Dwarf planets: all about these controversial almost-planets
If you know anything about the Pluto demotion controversy of 2006, you probably know that the former ninth planet of the solar system is now a lowly dwarf planet.
The decision by the International Astronomical Union (IAU) was certainly greeted with debate and criticism from the public, many of whom will likely tell you that they still consider Pluto a planet. Besides being a bit of a weird hill to die on for anyone who isn't a professional astronomer, it is also likely the result of a broader ignorance of dwarf planets as a category, which is a shame.
While some might feel like something was lost when Pluto was reclassified as a dwarf planet, we actually gained a far deeper understanding of a whole new kind of celestial body in our solar system, one that provides a crucial link to the early solar system and the era of planetary formation that ended about 4.5 billion years ago.
What is a dwarf planet?
According to the IAU, a dwarf planet is a celestial body that orbits the Sun, isn't the natural satellite of another body in the solar system, has not cleared the neighborhood around its orbit, and typically has enough mass and internal gravity strong enough to force its shape into a spheroid or near-spheroid shape.
"A dwarf planet may also orbit in a zone that has many other objects in it," the IAU added. "For example, an orbit within the asteroid belt is in a zone with lots of other objects."
This also applies to the Kuiper Belt out beyond the orbit of Neptune, home to nearly all the known dwarf planets or dwarf planet candidates.
What are the five dwarf planets in the solar system?
The first one of them is Pluto. The most well-known member of the dwarf planets, Pluto was discovered in 1930 and was originally considered the ninth planet in the solar system.
It is the largest dwarf planet by size and second-largest by mass. Typically orbiting out beyond the orbit of Neptune in the Kuiper Belt, the eccentricity of Pluto's 250-year-long orbit actually puts Pluto closer to the Sun than Neptune on occasion.
The surface of Pluto features large plains, glaciers, and mountains of nitrogen and water ice, including a distinctively shaped glacier that resembles the left hemisphere of a heart. Pluto has just 0.2% of the Earth's mass, and its mass is about a tenth that of the Moon's.
Despite its diminutive size, Pluto has five known moons of its own: Charon, Nix, Hydra, Kerberos, and Styx. Charon is especially interesting because its mass is near enough to Pluto's that the two bodies actually orbit each other as a binary system, with their joint center of gravity (its barycentre), lying in the empty space between the two, rather than within the mass of Pluto itself.
This gives Charon a pretty strong case for dwarf planet status in its own right, though, for now, the IAU designates it as a moon.
The second one is Eris. Discovered in 2003, Eris is actually what sparked the original debate that saw Pluto reclassified in 2006. This is because Eris is actually about 27% larger than Pluto by mass, and it was originally thought to be bigger in size as well. Eris's size was later refined to be slightly smaller in diameter than Pluto.
Eris's orbit is even more eccentric than Pluto's, taking 557 years to complete one orbit. Eris's aphelion, or the furthest point in its orbit from the Sun, is far beyond the extent of the Kuiper Belt, and its orbit crosses both Pluto's and Neptune's orbits.
Eris was given its name in 2005 to signify the controversy that its discovery sparked; Eris is the ancient Greek goddess of discord.
The first dwarf planet ever discovered, Ceres was identified in 1801 and was originally classified as an asteroid (the very first asteroid, in fact), though it ticks all the boxes as a dwarf planet so there is debate over whether it can be called an asteroid anymore. Asteroid or not, Ceres accounts for fully 25% of all the mass of the asteroid belt.
Ceres is also the only dwarf planet in the asteroid belt between Mars and Jupiter, making it the closest dwarf planet to us, and so the easiest to study up close. It was the first dwarf planet ever visited by a spacecraft (in 2015).
Ceres orbits the Sun in 4.6 years and orbits entirely within the mostly circular asteroid belt. It is a rocky world on the surface, but it is believed that a substantial percentage of Ceres's mass comes from a subsurface ocean of liquid water. Ceres does not have a moon, and it isn't massive enough to gravitationally "clear" its orbit in the asteroid belt.
Discovered in 2003, the dwarf planet Haumea stretches itself just under the wire to qualify as a dwarf planet. A key characteristic that dwarf planets must possess is enough mass that their shape is determined by the force of their own gravity.
Haumea isn't the typical spheroid shape of the other four dwarf planets but is instead shaped like an elongated sphere, similar to a rugby ball. This is likely because of its incredibly fast spin around its major axis, which takes just 3.9 hours to complete one rotation. As a result, Haumea is flattened somewhat by the angular momentum of this spin.
Haumea technically still qualifies, though, since the force of its own gravity is still the dominant force that determines its shape, with the spin-induced elongation distorting its shape after the fact.
Haumea has two moons that we know of, Hi‘iaka and Namaka, and it was recently discovered to have a small ring system, making it the only dwarf planet that is known to have one. Haumea is named for the Hawaiian goddess of birth and fertility and orbits out beyond Neptune, in the Kuiper Belt, with an orbital period of 285.46 years.
The most recent dwarf planet to be discovered, Makemake, named after the creator god of the Rapa Nui people of Easter Island, was spotted in 2005, just a year before the new dwarf planet category was created (Makemake wasn't officially classified as a dwarf planet until 2008).
Orbiting in the Kuiper Belt as far as 50 AU from the Sun, Makemake is an icy ball of rock, likely similar in composition to its neighbor Pluto, and is thought to be reddish in color as a result. It takes about 306 years to orbit the Sun, and it was found to have a small moon in 2015, which is unofficially nicknamed MK 2.
What makes a planet a dwarf planet?
There are three criteria that make a solar system body a dwarf planet.
First, it has to be a body orbiting the sun that is not a natural satellite of another body, like a planet. This would make it a moon, not a dwarf planet.
Second, it must be large enough that its gravity is strong enough to shape itself into a spheroid. Haumea is the only dwarf planet that pushes the boundary of what qualifies in this regard, but the other four dwarf planets are all appropriately sphere-shaped.
Finally, a dwarf planet cannot have gravitationally cleared its orbital path of debris the way a proper planet has. This is the defining distinction between a dwarf planet and a regular planet, and some have criticized this characteristic as arbitrarily defined to produce the desired result, namely to have a small number of planets in line with tradition.
"So, some scientists tried to develop a method to mathematically justify a small number of planets," argues Philip Metzger, a planetary scientist at the University of Central Florida's Florida Space Institute, "which was the criterion that a planet has to clear its own orbit. And this was really developed post facto to keep an orderly, small number of planets.”
Whether or not a body has cleared its orbit of debris doesn't provide any insight into the nature of the body, Metzger says, and this makes it a poor yardstick to measure a thing by. He also points out that until 2006, the idea that a planet had to have cleared its orbit in order to qualify as a planet was an entirely novel qualification that had never even been considered in the past when discussing what made a planet a planet.
“It’s a current description of the status of things,” said Metzger. “But if, for instance, a star passes by and disrupts our solar system, then planets are not going have their orbits cleared anymore.”
“It’s like defining ‘mammals,’” Metzger added. “They are mammals whether they live on the land or in the sea. It’s not about their location. It’s about the intrinsic characteristics that make them what they are.”
Still, there is clearly something qualitatively different between a dwarf planet like Ceres and even the smallest planet, Mercury, even if the distinction is difficult to precisely describe. Gut feelings are no substitute for actual science, however, so the cleared-its-orbit qualifier is at least better in this regard.
It also does tell us something about dwarf planets that makes them special in this context. In many ways, dwarf planets are relics of the past, leftover from a time when planetary formation was still occurring 4.5 billion years ago. Dwarf planets represent an intermediate stage in planetary development that all the major planets, including Earth, would have passed through on the way to their current state.
That is an important distinction that is worthy of qualification, even if it might mean we have to reimagine some cherished ideas we learned as kids.
Are there more dwarf planets out there?
Almost definitely. Already, there are dozens of possible dwarf planet candidates that are waiting to be confirmed by further study and observation, and while not all of them are going to be designated full-fledged dwarf planets, the odds are good that at least some of them will.
The solar system is a big place, and the Kuiper Belt, especially, is also really crowded. There are countless transneptunian objects out there that could check all the right boxes to join their neighbors Pluto, Eris, Haumea, and Makemake.
The asteroid belt between Mars and Jupiter, on the other hand, is pretty well mapped by this point. Only Ceres comes close to being a dwarf planet so we're not likely to get any new candidates from the asteroid belt.
Whether the qualifications for dwarf planet status continue to prove useful remains to be seen, of course. After all, up until 2006, Pluto easily qualified as a planet until new discoveries called that categorization into question.
With the accelerating rate of exoplanet discovery and new, more powerful instruments capable of giving us deeper insight into these exoplanets' characteristics and structure, we might find that new designations are required and that the current dwarf planet classification needs to be completely tossed out.
We won't know until some discovery forces the issue, as Eris's discovery did in 2005. If you're looking for reasons to hope that Pluto will get promoted from its current dwarf planet status and back to its former glory as a proper planet, it's something to look forward to, at least.