Some people are still unhappy about Pluto’s demotion to a dwarf planet, but it’s in good company — there are some fascinating celestial bodies in this category.

What makes a planet a planet? We’ve been debating this almost since planets were discovered. In 2006, the International Astronomical Union came up with the first-ever scientific definition of a planet. With that came a new classification for these heavenly bodies that weren’t quite planets, but were more than just asteroids. While this resulted in the demotion of Pluto — previously our ninth planet — it also meant that there was a whole new group of objects to discuss: dwarf planets. Before this definition, we’d found objects that were bigger than Pluto and had many of its characteristics. So astronomers reasoned that if Pluto was a planet, then they would have to be considered planets, too — or we’d have to come up with a new definition. NASA planetary scientist Alan Stern coined the term ‘dwarf planet’ in 1990, but these smaller bodies were also called things like planetoids or sub-planets.

What makes a celestial body a dwarf planet may differ depending on which astronomer or planetary scientist you ask. According to the IAU, a dwarf planet is a celestial body that orbits around the Sun and has enough mass to keep its shape spheroid (maintaining hydrostatic equilibrium) but hasn’t «cleared the neighbourhood of its orbit.» This means that planets have to be the dominant body in its orbit and there are no other bodies near it that are close in size (except for a satellite). Dwarf planets don’t do that, and a dwarf planet can’t be a satellite of another planet, either. Everything else, except for satellites, is considered a small Solar System body. This includes most asteroids, comets, and most trans-Neptunian objects — objects that orbit the Sun at a distance further than the planet Neptune.

Currently there are five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. We have only observed Ceres and Pluto and know with certainty that they fit the IAU definition. Eris was added because its discovery in 2005 showed that it was more massive than Pluto (since then, we’ve found that it may be about the same size as Pluto) and some astronomers and planetary scientists wanted to deem it the tenth planet because of that. Ceres was discovered before any other dwarf planets, in 1801. It’s an asteroid and the only dwarf planet that lies in the inner Solar System, located in the asteroid belt between Mars and Jupiter. It’s been known by many terms since its discovery, including planet and asteroid. It’s now considered both the largest known asteroid and the smallest dwarf planet. The other dwarf planets are located in the Kuiper belt, an area of the Solar System that stretches from the orbit of Neptune to about 7.5 billion kilometres (4.7 billion miles) from the Sun.

Soon after the IAU defined dwarf planets, it expanded the definition. Now any unnamed trans-Neptunian object with an absolute magnitude, or brightness, greater than +1 (meaning that they likely have a diameter greater than 838 kilometres or 521 miles) is assumed to be a dwarf planet. For reference, Pluto’s absolute magnitude is -0.7. That’s how Haumea and Makemake came to be considered dwarf planets.

Although there are just five confirmed dwarf planets, there are likely many, many more. Mike Brown, professor of Planetary Astronomy at the California Institute of Technology, keeps an updated list that includes «eight objects that are nearly certainly dwarf planets, 30 objects that are highly likely to be dwarf planets, 60 objects that are likely to be dwarf planets, 103 objects which are probably dwarf planets, and 394 objects which are possibly dwarf planets.» Not all scientists agree with Brown, but they do generally agree that there are at least 100 and possibly up to 200 dwarf planets. Most of these lie in the Kuiper belt. If we go beyond that, there could be thousands of objects that meet the standard for dwarf planets.

Incidentally, Alan Stern didn’t agree with the dwarf planet definition mandated by the IAU. Neither did a lot of other astronomers and planetary scientists. The vote took place on the last day of a conference and included just 424 astronomers, out of about 10,000 IAU members. Stern also argues that there are planets that haven’t «cleared the neighbourhood of its orbit,» including Earth, which has thousands of near-Earth asteroids orbiting along with it.

Dwarf planets inside and out.

Each dwarf planet is different, but they do have two things in common: a rocky core and icy mantle.

The definition of a dwarf planet doesn’t include anything about their structure, but from what we know they do seem to have a lot of similarities. Aside from former planet Pluto, we know the most about Ceres. Ceres is the largest asteroid in the asteroid belt between Mars and Jupiter. Some believe that it has a 100-kilometre (62-mile) thick water-ice mantle atop a rocky core. If this is true, then Ceres could have more fresh water than Earth. There could also be a layer of liquid water underneath.

Ceres’s crust is likely to be a thin, dusty layer of carbonate and clay minerals, similar in many ways to other asteroids. The Hubble Space Telescope, as well as the Keck Observatory Telescope on Earth, have shown the presence of features such as craters, but that’s about all we know of the surface. If Ceres has an atmosphere, it’s a weak one, with water frost. Surface temperatures are estimated at -38°C (-36°F). Because of the presence of so much water ice, some scientists speculate that Ceres is a good candidate for extraterrestrial life.

Unlike Ceres, Haumea is thought to be made up of solid rock, with a thin layer of crystalline ice.

This isn’t typical of objects in the Kuiper belt, which are more likely to have a thicker mantle of ice. But Haumea also has two small moons, Namaka and Hi’iaka, so the collision that possibly created the whole system may have removed most of its ice. The ice layer makes it appear bright white — Haumea is the third brightest object in the Kuiper belt behind Pluto and Makemake.

Despite that brightness, we have less information on Makemake’s structure. It does likely have the rocky core and icy mantle, and its reddish colour indicates the presence of methane.

Eris is probably about the same size as Pluto, but its size doesn’t mean that we know a great deal about its make-up. It’s around twice as far from the Sun than Pluto, so getting details has been difficult. Speculation is that it’s similar to Pluto in composition, again with a rocky core, icy mantle and gases, such as methane, on the surface.

The Kuiper belt.

The Kuiper belt is a region of objects located beyond the orbit of Neptune at about 4.5 billion km (2.8 billion miles) to about 7.5 billion km (4.7 billion miles) from the Sun. Like the asteroid belt, the Kuiper belt contains objects leftover from the formation of the Sun. But the Kuiper belt is much larger than the asteroid belt.

It was discovered in 1992, after years of speculation by astronomers about the existence of trans-Neptunian objects. The name ‘Kuiper belt’ comes from astronomer Gerard Kuiper, who oddly enough did not believe that there was a belt of objects located beyond Neptune’s orbit. He believed that objects leftover from the Sun’s formation likely went out towards the Oort cloud — a hypothetical cloud of tiny objects located almost a light year from the Sun — or even out of the Solar System entirely. In Kuiper’s time, however, Pluto was believed to be about the size of Earth.

There are more than 1,000 known objects in the Kuiper belt, and as many as 100,000 may exist. They are called Kuiper belt objects or KBOs. The belt’s proximity to Neptune affects many of the objects, causing orbital resonances. This means that one object’s gravity affects the other in a regular, periodic way. In some cases, it’s a mean motion resonance, or an exact ratio. For example, about 200 objects in the belt are in a 2:3 resonance with Neptune. They orbit the Sun twice for every three Neptunian orbits. This group includes Pluto and its satellites. Eris may be in a 17:5 resonance, while Haumea is believed to be in a 12:7 orbital resonance with Neptune.

A section called the classical Kuiper belt lies between the 1:2 and 2:3 resonances. Here Neptune’s gravity does not exert enough of an influence on the KBOs to create resonances. The classical Kuiper belt comprises about 67 per cent of all known KBOs. Objects here are sometimes called cubewanos, after the latter half of the name for the first trans-Neptunian object found after Pluto and Charon: (15760) 1992 QB1. The dwarf planet Makemake is a cubewano.

How big is a dwarf planet?

Measuring a dwarf planet is more complicated than it seems.

As the name ‘dwarf planet’ indicates, size has a lot to do with the designation. To be a dwarf planet and not an asteroid or other small Solar System body, an object has to be big enough to have sufficient gravity to pull the object into a stable spheroid (Haumea, with its ellipsoid shape, is an exception because it’s considered stable). This can’t be defined by a specific measurement, because it varies depending on both the object’s composition and its history. For example, astronomer Mike Brown believes that rockier bodies reach hydrostatic equilibrium at about 900 kilometres (559 miles) and icier ones, between 200 and 400 kilometres (124 and 249 miles).

Compounding the issue is the fact that it can be difficult to measure the size of distant objects. We estimate sizes of Solar System objects by measuring their absolute magnitude (brightness), as well as their albedo (reflectivity). Absolute magnitude allows astronomers to measure the brightness of Solar System objects as if they were all the same distance from the Sun and the Earth and at the same angle. A negative absolute magnitude indicates a bright object, while positive numbers indicate dimmer objects. Albedo is a ratio of reflected sunlight, so an albedo of 1 would be a perfect reflection of a white surface and zero would be no reflection of a perfectly dark surface. The presence of satellites or other objects around it also helps determine an object’s mass. Yet all of the measurements are estimates, with varying margins of error.

«It can be difficult to measure the size of distant objects»

Initially the IAU did not establish limits for dwarf planet size. Later, it clarified that dwarf planets must have an absolute magnitude brighter than +1. This means that its diameter will be greater than 838 kilometres (521 miles), assuming an albedo greater than or equal to 1.

Estimates of Pluto’s diameter have varied by as much as 70 kilometres (44 miles) depending on the instrument used and the haze in its atmosphere. When Eris was discovered, its diameter was estimated to be 2,397 kilometres (1,500 miles) -making it larger than Pluto. Later it was revised, and given the margins of error, these dwarf planets are considered to be roughly the same diameter. Eris is the more massive of the two, with a mass about 0.27 per cent that of the Earth’s mass. It also has an albedo of 0.96, one of the highest in the Solar System, and an absolute magnitude of -1.19.

When it comes to dwarf planet candidates, there is some controversy. Since the IAU has decreed that a dwarf planet must have an absolute magnitude brighter than +1, which potentially rules out some otherwise good candidates if you consider Mike Brown’s list of possible dwarf planets. For example, dwarf planet candidate Sedna has an absolute magnitude of 1.8 and the latest measurements estimate a diameter of 995 kilometres (618 miles), give or take about 80 kilometres (50 miles). This is large enough to be spherical. The largest estimated unnamed object in the Solar System is a trans-Neptunian object currently named 2007 0R10. It has a very reddish surface and an estimated absolute magnitude of 2, but a diameter between 1,070 and 1,490 kilometres (665 and 926 miles). The object is just too far away to get a better measurement and to be sure of hydrostatic equilibrium.

So, although there are just five officially confirmed dwarf planets currently, many astronomers and planetary scientists agree that in reality far more objects should probably be classified as more than just KBOs. It’s likely a matter of getting better measurements of the candidates as well as the IAU making some more changes.

Exploring the Kuiper.

NASA’s New Horizons spacecraft will attempt to learn more about the Kuiper belt and its objects.

The New Horizons spacecraft was launched by NASA in January 2006 and is currently a little over two years away from approaching Pluto, with an estimated arrival date of July 2015. The probe is on a mission to be the first to visit Pluto as well as its moons, Charon, Nix, Hydra, S/2011 P 1, and S/2012 P 1. Afterwards, New Horizons will attempt to explore other objects in the Kuiper belt, but those objects are yet to be determined.

The spacecraft has been dubbed «the fastest spacecraft ever launched,» and scientists were in a hurry to get it to Pluto as soon as possible for a reason. Pluto has been steadily moving away from the Sun, and its atmosphere will eventually freeze. The sooner we can get to the dwarf planet, the better we’ll be able to study its atmosphere. The rush also has to do with the amount of sunlight that Pluto and Charon receive — NASA wants to reach it while most of the planet is in sunlight and before it becomes more difficult to take photos.

New Horizons will pass within 10,000 kilometres (6,200 miles) of Pluto and 27,000 kilometres (17,000 miles) from Charon. It will provide the best photographs yet of both, and also take detailed measurements of Pluto’s surface and atmosphere.

This part of the mission will take about 24 hours, or a full Earth day. Afterwards, New Horizons will visit a KBO, one that is 50 to 100 kilometres (30 to 60 miles) across. It will map the object, look for an atmosphere and take photographs and other measurements. The KBO part of the mission is expected to take place between 2016 and 2020.

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