So what is a planet, anyway?
Published on Jan 16, 2006 at 7:40 pm.
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Filed under planets.
So, just what /is/ a planet anyway?
Seeking Solace and Sciencewoman asked questions pertaining to the definition of a planet (as in the new object discovered beyond Pluto) — a topic that came up on Tom’s page, too. Fly Girl asked me for a beginner’s topic for those of my readers who aren’t astronomers. So, I figured that a good topic might be defining a planet. However, this will be a long post, I am afraid.
Defining a planet seems a simple task. After all, we all know what a planet is, right? Well, maybe it isn’t so simple. So, I looked in a dictionary, and it said that a planet is a large non-luminous body orbiting the Sun, as in the nine planets of the Solar System. Great. We define a planet as something that is a planet. That is real useful. So, how do we know if the new object announced last summer is a planet or not based on this definition?
So, let’s go back. The word planet comes to us from the Greek, and it means wanderer. The ancients noticed that most of the stars stayed put, and so they made patterns in the sky that stayed the same for as long as anyone could remember. There were five exceptions, though. These stars appeared to wander through the constellations. They were Mercury, Venus, Mars, Jupiter, and Saturn. These were the planets. Sometimes included on the list is the Moon, since it is also in different constellations on different days. Even the Sun moves around in the sky, sometimes being high in the sky, and sometimes low. So, that gives seven objects that move around. We have seven days of the week. Saturn’s day, the Sun’s day, the Moon’s day, and so forth. The others are named after non-Roman gods, but in Latin, French, or Spanish, we see that Tuesday is Mars’ day, Wednesday is Mercury’s day, Thursday is Jupiter’s day, and Friday is Venus’ day.
OK, this is nice, but doesn’t help our quandary. Copernicus’ idea that the planets and the Earth all orbit the Sun led Giordano Bruno to presume that meant that Earth was a planet and that the planets were worlds like Earth. When Hershel discovered Uranus, there was little doubt that he had found a new planet. But, comets had been known since ancient times, and no one really argued that they counted as planets. Then, Ceres, Vesta, and other small objects were found with orbits between those of Mars and Jupiter. They became popularly called asteroids, though astronomers refer to them as minor planets. The largest asteroid is only a tiny fraction of the size of a planet: not quite a thousand miles across, so they seemed to be surely a different class of object from a planet. Then Pluto was discovered. At first, it was mistakenly thought to be larger than Mars, and thus clearly a planet. Eventually, though, we found it only a bit larger than the largest asteroid. But, it was on the list by that point, and there was nothing between the size of the largest asteroid and Pluto. Then, we found other Kuiper Belt Objects (see my previous two posts). Some of these were nearly half the size of Pluto. This raised a question as to how big does something have to be to be a planet. Eventually, we found objects such as Quaoar, which were over half the size of Pluto, and then Sedna, nearly 80% the size of Pluto. Finally, we found an object, provisionally designated 2003UB313 which is actually a bit larger than Pluto. So, is it a planet? Is Pluto a planet? Hmm. We are back to our lousy definition of a planet.
So, perhaps we can look at another definition. Since asteroids are too small to be planets, then perhaps size is the key. But what size do we use? Is it fair to pick an arbitrary size, say Pluto sized or larger, so that Pluto and 2003UB313 are planets, but Sedna isn’t? What about an object 97% the size of Pluto, and the same composition and structure. Is it a planet? How about 93% the size of Pluto, or 88%, or 75%? Where do we draw the line? What about saying that a planet has to be bigger than our Moon to count? That leaves both Pluto and 2003UB313 off the list? Is it fair to pick an arbitrary size and make that the definition? Probably not.
OK, then perhaps some other size should be picked. The larger an object is, the more gravity it has. Small objects have low gravity. With low enough gravity, then the tidal forces in the Solar System will pull an object apart. If we define a planet as something whose on gravity holds it together, then the Solar System has several million planets, including all the asteroids and comets. OK, that is too generous. Most of these small objects, are irregular in shape. It takes a certain amount of gravity to pull protrusions down and to make an object more or less spherical. This process is called isostasy. So, do we define a planet as being large enough for isostasy to cause the body to be spherical? Using this definition, nor only are Pluto and 2003UB313 planets, but so are Sedna, Quaoar, Ceres, etc. There are well over a dozen such objects.
To make it even more confusing, what do we do with the Moon, or the major moons of Jupiter and Saturn? They are larger than Pluto, and they have been pulled into spherical shapes. Furthermore, Io and Europa have volcanism, there is rain on Titan (granted its methane, not water), Triton has geysers, and there is something that looks a lot like plate tectonics going on with Ganymede and perhaps Enceladus. Titan and Ganymede are even larger than the planet Mercury, and Callisto and Triton are only slightly smaller. Do these count? Is it fair that they meet all criteria for a planet except that they orbit another planet? How about Charon, Pluto’s moon? It is over half the size of Pluto? Is this a planet and moon, or a binary planet? Furthermore, our own Moon is only slightly smaller than Mercury. It would surely count as a planet if it were in orbit around the Sun. Why does the Earth-Moon system count as a planet and moon rather than a double planet? If Pluto counts because of its size, then the Moon should, too, right? The Moon is still significantly larger than 2003UB313.
So, perhaps another definition should be used. We know that Earth is rocky, as are Mercury, Venus, and Mars. Pluto, though, is composed mostly of ices (we think), with rock being a small percentage of the planet. So perhaps, we should say that a planet is a large chunk of rock. Well, that still leaves the Moon on the list, but leaves Jupiter, Saturn, Uranus, and Neptune (the four largest objects in the Solar System other than the Sun, itself) off of the list! These bodies are called gas giants because hydrogen and helium compose a major part of the planet’s mass. In fact, these planets don’t even have solid surfaces. The deeper you get into the planet, the higher the pressure, until hydrogen is compressed into a liquid. Hmm. Maybe, some other definition would help.
How about we consider how planets form? There are two models, nebular condensation and accretion. Under nebular condensation, the planets start forming when the Sun is forming. The final stages of the Sun’s formation consist of a vast disk of material swirling around feeding the forming Sun. This is called an accretion disk. If planets are forming in the disk, it is a proplyd. Nebular condensation says that instabilities cause portions of the disk to get thick, and then the gravity of those thicker patches pulls in more material until you have a planet. The accretion model says that small pieces of material in the proplyd come together to form small bodies loosely held together. Some of these run into each other, forming larger objects called planetessimals. The planetessimals collide to form planets. Close to the forming Sun, it is too warm for ice to be stable, so the small objects are mostly rocky. Farther from the Sun, you get a mixture of rock and ice. Water is very common in the universe, so there would be perhaps more stuff in the outer Solar System to make planets with, and so they would be bigger. If the planet is big enough, then it has the gravity to hold onto hydrogen and helium, the two most common elements in the universe. Cool. This model seems to fit what we see. However, when you work through the mathematics, you see that Jupiter and Saturn are too big for this model. So perhaps Jupiter and Saturn formed through nebular condensation and the Uranus, Neptune, and the inner planets formed through accretion. Doing a bit more computation, you find that as Uranus and Neptune get bigger, their gravity interacts with one another, and they migrate farther outward in the Solar System, throwing vast amounts of material and smaller bodies even farther out, where they are too thinly distributed to ever come together to form anything larger. This populates the Kuiper Belt. Studies of asteroids and comets to date show that many, if not most, of these bodies are very loose collections of material, barely held together. This fits with the accretion model. However, some asteroids seem to come from larger bodies, perhaps as large as planetessimals that collided destroying one another. Under this model, Pluto and 2003UB313 would simply be a couple of these icy things tossed out by the migration of Uranus and Neptune.
So, if accretion is how you define a planet, then Jupiter and Saturn are again off the list, but you might argue that Ceres and perhaps Vesta (two very large asteroids) should be on the list. We don’t really know the structure of these bodies, but they may be closer to planetessimals than aggregates like Ida, Mathilda. or Eros, three asteroids that we have recently sent unmanned spacecraft to study. Even more problematic is that the moon systems of Jupiter and Saturn probably formed from accretion during the formation of those bodies, so that means that Titan, Ganymede, and several others should be on the list of planets. If you say that nebular condensation makes a planet, then only Jupiter and Saturn make the list! Then, you have another issue. Jupiter basically has the same composition as the Sun. If it were much larger, then Jupiter would have enough mass to compress its interior to the point that nuclear fusion starts and it would be a star. But, if a star has too little mass, it won’t fuse hydrogen. Such a failed star is called a brown dwarf. At what point do you differentiate a small brown dwarf from a very large planet? We have found planets and brown dwarfs around other stars. Nearly 150 planets have been found around other stars, and many dozens of brown dwarfs have been identified. So far, we have found planets no more than tens of times the mass of Jupiter, and brown dwarfs nearly 100 times the mass of Jupiter, but we can easily imagine objects in between in size. Are they planets or brown dwarfs?
So, have I cleared things up? Yeah, right. Sadly, astronomy, one of the oldest sciences, has never really developed a definition of planet. We are working on this, and hope to have one soon, but whatever definition is picked will surely upset somebody. Personally, I don’t think of Pluto as really a planet. It is a giant comet nucleus-type-thingy. That leaves 8 other objects that are clearly planets. But, I tend to think of the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton as being planetary in nature, due to their size and structure. Triton may be a bit of a stretch, since, though it is nearly the size of Mercury, it seems to be a giant version of Pluto itself. It has a bizarre orbit around Neptune, too, indicating that it may be a captured object.
-Astroprof
Astroprof’s Page » Defining Planets (Part I) on February 2, 2009 at 4:49 pm: 1
[…] all of the other bodies just like Pluto. And the matter did not start with the 2006 IAU meeting. I had already written about this over half a year before the decision, and I even gave a public presentation in January of 2006 […]