Date: September 4, 2011
Title: The Dwarf Planets of the Outer Solar System
Podcaster: Joe Scaife
Description: Pluto, Haumea, Makemake and Eris are the dwarf planets of the outer solar system, with dozens more candidates set to join them in future. This podcast explores these worlds, and finds out why they’re just as unique and interesting as the larger major planets we’re familiar with.
Bios: Joe Scaife, known on the Internet as Procyon-Vulpecula, is a 20-year-old planetary science student from Manchester, UK. Ever since he was a child, Joe has been fascinated with the universe and in particular our neighbouring worlds in the Solar System. He holds a particular fascination with the newly discovered icy worlds in the outermost parts of the Solar System, and this is his first podcast.
Sponsors: This episode of “365 Days Of Astronomy” has been brought to you by Distant Suns 3, the award winning personal planetarium software for the iPad and iPhone. Unleashing your inner astronaut since 2008.
Transcript:
Hi, I’m Joe Scaife, a Planetary Science student at the University of Manchester in the UK.
It’s now been just over five years since the International Astronomical Union got together in Prague and after a tense and heated debate controversially defined the word “planet” for the first time. Their definition split everything that orbits the Sun into three groups: “planets,” “dwarf planets,” and “small solar system bodies.” While the definition was highly controversial, and debates continue to rage about whether dwarf planets should be considered a sub-class of planets or a different kind of entity altogether, all astronomers and planetary scientists agree that dwarf planets are intriguing and fascinating worlds.
So what is a dwarf planet? A “dwarf planet” is officially defined as an object that orbits the Sun, has enough mass for its own self-gravity to crush it into a rounded shape, but has not cleared the neighbourhood around its orbit, like the major planeta have. Take Ceres, for example, the largest object in the asteroid belt. Ceres is massive enough to be pulled into a rounded, near-spherical shape, like a planet. As far as we can tell, it has internal layers, with a crust, mantle and core. It may have once been geologically active, has surface features and possibly even has a thin atmosphere. Yet despite its planet-like attributes Ceres orbits in the middle of the asteroid belt, sharing its orbit with millions of smaller chunks of rock and metal, and so cannot be said to have cleared out its own orbital path like a larger object would be. Ceres is therefore a planet-like object in the middle of a belt of debris: a dwarf planet. Ceres appears to be the only dwarf planet in the asteroid belt, although its smaller cousins Vesta and Pallas are borderline. No, most of the dwarf planets lie far beyond Neptune, in the deep frozen Kuiper Belt.
The most famous of these Kuiper Belt dwarf planets, Pluto, was discovered back in 1930, long before any other Kuiper Belt objects were find. For a long time, Pluto was the black sheep of the planetary family. Smaller than Earth’s moon, Pluto has a tilted, inclined orbit, that sometimes takes it inside the orbit of Neptune – although there is no possibility the two worlds can ever collide. Unlike the inner, rocky terrestrial planets, or the outer gas giants, Pluto is made up of a unique mixture of rock and various ices, more similar in composition to a moon of a gas giant or a gigantic comet than any planet then known. With the discovery of the Kuiper Belt, however, Pluto makes sense. Orbiting amongst its small, icy cousins, with eccentric orbits, Pluto is no longer a misfit, but a big fish in a small pond, a perfectly ordinary dwarf planet – a class that far outnumbers the terrestrial or gas giant planets we’re familiar with.
Pluto itself appears to be made of a rocky core, surrounded by a mantle of ice. We would assume this is water ice, because water ice is so common out here, although no water ice has been found in Pluto’s spectrum. The surface is instead covered with exotic frosts, snowfall that we on Earth are familiar with as gases: carbon dioxide, methane, and even nitrogen. The reason Pluto’s surface is covered in frozen gases is because the dwarf planet appears to have a thin and tenuous atmosphere, that freezes onto the surface when it recedes too far from the Sun. We have seen little detail on its surface, but what fuzzy photos we can get from our best telescopes like Keck and Hubble reveal Pluto as a patchwork of bright and icy regions alternating with dark, reddish-brown patches. These are probably tholins – hydrocarbons, and other tarry organic materials formed by the action of ultraviolet light upon simple carbon compounds like methane and carbon dioxide. Not that there is much sunlight out here, though; from Pluto, the Sun is just a bright star in the sky, and sunlight here is 900 times weaker than it is on Earth.
Pluto has four moons, three of them being tiny and asteroid-sized. Its largest companion, however, named Charon, is about half Pluto’s size. It’s so big and massive relative to Pluto that the two worlds orbit a common point between them, rather than Charon orbiting Pluto, and so they have been described as a double dwarf planet. Charon, however, is not like Pluto. Its surface is less contrasting, and more neutrally coloured. It appears to be too small to hold on to an atmosphere, so its surface doesn’t contain volatile frosts; its water ice is exposed for all to see. And Charon’s density is less than Pluto’s, and not much more than that of water ice. A small, dull-coloured, less dense companion, lacking in atmosphere and volatiles, orbiting a larger, vibrant and colourful world with an atmosphere and a complex climate? The parallels between the Pluto-Charon system and the Earth-moon system are so striking that most astronomers agree they formed in much the same way – through a titanic collision. Another Kuiper Belt object once struck Pluto with such force it blasted out chunks of its icy mantle, which collected together to form the its faithful companion. A giant impact would explain another curious feature of Pluto – the fact that it orbits the Sun more or less “tipped on its side,” like Uranus.
The next dwarf planet out that the IAU currently recognises is Haumea, an amazing little world. Although it has enough mass to pull itself into a spherical shape, and therefore counts as a true dwarf planet, Haumea is actually far from spherical. The dwarf planet rotates so quickly the a day on Haumea lasts just four hours. This rapid spin has caused the initially round world to bulge out at its equator so much, it resembles an egg or a rugby ball more than it does a round planet. What caused Haumea to spin so quickly? Haumea has two tiny moons, Hiaka and Namaka, both made of water ice. Haumea is also bright and shiny, as if it’s covered in ice, and its spectrum reveals its surface is indeed made of ice. Yet when we use the orbits of the moons to calculate Haumea’s total mass – and therefore its density – we find Haumea is as dense as a rock. Haumea must be made up almost entirely of rock, with only a thin frosting of ice on its surface. What has happened to Haumea? Astronomers think that, like Pluto, Haumea once suffered a titanic collision, which blasted away its outer, icy layers, leaving behind a rocky core. The impact was at such an angle that it set the dwarf planet spinning like a kicked football, spinning so fast it stretched itself out into its current egg-like shape. Haumea’s moons are thought to be icy fragments left over from that collision, and astronomers have identified several small icy objects on orbits similar to Haumea’s that are believed to be remnants from that collision.
The next official dwarf planet out, Makemake, was discovered around Easter 2005, and was nicknamed “Easter Bunny” by the discovery team led by Mike Brown.. When it came to choosing an official name, Brown named it after the fertility god of Easter Island, giving two subtle connections to its temporary nickname. Makemake itself is unique amongst the dwarf planets in not having any known moons. Because it doesn’t have any moons orbiting it, it’s hard for scientists to measure Makemake’s mass and therefore density accurately, but it’s reasonable to assume it consists of a rocky core overlain by an icy mantle, if it’s anything like most other large Kuiper Belt objects. Makemake is just a little smaller than Pluto, and has a highly elliptical orbit, too. Although no atmosphere has been detected yet, Makemake’s surface is a deep reddish-brown colour, due to tarry organic tholin deposits on its surface, so we suspect Makemake may have a temporary atmosphere that, like Pluto’s, freezes as it recedes from the Sun.
The farthest currently recognised dwarf planet is Eris, discovered in 2005 – again, by Brown’s prolific team. Eris was named for the Greek goddess of discord and strife, who stirred up conflict among men and started the Trojan War, because its discovery ignited the ongoing controversy over what is and isn’t a planet. Eris is 27% more massive than Pluto and was initially thought to be substantially larger, so if Pluto is a planet, surely Eris must be too? As it turns out, Eris may in fact be smaller than Pluto – new measurements have shown that Eris and Pluto are around the same size, and it’s currently impossible to tell which is larger. However, since Eris is much more massive, it must be substantially denser than Pluto – for reasons nobody understands. Eris is probably mostly made of rock, with a relatively thin icy mantle around it. Eris was initially thought to be much bigger than Pluto based on its brightness, so for Eris to be as small as it is means its surface must be extremely reflective. It reflects 99% of all the light falling on it, giving it a blindingly bright, white, shiny surface. The reason for this can be seen when we look at Eris’ orbit. Eris is a scattered disc object – that is, a Kuiper Belt object that at some point passed too close to Neptune and was flung out into a huge, eccentric orbit, taking it well outside the main Kuiper Belt, by the giant planet’s gravity. Eris is currently at its farthest point in its orbit, far beyond the Kuiper Belt, at 97AUs – that’s 97 times further from the Sun than Earth is, and three times further than Pluto. At that distance, Eris is just a few degrees above absolute Zero – the coldest temperature possible. Its atmosphere freezes to the ground, covering whatever surface features it has in a thin layer of bright, reflective frost. Eris has one tiny, icy moon, named Dysnomia after Eris’ daughter in Greek Mythology and the demon spirit of lawlesness.
The International Astronomical Union currently recognises five dwarf planets – Ceres in the asteroid belt, and Pluto, Haumea, Makemake and Eris out in the Kuiper Belt. Yet astronomers reckon there may be several hundred dwarf planets out there – there are certainly many more than the IAU currently recognises. Why will the IAU commit to only five? The reason is that dwarf planets out in the Kuiper Belt are small, far away, dim and hard to observe. It’s very difficult to judge their shapes accurately, even if we can calculate they must be large or massive enough to pull themselves into a rounded globe. So while the IAU has been extremely cautious about putting them on the dwarf planet roster so far, scientists are convinced that many more objects they know of must be dwarf planets too. Quaoar, Sedna, Orcus… there are many other exciting dwarf planets out there, but sadly, time constraints restrict me to talking about the IAU’s official list. I hope I’ve inspired you to go look at these other worlds.
However many dwarf planets there are, scientists are still coming to terms with this new class of object in our solar system. The dwarf planets are largely unexplored, but that will all change in 2015, when NASA’s Dawn spaceprobe orbits Ceres in the asteroid belt, and its New Horizons probe will fly past Pluto and Charon before picking another Kuiper Belt object to explore. Now we have discovered an exciting selection of these new worlds, our telescopes and space probes will have plenty to look at over the next few years as we explore the new frontier that the dwarf planets of the outer solar system represent.
End of podcast:
365 Days of Astronomy
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