INTRODUCTION TO DWARF PLANETS
Right now, unless you’re somehow, watching this from space, you are situated on a planet, a giant sphere of material and heat. Ours is made mostly of rock, as are the three other planets in our backyard. Further away from the sun lie four huge balls of hydrogen and helium in different states. These constitute the eight planets of the solar system four rocky terrestrial planets and four gas giants. However, this is far from the full picture the solar system is also home to another type of world between and beyond the orbits of the eight conventional planets lie dozens and dozens of small icy worlds orbiting in the outer reaches of the solar system. These are known as minor planets or at least they used to be, the best example is of course Pluto. In 2006 Pluto was reclassified as a so-called dwarf planet. Voiding its status as the solar system’s ninth planet which it had held for many decades. Pluto ultimately is just too small to be considered on the same level as the Earth or Saturn. However, there is more to it than just Pluto‘s size. The actual story behind Pluto‘s reclassifications started with the discoveries of several similar remote minor planets and now Pluto sits among a family of almost worlds dwarf planets which despite being much smaller than the earth still exhibit fascinating and unlikely properties. Today we will experience these frozen far out worlds first hand.
MINOR PLANETS
The term minor planet is a broad classification defined as any astronomical object in orbit around the sun or another star which is neither a planet nor a comet. Most of the time this is because it is either too small to be a planet or it doesn’t have the same impact on its surroundings as the other planets do. While Pluto is the obvious one there are almost 8,00,000 known minor planets in the solar system alone which are archived by the minor planet centre, up until about 15 years ago the terms minor planet planetoid and asteroid were all used synonymously referring to a body not large or round enough to be considered a conventional planet. However, since the term was coined, we have reached and studied the outer solar system, a place where rocky and icy material is much more abundant leading to discoveries, which have blurred the lines between what can be considered a planet and what can’t. As a result, the terminology has become more complicated over time and the need for clarifications arose and so in 2006 the international astronomical union voted to amend their definition of a planet before this. A planet had to satisfy two simple criteria,
- the first being that it needs to orbit around the sun obviously, and
- the other thing is that it needs to be in hydrostatic equilibrium in other words large and heavy enough that it becomes rounded by its own gravity giving it a spherical shape.
Under these criteria, Pluto was discovered and named the solar system‘s ninth planet which it remained for many decades. But with the definition change came a third criterion. Now a planet must also clear out its orbital path as they travel around the sun the large planets either clear out or consume material in their way dispersing the rubble of the early solar system and clearing themselves a region of space which they dominate and influence under this revised definition. However, Pluto doesn’t fit the bill the reason being because it orbits within a large band of debris near the outer rim of the solar system and thus doesn’t clear out its region of space, thus it lost its planetary status. To the disappointment of many following the rule change the IAU then reclassified the term minor planet into two new sub-categories, dwarf planets and small solar system bodies or SSSB. Dwarf planets like Pluto are the nearly worlds which are large enough to be rounded but don’t dominate their surrounding space. A small solar system body is anything else that isn’t a comet like a large asteroid or the micro satellites which orbit such asteroids or perhaps the icy objects beyond the reaches of Neptune known as Trans-Neptunian objects or TNO. As a result, the term minor planet has become outdated but it is still used to encompass both types today. However, we are going to be focusing for the most part on dwarf planets while not as large or structured as their planetary counterparts they can still house outstanding properties the likes of which we never expected to find on worlds. So far away from the sun and while most of these worlds lie billions of kilometers away and are thus very difficult to reach and study.
CERES
The first dwarf planet ever to be discovered and visited Ceres. Ceres was discovered in 1801 by Giuseppe Piazzi, who noted it while he was searching for stars. Initially he mistook it for a comet and named it Ceres Ferdinandea. However, within a year astronomers had identified it as another planet and shortened its name to ceres named after the roman goddess of corn and harvests it lies over 400 million kilometers away from the sun. Within the asteroid belt a ring consisting of pieces of rock between the orbits of Mars and Jupiter. After initially being mistaken for a comet and then for a planet ceres was reclassified as just another asteroid among the many others in the belt. However, it sticks out as an outlier against its rocky neighbours at just under 1000 kilometers in diameter. It is by far the largest object within the belt and also accounts for about a third of the belt’s total mass and so it was among a host of objects to be reclassified as dwarf planets in 2006. Putting aside the anthropocentric terminology however it seems strange doesn’t it a dwarf planet here in a sparsely populated belt where nothing else like it exists too large to be an asteroid but too small to be another planet. So, in 2015 we visited the world in an attempt to better understand its origins NASA’s dawn spacecraft made flybys of series sending back detailed images and giving us clues to its past.
It is composed of mostly rock and water ice its core and mantle are thought to constitute a layer of ice which if it exists would mean that ceres has more water than the earth in a solid form. At least but buried beneath its icy cratered surface is thought to have pockets of liquid water surviving from a subsurface ocean which once flowed above ground on the surface of the dwarf planet. It is now thought that ceres is the remains of an embryonic planet small rocky body which was accumulating material in the early solar system, just like the earth it is now. It believed that, ceres may have had an ocean of magnesium salt water flowing on its surface billions of years ago however its destiny was cut short when its neighbouring planets Jupiter in particular scattered and consumed the vast majority of material in the area, the same material that grew the terrestrial planets and the gas giants as such ceres was unable to grow larger and so was far too small to hold on to its primordial atmosphere and so the budding proto-planet died and was cast into the asteroid belt destined to float silently among rubble and debris when its atmosphere was lost to space. So, the ocean and its remnants are now buried beneath the surface in its last surviving pockets. We can see evidence of these liquid water pockets in these shiny bright patches on the surface. These highly reflective mounds are actually deposits of magnesium salt residue left over from liquid water eruptions in the mantle which spilled out onto the surface. Whether anything could survive in these underground pockets of water remains a mystery one thing’s for sure though ceres is a true gem hidden within a dead and desolate region of the solar system.
THE KUIPER BELT
One of the reasons there aren’t any other dwarf planets in the asteroid belt is because the belt is so sparsely populated. Most of its material has been depleted and the few objects that remain are scattered millions of kilometers apart. However, significantly further away than the asteroid belt. Beyond the orbit of all eight of the planets lies another far larger belt of material the Kuiper belt. The Kuiper belt is a thick rotating disk of icy objects and debris similar in structure to the asteroid belt. But much larger and more densely populated the objects in the Kuiper belt are thought to have formed very early on in the solar system’s life. Rocks began appearing out of the dust around the young sun which condensed to form the planets and any primordial rocky pieces that were not consumed were scattered and much of it drifted and settled into this massive belt. Unfathomably far away from the sun because the objects within are so far away from the sun’s light they have only been warmed and irradiated slightly in the billions of years. Since their formation and so are thought to be well-preserved artifacts which can allow us to examine what the solar system was like back then the Kuiper belt begins at just beyond the orbit of Neptune around 30 astronomical units away so that’s 30 times further than the earth is from the sun and it extends out to around 50 times further away. Thus, it is so remote and vast that it is a place that we have only just begun to explore and understand objects within the Kuiper belt are referred to quite simply as Kuiper belt objects or KBO’s scattered within. The Kuiper belt’s expanse are hundreds of millions of cometary nuclei hundreds of thousands of objects larger than 100 kilometers in diameter and perhaps hundreds of small cold dwarf planets, which have yet to be discovered and crucially because they all exist and orbit within the bounds of this belt, they do not clear out their orbital paths or dominate the space around them effectively excluding them from being planets. However, that doesn’t stop them from having intriguing planet-like features such as transient atmospheres which exist as gas in an atmospheric layer when faced with the light of the sun before collapsing down into frost on the surface during the night.
PLUTO
Our most distant honorary planetary neighbour Pluto. Pluto was discovered in 1930 and it is named after the roman god of the underworld. It is classed as a Kuiper belt object and a dwarf planet today but it was long considered to be the solar system’s ninth planet. It is smaller than all eight of the solar system’s main planets and orbits considerably further out averaging a distance of about 39 astronomical units. It orbits the sun in both an elliptical and tilted orbit which can take it as far away as 50 astronomical units to the very distant outer edge of the Kuiper belt. Its small size low visibility and sheer distance from the earth makes it a very tricky world to study. Until about five years ago we only had very low detail observations of the dwarf planet but all this changed in 2015 when NASA’s new horizons mission flew by Pluto revealing it in brilliant detail for the first time.
It is thought to have a rocky core surrounded by a water ice mantle. Topped by an icy crust because it is so far away its surface temperatures can fall to as low as -240°C, so cold that nitrogen and methane gases freeze down onto its surface and into frozen nitrogen. Ice sheets across its vast plains for the most part Pluto is a scarred cratered world except for in one unusual place like nowhere else found in the solar system. Pluto’s volatile ices sublimate directly into a gas which expands into an atmospheric layer of nitrogen, methane and carbon monoxide. Because it has such low gravity its atmospheric layer is held down much less tightly than it is on earth allowing its atmosphere to extend much further out into space. And as the new horizon spacecraft flew away from Pluto it turned back and saw this inflated atmospheric layer in all of its breathtaking glory. Pluto also has no less than five moons the largest Sharon is about half the size of Pluto itself, making it the largest moon relative to its parent in the solar system, and also one of the closest. So close in fact that the gravitational center of the Pluto system is not actually Pluto itself and thus Pluto and Sharon are generally referred to as a barycenter the other four moons Nyx, Hydra, Styx and Kerberos are all much smaller resembling fragments more so than moons we’re not sure how the Pluto-Sharon system came to be but the leading theory is that it was formed by a collision of Kuiper belt objects.
ERIS
Out of three newfound dwarf planets it was Eris, that caused the stir Eris is the second largest dwarf planet in the solar system after Pluto, but it is also the most massive. It is estimated to be about 2,326 kilometers in diameter, only about 50 kilometers less than the dwarf planet’s sibling and so virtually the same size when viewed on a global scale. Scientists think that it is probably rocky and icy with the terrain similar to the one found on Pluto. It was first spotted by brown in 2003 and was later confirmed in 2005 and when it was it was set to be classified as the solar system’s 10th planet. However, this raised an interesting dilemma, if we allow Pluto, Eris and any other dwarf planets to be planets then before long the solar system would have 20 or 30 planets. The vast majority of which would just be rounded Kuiper belt objects and so this led to the addition of the third criterium.
In the IAU’s definition of a planet effectively excluding all Kuiper belt objects from consideration both Pluto and Eris were classified as dwarf planets and Eris was named after the Greek goddess of strife and discord. We don’t know much about what its surface is actually like purely because it is so far away is. Over three times further away than Pluto and its orbit takes it far beyond the Kuiper belt to the cusp of interstellar space at almost 100 astronomical units away. Because it is so far away from the Sun gases like nitrogen and methane will have almost certainly frozen down onto its surface like they do on Pluto. One thing we do know for certain though is that Eris has one small moon named Dysnomia it has a nearly circular orbit around Eris completing a lap of the dwarf planet once every 16 days. This satellite has allowed us to make estimations of the dwarf planet’s mass which is how we managed to conclude that it is more massive than Pluto and thus the most massive dwarf planet in the solar system.
HAUMEA
The second dwarf planet discovered by brown was Haumea. It was first noted in late december of 2004 originally being nicknamed Santa. Before being officially named after the Hawaiian goddess of fertility and childbirth. It is about a third as massive as Pluto and it’s hard to know whether to class it as a dwarf planet or a small solar system body. Unlike the others mentioned so far it is not a spherical world, instead it has an elongated egg shape, estimated to be around 1,600 kilometers in diameter. Its rapid rotation and high albedo are clues as to its origins and we now believe that the minor planet is perhaps the largest constituent of a collection of Trans-Neptunian objects left over from a large collision of debris.
Before this, Haumea was perhaps a rounded world more analogous to Pluto and Eris, but this collision is thought to have blown off Haumea’s icy mantle. Resulting in a relatively unstructured and un-shapely body composed of silica rock. We cannot make-out the surface in much detail, but its colour changes as it rotates relative to us, indicating that there is a dark patch on its surface like on Pluto. It also has two very small moons Hiyaka and Namaka which are also thought to be remnants of the same collision. Perhaps its most unusual and captivating feature is that it has a faint ring around it. Making it the first Trans-Neptunian object with a ring system to be discovered the ring’s width is about 70 kilometers and it extends the system’s total diameter to around 2,300 kilometers wide pretty impressive complexity for such a small immature world.
MAKEMAKE
Lastly, we have Makemake the third dwarf planet to be discovered by brown in 2005. It is slightly smaller than Haumea about 1,400 kilometers in diameter, but unlike the former it is rounded by its own gravity while small it is the brightest Trans-Neptunian object after Pluto. But it managed to evade detection for many years thanks to its unusual orbit which lies out of sync with the solar system’s ecliptic plane. At this moment in time, it is over 50 astronomical units away near the outer edge of the Kuiper belt, which is about as far away from the sun, as it gets along its journey analysis of its spectral data revealed that its surface is most likely covered in grains of frozen methane. There is also evidence for small quantities of nitrogen ice, but nowhere near as much as on Pluto indicating that it has lost its nitrogen over time. The presence of methane and nitrogen together suggests that the world may have a transient atmosphere like Pluto. If so, this would explain the nitrogen depletion because Makemake is particularly small it cannot retain its atmosphere when it expands especially the nitrogen-methane is lighter than nitrogen but it freezes at much higher temperatures, meaning much less methane thaws when exposed to the sun. Nitrogen on the other hand is easily excited by the sun’s light and only the difference of a few degrees causes it to sublimate into a gas, where it is lost to space thanks to the low escape velocity of the dwarf. Makemake also has a tiny moon nicknamed mk2. Like the others it is thought to have formed from a collision but unlike its parent its body is as dark as charcoal, perhaps because it is simply too small to have been able to retain its reflective ices which sublimated and escaped into space long ago along with Makemake’s nitrogen ice.
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