Recently scientists announced the possibility of a planet larger than Earth in the farthest reaches of the solar system (Trujillo & Sheppard, 2014, Nature, 507). That's really cool, but before we address this fascinating possibility we must first address a far more pressing and vaguely related question: what happened to Pluto?!
The reclassification of Pluto from full-fledged planet to dwarf planet is often taken as something akin to discrimination against small solar system objects and a personal insult to Pluto-fans everywhere. Rumor has it, Neil deGrasse Tyson was attacked by a mob of angry 4th graders after the announcement came out. I'm all about standing up for the little guy, but I've also noticed no one every protests in favor of Eris, Sedna, Makemake, or Quaoar being upgraded to full planet status.
Wait...Quaoar?
Let's look at a little history of Pluto and friends:
Back in the early 1900's it was rumored that there might be a 9th planet beyond Neptune. Astronomers thought Neptune's orbit was being perturbed by some more distant body - they were wrong, but it inspired them to start looking for the 9th planet - finding a planet was a sure-fire way to become a famous astronomer (or just plain famous). One person in particular who was keen on finding the 9th planet was American astronomer Percival Lowell (founder of the Lowell observatory in Arizona and inventor of Martians - really). Lowell devoted the latter part of his life to discovering the elusive planet, but died without achieving success. Lowell's successor was former farm-hand turned astronomer, Clyde Tombaugh. Tombaugh switched from farming to observatory maintenance man at an early age, and although uneducated, became a skilled astronomical observer. After Lowell's passing, Tombaugh continued Lowell's search, and by a stroke of luck discovered Pluto in 1930.
Much of Pluto's popularity comes from being the only "planet" to have been discovered by an American - Uranus and Neptune were discovered by a German-born Englishman, and a Frenchman respectively, while the rest were figured out by cavemen. At the time, it seemed only natural to consider Pluto a planet because all relatively large objects orbiting the Sun were considered planets. We also had no sense of Pluto's size. Interestingly, when the largest asteroid Ceres was discovered back the 1800's, it too was considered a planet for several months until astronomers realized just how small it was.
Pluto was an official planet from the time of its discovery until 2006 when the International Astronomical Union (IAU) changed its designation, although astronomers were questioning its status as early as the 1970's. To understand why the change was made, let's consider the demographics of the solar system.
Most of the solar system can be divided into about six categories: starting in the inner solar system we have the four rocky planets, just beyond these the asteroid belt, followed by the four gas giant planets in the outer solar system, beyond which is the Kuiper belt, the scattered disk, and the Oort cloud. The inner rocky planets are relatively small and similar in composition to Earth, while the outer planets are much larger and primarily composed of hydrogen and helium. The asteroid belt and Kuiper belt are bands of small rocks and chunks of ice, respectively that are the "leftovers" from the formation of the solar system. The scattered disk is a widely scattered collection of objects that were ejected from Kuiper belt by collisions or close encounters, while the Oort cloud is a hypothetical collection of countless comets encircling the solar system 10,000 times farther from the Sun than Earth.
With the exception of the Oort cloud (which is spherical), and to a certain degree the Kuiper belt and scattered disk (where slightly tilted orbits are common), the solar system is quite flat with all planets and most asteroids and comets orbiting the Sun in a single plane. The eight planets also have very circular orbits. Pluto orbits the Sun within the Kuiper belt, and is neither rocky nor gaseous like the eight planets, but is instead primarily composed of ice like its neighbors. Pluto also has a highly elongated and slightly tilted orbit.
With the exception of Pluto, the solar system beyond Neptune was virtually unknown prior to the late 1970's - astronomers had known that comets came from beyond Neptune as far back as the 1700's, but no object other than Pluto had ever been seen. Discoveries of small icy objects beyond Neptune starting in the 70's confirmed the existence of the Kuiper belt. Then, in 1992, the discovery of Eris changed everything. Eris is a Pluto-sized object in the scattered disk and was almost considered the 10th planet. But then within a period of less than 20 years, many more Pluto-like objects were discovered in the Kuiper belt and scattered disk, including Sedna, Makemake, and Quaoar (pronounced kwa-wahr). Now there was a problem - based on the discoveries already made and studies of the Kuiper belt, astronomers predicted that there could be hundreds of these Pluto-sized objects beyond Neptune. 100 new planets? "My Very Educated Mother Just Served Us Nine Pizzas" was going to turn into the longest mnemonic ever.
The real problem was not the number of objects, but that, while they shared many characteristics in common with each other, such as size, composition and orbits, they were very different from the eight rocky and gaseous planets. Never before did we have to consider how we defined what a planet was. It is the task of the IAU to name and define astronomical objects and they now had the undesirable task of having to come up with the definition of a planet. (Undesirable because it was such a heated topic.) Being a scientific organization, the definition needed a scientific basis and must be applied universally - no "grandfathering in" or arbitrary weeding out. After much deliberation, the final definition they settled on was as follows:
A Planet must:
The reclassification of Pluto from full-fledged planet to dwarf planet is often taken as something akin to discrimination against small solar system objects and a personal insult to Pluto-fans everywhere. Rumor has it, Neil deGrasse Tyson was attacked by a mob of angry 4th graders after the announcement came out. I'm all about standing up for the little guy, but I've also noticed no one every protests in favor of Eris, Sedna, Makemake, or Quaoar being upgraded to full planet status.
Wait...Quaoar?
Let's look at a little history of Pluto and friends:
Back in the early 1900's it was rumored that there might be a 9th planet beyond Neptune. Astronomers thought Neptune's orbit was being perturbed by some more distant body - they were wrong, but it inspired them to start looking for the 9th planet - finding a planet was a sure-fire way to become a famous astronomer (or just plain famous). One person in particular who was keen on finding the 9th planet was American astronomer Percival Lowell (founder of the Lowell observatory in Arizona and inventor of Martians - really). Lowell devoted the latter part of his life to discovering the elusive planet, but died without achieving success. Lowell's successor was former farm-hand turned astronomer, Clyde Tombaugh. Tombaugh switched from farming to observatory maintenance man at an early age, and although uneducated, became a skilled astronomical observer. After Lowell's passing, Tombaugh continued Lowell's search, and by a stroke of luck discovered Pluto in 1930.
 |
| Discovery images of Pluto - Pluto is the tiny star indicated by the arrow that moves from one night to the next. The difficulty of discovering outer solar system objects is they appear as nothing more than tiny stars (not visible to the naked eye) that move relative to the background stars. |
Much of Pluto's popularity comes from being the only "planet" to have been discovered by an American - Uranus and Neptune were discovered by a German-born Englishman, and a Frenchman respectively, while the rest were figured out by cavemen. At the time, it seemed only natural to consider Pluto a planet because all relatively large objects orbiting the Sun were considered planets. We also had no sense of Pluto's size. Interestingly, when the largest asteroid Ceres was discovered back the 1800's, it too was considered a planet for several months until astronomers realized just how small it was.
Pluto was an official planet from the time of its discovery until 2006 when the International Astronomical Union (IAU) changed its designation, although astronomers were questioning its status as early as the 1970's. To understand why the change was made, let's consider the demographics of the solar system.
Most of the solar system can be divided into about six categories: starting in the inner solar system we have the four rocky planets, just beyond these the asteroid belt, followed by the four gas giant planets in the outer solar system, beyond which is the Kuiper belt, the scattered disk, and the Oort cloud. The inner rocky planets are relatively small and similar in composition to Earth, while the outer planets are much larger and primarily composed of hydrogen and helium. The asteroid belt and Kuiper belt are bands of small rocks and chunks of ice, respectively that are the "leftovers" from the formation of the solar system. The scattered disk is a widely scattered collection of objects that were ejected from Kuiper belt by collisions or close encounters, while the Oort cloud is a hypothetical collection of countless comets encircling the solar system 10,000 times farther from the Sun than Earth.
With the exception of the Oort cloud (which is spherical), and to a certain degree the Kuiper belt and scattered disk (where slightly tilted orbits are common), the solar system is quite flat with all planets and most asteroids and comets orbiting the Sun in a single plane. The eight planets also have very circular orbits. Pluto orbits the Sun within the Kuiper belt, and is neither rocky nor gaseous like the eight planets, but is instead primarily composed of ice like its neighbors. Pluto also has a highly elongated and slightly tilted orbit.
With the exception of Pluto, the solar system beyond Neptune was virtually unknown prior to the late 1970's - astronomers had known that comets came from beyond Neptune as far back as the 1700's, but no object other than Pluto had ever been seen. Discoveries of small icy objects beyond Neptune starting in the 70's confirmed the existence of the Kuiper belt. Then, in 1992, the discovery of Eris changed everything. Eris is a Pluto-sized object in the scattered disk and was almost considered the 10th planet. But then within a period of less than 20 years, many more Pluto-like objects were discovered in the Kuiper belt and scattered disk, including Sedna, Makemake, and Quaoar (pronounced kwa-wahr). Now there was a problem - based on the discoveries already made and studies of the Kuiper belt, astronomers predicted that there could be hundreds of these Pluto-sized objects beyond Neptune. 100 new planets? "My Very Educated Mother Just Served Us Nine Pizzas" was going to turn into the longest mnemonic ever.
The real problem was not the number of objects, but that, while they shared many characteristics in common with each other, such as size, composition and orbits, they were very different from the eight rocky and gaseous planets. Never before did we have to consider how we defined what a planet was. It is the task of the IAU to name and define astronomical objects and they now had the undesirable task of having to come up with the definition of a planet. (Undesirable because it was such a heated topic.) Being a scientific organization, the definition needed a scientific basis and must be applied universally - no "grandfathering in" or arbitrary weeding out. After much deliberation, the final definition they settled on was as follows:
A Planet must:
- Orbit the Sun
- Have a large enough mass to obtain hydrostatic equilibrium (be spherical)
- Have cleared the neighborhood around its orbit
Number 1 is obvious, while number 2 rules out objects that are not round (like most asteroids) - generally any object larger than 1000 km in diameter will be spherical due to gravitational forces. Pluto (as well as many other solar system objects, like the asteroid Ceres) meets these first two criteria. The last rule requires that a planet be large enough to gravitationally dominate its orbit - any other objects sharing the orbit will either eventually fall into the planet or be kicked out of the orbit by gravitational encounters. This rule eliminates all but the eight planets we know and love today. Pluto, Eris, Ceres and the rest all share their orbits with millions of other objects and are by no means the dominant bodies in their orbits. Any object that satisfies the first two requirements while not meeting the third is now classified as a dwarf planet.
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| Eight of the largest dwarf planets (a trans-Neptunian object is anything beyond Neptune), several of which have their own moons. Sizes are shown to scale with part of Earth shown for comparison. |
So what about Planet X? First, the term "planet x" is the colloquial term used for any newly discovered, but unconfirmed or unnamed planet - as you might have guessed, it's not a term that's actually used very often, except perhaps by overeager journalists. Astronomers are known for coming up with their own unofficial names too - for a period of a time, there was a dwarf planet known as "Easter bunny". In this situation several dwarf planets and smaller objects, including the large dwarf planet Sedna, have been found to have very unusual orbits. These planets orbit extremely far from the Sun in a no-man's land between the scattered disk and the Oort cloud. They also have extremely elliptical orbits - Sedna ranges from 76 AU (an AU is an astronomical unit, or the distance between the Earth and the Sun) from the Sun at closest to more than 400 AU at its most distant. Aside from their unusual locations and orbits, what is particularly surprising is that all of these objects orbit in basically the same plane - because scattered disk objects were flung into their current locations by random interactions with other objects, they tend to have fairly randomly oriented orbits. One hypothesis as to both how these Sedna-like objects got to their current locations why their orbits are so similar is that they are being affected by gravitational tugs from a planet larger than Earth 250 AU from the Sun. Naturally, it is now being referred to as Planet X. I want to emphasize that such a planet has not been observed - in fact it may not even be possible to observe such a planet due to the extreme distance (Pluto, by comparison is a mere 40 AU from the Sun on average) - this is simply one possible explanation for the observed behavior of these objects. Some astronomers say there may even be dozens of planet-sized objects in the Oort cloud
So at present there are still just 8 planets and a dozen or so dwarf planets. But I still think the possibility of more planets in our solar system is totally cool.
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