It began in August of 2006 at a meeting of the International Astronomical Union (IAU). Pluto was officially reassigned to the newly defined category of dwarf planets. It was joined in September of the same year by Xena, now officially named Eris (pronounced eeriss) and the former asteroid, Ceres. We are now up to 5 and counting, as the number will likely continue to grow. For detailed definitions see the previous Astronomy Highlight, “What is a Planet”. Meanwhile, a dwarf planet could be very roughly defined as a planet-like object, but one which has not cleared its orbit of debris.

So, it is mainly this one thing, debris, that separates the dwarf planets from the planets. Lets have a look at these relatively small objects, and where they live.

The five are, in order of distance from the Sun: Ceres, Pluto, Haumea , Makemake, and Eris. Soon we will need to work out a new rhyme to help us keep them straight. To understand these celestial bodies, we need to explore their origins. They reside in 2 completely separate regions of the Solar System known as the Asteroid Belt and the Trans-Neptunian region, which includes the Kuiper Belt. These are, as the names hint at, belts or bands of stuff circling the Sun.

Before continuing, a few words about distance: an Astronomical Unit, one AU, is simply the average distance of Earth from the Sun and therefore is a convenient and meaningful way for us to visualize distances inside the Solar System. It is an average distance and not a precise one because Earth’s path around the Sun is not a perfect circle, rather an ellipse which carries it sometimes closer and sometimes farther away. One AU is approximately 93,000,000 miles or 150,000,000 kilometres.

The Asteroid Belt of rocky objects lies between the orbits of Mars and Jupiter, at a distance of from approximately 2 – 4 AU. We believe it to be a potential planet that never coalesced out of the disc of material around the Sun from which the other planets formed. This is thought to be in part due to the gravitational forces exerted on the material in the Belt by massive Jupiter, orbiting the Sun at around 5 AU. The Asteroid Belt consists of millions of bits of rock ranging from dust particles to the size of a small planet, and is very sparsely populated, unlike the Hollywood image as a hiding place or obstacle course for spacecraft.

The Kuiper Belt is a rather different region. Lying primarily beyond the orbit of the most distant planet, Neptune, it is more of a donut-shaped band of frozen nitrogen and volatiles like water, methane and carbon monoxide; leftovers from the early days of the formation of the Solar System. The Kuiper Belt appears to begin at about 35 AU from the Sun and extends as far as 55 AU. It is part of a much larger Trans-Neptunian region of icy rubble that may stretch to around 100 or more AU from the Sun.

The Asteroid Belt dwarf planet

Ceres, the third dwarf planet to be recognized, was actually designated as a planet when it was first discovered in 1801. Later, after the discovery of several similar objects in a similar orbit, it was changed to asteroid status. In September 2006 it was put into the new category of dwarf planets, along with Pluto and Eris. Ceres is a rocky sphere largely made of carbon and silica, and possibly water ice, orbiting the Sun at 2.8 AU. Ceres’ year is about 4 ½ Earth years long and its day a short 9 Earth hours and 4 minutes. It appears to have a thin atmosphere. The largest member of the Asteroid Belt, it contains approximately 1/3 of the mass of the entire Belt. No natural satellites have been found in orbit around Ceres.

Ceres, the first asteroid to be detected, in 1801, is about the size of Texas. With a warm surface, Ceres contains water-bearing minerals, and possibly a very weak atmosphere and frost. This photo was taken with the Hubble Space Telescope in January of 2004.

Three smaller asteroids, similar but less rounded, Vesta, Pallas and Hygiea, are potential candidates for dwarf planet status.

The Trans-Neptunian dwarf planets

Pluto is a small ball of rock and ice, about a fifth the mass of our own moon, with three icy moons of its own that we know of. It has an atmosphere of methane and carbon monoxide. Its surface is covered with frozen nitrogen, and it resides in the near edge of the Kuiper Belt, taking around 248 Earth years to orbit the Sun. The orbital path is very elliptical, carrying Pluto from as close as 30 AU to as far as 49 AU from the Sun. One of its moons, Charon, is more than half the size of Pluto making them actually more of a double, or binary, dwarf planet system. The other two known moons of the system, Hydra and Nix, are tiny in comparison.

Pluto became, with much uproar and controversy, the first member of this new category of Solar System objects at the August 2006 meeting of the International Astronomical Union. This decision left some people feeling unsatisfied, as though Pluto had been demoted. Actually, instead of being the planet that we knew the least about, being so far away from us, Pluto became the dwarf planet that we know the most about, “King of the Kuiper Belt” as it was put to me by a young girl on a tour of our local observatory.

In 2015, when the New Horizons spacecraft flies by Pluto we hope to learn a great deal more about its makeup, atmosphere and behaviour.

When Pluto was first discovered in 1930, it was thought to be the most distant thing in the Solar System. Over the years several scientists predicted the existence of the icy belt to which it actually belongs, and only as late as 1992 was the Kuiper Belt demonstrated to actually exist. In 2006 the Hubble Space Telescope captured this beautiful image of Pluto and its moons:

Dwarf Planet Pluto (large), moon Charon (smaller by about half) and two tiny moons, Nix and Hydra.

Haumea, next in order of distance from the Sun, is the latest addition to the growing list of dwarf planets. This icy object orbits the Sun in the Kuiper Belt, once every 285 Earth years, along with its two moons, Hi’iaka which takes 49 Earth days to complete one circuit around its parent dwarf planet, and Namaka with a 34-day orbit. Its average distance from the Sun is approximately 43 AU. Haumea is named for the patron goddess of the island of Hawai’i where it was discovered, and its moons are named for her daughters. One Haumean day appears to be just 4 Earth hours long as it spins quite rapidly. It contains water ice and at least one of its moons is covered with water ice. Observations of Haumea suggest that it has an oval shape, twice as long as it is wide.

An artist’s conception of Kuiper Belt resident, Haumea, and its two moons, Hi’iaka and Namaka. Haumea is the fifth Solar System body to be appointed to the category of dwarf planet.

Makemake, became a dwarf planet in June of 2008, before Haumea, but is more distant from the Sun. It is spherical and probably slightly larger than Haumea. Apparently unaccompanied, Makemake orbits the Sun once every 310 Earth years at an approximate average distance of 46 AU, in the farther side of the Kuiper Belt. The presence of methane and possibly nitrogen have been detected on its surface.

Eris, the largest dwarf planet, spends most of its days far beyond the Kuiper Belt. Its long narrow elliptical orbit carries it, along with its one known moon, Dysnomia, from 38 AU to as far as 97 AU (more than twice as far) from the Sun and requires 557 Earth years to complete one circuit, a very long Eridian year. Eris is what we call a Trans-Neptunian Object (TNO) spending very little of its time in the realm of the Kuiper Belt. It is more than 25% more massive than Pluto but residing at a far greater distance, hence was not detected until 2003, almost three quarters of a century after Pluto. Like Pluto, the surface of Eris appears to have methane ice.

Dwarf planet Eris (Xena) and its one known moon, Dysnomia.
Image by Hubble Space Telescope.

Four more TNO dwarf planet candidates are Sedna, Orcus, Quaoar, and Varuna.

It was the discovery of Eris that largely motivated the decision of the IAU to render a formal definition of a planet, and to create the new category of dwarf planet to handle these newly discovered objects that orbit the Sun with thousands of their less formed rocky and icy companions.

One of the questions I am frequently asked is “What happened to poor Pluto?” Some of us appear to have taken its reclassification personally. The debate about whether Pluto should have the status of a planet is not a new one but gained momentum steadily as technological advances enabled us to learn more about it and about the region of space it occupies. The discovery in 1978 of Pluto’s moon Charon, and confirmation in 1989 of Pluto’s atmosphere, eased the difficulties a little in the minds of many people. The scientific debate was re-fueled, however, with the discovery in 2003 of a larger-than-Pluto object, informally named Xena by its discoverer, very far out in the Kuiper Belt. The existence of this belt or ring of small icy worlds and frozen rubble left over from Solar System formation was confirmed in 1992. Since then scientists have accepted that Pluto and Charon are members of the Belt which orbits the Sun outside the orbit of Neptune, and which potentially contains very many similar objects.

What it all comes down to is how you define “planet” and the fact is that we didn’t used to define it, not formally. I might add, in the end it also came down to who exactly had the authority to decide the definition on behalf of the astronomical community and all the rest of us.

Solar System Montage from various JPL managed projects. Pluto was not represented as its small size and great distance preclude detailed photographs.

If you asked “What is a planet?”, most people would probably have answered by naming a few. And that certainly was fair enough in the absence of a definition. If you asked for a proper definition, you might have heard that a planet is round, and orbits the Sun or other star, and may or may not have some moons, rings, and an atmosphere – basically a description of the planets that we know. You might also have learned that the word “planet” comes from the Greek meaning “wanderer” - used by ancient Greeks to describe the heavenly bodies that appeared to wander amongst the other stars. The paths of the planets wander against the background of the stars because they are in orbit around the Sun, and so, from our perspective appear to go back and forth across the sky.

The General Assembly of the International Astronomical Union (IAU) met in Prague in 2006, and high on their agenda was to resolve the whole planet issue by voting on a formal definition An IAU subcommittee worked for over a year to devise a definition that would meet with the approval of a majority of members. Their proposal would not have surprised most people and Pluto would have remained officially a planet.

Ceres, the first asteroid to be detected, in 1801, would have gained full planet status under the IAU proposed definition. About the size of Texas and with a warm surface, Ceres contains water-bearing minerals, and possibly a very weak atmosphere and frost. This photo was taken with the Hubble Space Telescope in January of 2004

However, in a surprising upset the IAU voted for a definition that excluded Pluto as a planet, preferring to define a new category of object to be called a Dwarf Planet. Note that Dwarf Planets are not Planets, not a subset of the Planet category. The definition of a planet is: a celestial body that (1) is in orbit around the Sun, (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (3) has cleared the neighbourhood around its orbit. Note that the first criterion means that this definition is only applicable within our Solar System. Extra-solar “planets” will require their own categories as we are able to learn more about them.

The IAU confirmed three Dwarf Planets - Pluto, Ceres, and Xena which was later officially named Eris (pronounced eeriss). These three bodies and a recent addition, Makemake, satisfy the first two criteria for a planet, but not the third. The definition of a Dwarf Planet includes the first two criteria, but the third is that the body has not cleared the neighbourhood around its orbit. While there remains a lot of debris in orbit around a number of the planets (including Earth), the four dwarf planets lie in the rocky Asteroid Belt (Ceres) and the icy Kuiper belt, orbiting with hundreds of thousands of similar objects.

The fate of Pluto’s moon Charon remains less clear as, unlike the moons of the planets, it does not meet the strict criteria for a satellite. It is more than half of Pluto’s size (diameter) and its orbit with Pluto would seem to satisfy the requirements of a Double Dwarf Planet System.

The implications of 8, not 9, planets appear to be unsettling for many; teachers, publishers, astrologers, and, not least, the director of the NASA New Horizons robotic mission currently on its way to Pluto. A rather large (over 300) group of scientists including several planetary specialists, formalised a petition stating that they did not agree with and would not use the new definition. However, nearly two years later, the dust seems to have settled and the new definitions are in use.

The bottom line – definitions and categories are things that assist us humans in our scientific search for knowledge. They are tools that we use, not some kind of absolute truth. The truth probably is that Pluto has little regard for how we, a select group of inhabitants of the rocky water world third from the Sun, choose to categorise it. It is still there. It appears to not have changed a bit as a result of all our agitation and discussion. Accompanied by its icy moons, it still orbits the Sun, in a cold dark outer region of the Solar System, inspiring our spirit of adventure and our insatiable desire to know our universe.

March 2006 Hubble Space Telescope photo of Dwarf Planet Pluto (large), moon? Charon (smaller by about half) and two tiny moons, Nix and Hydra