification was over-complicating the issue.
Even now, most images of the solar system depict Pluto as the ninth and final planet, ignoring those equal or greater in size. Perhaps, if more people knew just how many ‘planets’ there are in the Solar System, they would understand the need for detailed classification.
New objects are being detected in the Solar System all the time, and while none are deserving of the title ‘planet’, they are deserving of the title ‘dwarf planet’ at the very least.
Before we take a look at some of these new planetoids, lets quickly review what the difference between a planet and a dwarf planet is.
What is a Dwarf Planet?
As of 2006, planetary objects now break down into three groups; planets, dwarf planets and moons.
To be classified as a planet, objects must tick the following three boxes:
1) Have sufficient mass to have become roughly spherical in shape (as opposed to smaller, less evenly shaped objects like asteroids)
2) Be in orbit of the sun, and not orbit another planetary body (as opposed to moons)
3) Have cleared the ‘neighbourhood’ around its orbit of other large bodies (the other objects either being scattered or becoming moons)
To be classified as a dwarf planet, objects need only fulfill the first two of these criteria – they must be roughly spherical and must not be moons. Being a dwarf planet therefore has little to do with size, and everything to do with having a shared (or unusual) orbit. However, it just so happens that all of the dwarf planets in our Solar system are smaller than the smallest planet, Mercury.
What is Pluto?
By this classification (and it is the official classification), Pluto is a dwarf planet. Pluto shares its orbital region with several other dwarf planets, known collectively as Plutinos, which are all in orbital resonance with (meaning their orbit is affected by) the planet Neptune.
Neptune is now the farthest actual planet from the sun, although there are many dwarf planets beyond this. Pluto isn’t even the largest of the dwarf planets; its neighbour Eris holds that distinction.
The 12 Dwarves
Time now to meet the new kids on the block – the dwarf planets.
So far there are 5 confirmed dwarf planets. These are Pluto, Eris, Ceres, Haumea and Makemake. However, there are at least 7 further dwarf planets awaiting official classification, including Orcus, Quaoar, Varuna, 2002 TC302, 2007 OR10, Ixion and Sedna.
With most of these planetary bodies having only been discovered in the past decade, we don’t yet know a great deal about them. Being considerably smaller than the Earths Moon, chances are they won’t be particularly interesting places to visit. However, their very existence is interesting, as it means that the outer Solar System is a much busier place than previously thought.
Lets take a closer look at some of these dwarf planets…
The nearest dwarf planet to the sun is not Pluto but Ceres, which orbits between Mars and Jupiter in the asteroid belt.
Ceres is the largest object in the asteroid belt (comprising about 1/3 of the belt’s mass) but is still considerably smaller than the Earth’s moon (less than 2% of the Moons mass, and 28% of its surface area). It orbits the sun once every 4.6 years.
Ceres is thought to have a rocky core, surrounded by a mantle of water ice (containing 200 million cubic kilometres of water). It’s surface is covered in carbonate minerals and clays.
All things considered, the dwarf planet Pluto has been a bit of a disappointment.
Despite the classification fiasco, in which the dwarf planet lost all of its credibility, Pluto remains the second most massive dwarf planet in the solar system (after Eris). Like Ceres, Pluto is also thought to have a rocky core and a mantle of water ice, but Pluto is also coated with a frosting of nitrogen ice at its surface. Pluto also possesses a very thin atmosphere of nitrogen, methane and carbon monoxide.
Pluto has three moons; the larger Charon, and the miniscule Nix and Hydra.
Orcus is often said to be the anti-Pluto because its orbit very similar to, but essentially the opposite of, the orbit of Pluto. If you think back to what you learned in science class, you’ll remember that Pluto has a highly elliptical (oval) orbit, which is highly inclined relative to the ecliptic (the orbital plane of the Earth). This is true of Orcus too. However, the orbit of Orcus is ’tilted’ the other way, mirroring that of Pluto. Also, when Pluto is at its aphelion (farthest distance from the sun), Orcus will be at its perihelion (closest to the sun) and vice versa.
The name Orcus also reflects this relationship, Orcus being the Etruscan name for the Roman god Pluto.
Orcus has one moon, Vanth. Vanth may be as much as 1/4 the size of Orcus. Again, this is similar to size relationship between Pluto and its moon, Charon.
Haumea, as its name suggests, is a very unusual dwarf planet. It was named for the Hawaiian goddess of childbirth, and has an unusual ‘squashed’ shape. The makeup of this planet is still something of a mystery.
Quaoar is a rocky dwarf planet whose origins are something of a mystery. One theory is that Quaoar may be the rocky core of a previously larger object, similar to Pluto. The theory suggests that Quaoar may have lost its icy mantle in a collision with another large object.
Both water ice and methane are thought to exist on the surface of Quaoar. The dwarf planet has one moon, named Weywot.
Makemake is the third largest dwarf planet in the solar system. Discovered at Easter,2005, it was temporarily nicknamed “Easterbunny”, and later named for the creator god of the inhabitants of Easter Island.
Makemake has a highly inclined orbit, at 29 degrees relative to the ecliptic. It’s surface is covered in methane.
Eris is the largest of the dwarf planets. When it was discovered back in 2005 it was briefly named Xena. Eris has both a highly elliptical orbit (with an aphelion of 97.5 AU and a perihelion of 37.7 AU) and a highly inclined orbit (44 degrees – the highest of any dwarf planet).
Eris is thought to have a composition similar to that of Pluto, with large amounts of water ice beneath its crust. However, there is evidence to suggest much larger concentrations of methane, both on its surface and beneath.
Eris has only one moon, named Dysnomia
Like Haumea, Varuna has the appearance of having been ‘squashed’ at its poles. This is the result of its unusually rapid rotation – a Varunian day lasting only 3.17 hours.
Sedna is the farthest dwarf planet from the sun, with a highly excentric orbit. In its 11,000 year trip around the sun, Sedna will reach a distance of nearly 1000 AU. That’s 1000 times the distance between the Earth and Sun. Only comets are known to orbit beyond this. You can read more about Sednas incredible orbit and how it might be useful to science fiction writers here.
The surface of Sedna is thought to feature frozen water ice and methane. Amazingly, despite its extreme distance from the sun, some scientists think that internal heating may make it possible for liquid water to exist beneath the surface.
2007 OR10 is the largest object in the solar system without a name, although its discoverers have given it the nickname “Snow White”. Like Quaoar, both water ice and methane are thought to exist on its surface.
So, to answer your question, there are 8 planets in the Solar System. But there are many more dwarf planets. In fact, there may even be some dwarf planets out there that we have not yet found.