Defining Planets (Part II)
Published on Feb 4, 2009 at 11:43 am.
7 Comments.
Filed under planets.
In my last post, I gave a sort of history of how we got to having 9 planets in the Solar System, according to science textbooks for most of the Twentieth Century. But, Pluto was a problem from the very beginning.
Already, by the beginning of the Twentieth Century, astronomers knew that there was a difference between the planets of the Inner Solar System versus the planets of the Outer Solar System. Mercury, Venus, Earth, and Mars were small worlds mostly made of rock, with a thin atmosphere in some cases. You might wonder at my calling Earth “small,” but it really is small when compared with the worlds of the Outer Solar System. Nearly 1000 Earths would fit inside Jupiter, and over 840 Earths would fit inside Saturn. Uranus and Neptune would each hold over 50 Earths.
The other big difference between the inner planets and the outer planets is the nature of their atmospheres. Mercury has so little as to be essentially a vacuum. Mars’ atmosphere is very scarce. Venus’ atmosphere is nearly 90 times denser than Earth’s, but still the atmosphere extends only hundreds of miles up and adds little to the apparent size of Venus. The inner planets have atmospheres of nitrogen, oxygen, and carbon dioxide. There area few other gases hanging around, but those make up the bulk of the atmospheres. The outer planets, though have atmospheres that are composed mostly of hydrogen and helium. In fact, there is so much hydrogen and helium that the atmospheres of these worlds compose a major percentage of their mass and apparent size. For Jupiter and Saturn, hydrogen and helium are the principal components of those worlds, and even for Uranus and Neptune hydrogen and helium are major components. Clearly something is different between these worlds. The outer planets, being so dominated by hydrogen and helium are often called “gas giants” to differentiate them from worlds that are mostly solid bodies with comparatively thin atmospheres.
When Pluto was discovered, the faulty mathematics that had predicted it had suggested that it was a massive body. The gas giants fit the bill, having much greater masses than the smaller rocky worlds. However, gas giants are large. If Pluto had been a gas giant, then it would have appeared as a small disk in telescopes when viewed at high magnification. Instead, it was just a dot. Eventually, astronomers had to accept that Pluto was much smaller than they had been expecting. More careful study showed that the mathematics predicting a massive body beyond Neptune were in error. So, Pluto didn’t have to be so large, after all. That didn’t cause anyone to remove Pluto from the list of planets. It just made astronomers realize that it was a much smaller planet then they had thought.
Over the years, the size of Pluto began shrinking in textbooks. Obviously, Pluto was not shrinking. Rather, the ability to measure its size was getting better, and Pluto was found to be much smaller than anyone had thought. As I said, when Pluto was first found, astronomers were expecting a gas giant. Clearly, Pluto didn’t fit the bill. And, when a more careful mathematical analysis showed no disturbance in Neptune’s orbit after all, that limited Pluto’s mass (and size) even more. At the time, astronomers knew of only four kinds of objects in the Solar System: small rocky planets (the terrestrial planets), large gas rich planets (the gas giants), tiny chunks of rocky material (asteroids), and tiny chunks of icy material (comets). If Pluto were as tiny as an asteroid or a comet, it would never have been seen. Comets at the time were not well understood, but Pluto didn’t look like any that we knew at the time. Asteroids were not known to exist so far from the Sun. It obviously wasn’t a gas giant. So, that left a rocky terrestrial planet in a strange orbit in the Solar System where no other terrestrial planets were known. Rocky surfaces reflect only so much light. So, in order to reflect as much light as it does, Pluto was thought to be about Mars sized (give or take a bit). I remember growing up I had a Solar System book that had cardboard cutouts of the planets to scale. I had them hanging up in my room. Pluto was about the size of Mars. Even at that time it was out of date, though, because astronomers had already realized that Pluto was much smaller than Mars. Word just hadn’t reached the makers of the book.
Pluto’s surface is covered in ice and frozen gases. In fact, ice is likely the major constituent of that world. In that, Pluto is rather more like the comets than had first been thought. That, too, doesn’t boot it from the list of planets, though. After all, if the asteroids are rocky, that makes Earth more like asteroids than it is like Pluto, Neptune, or Saturn. But, ice is far more reflective than rock. So, Pluto can be much smaller and still reflect as much light as it does. Soon, it became apparent that Pluto was quite a bit smaller even than Mercury. Then, in 1978, James Christy discovered that Pluto had a moon. That moon, Charon as it was eventually named, turned out to be the key to really discovering Pluto’s true size. As Charon orbits Pluto, it sometimes moves in front or behind Pluto. That happened for about 5 years between 1985 and 1990. Studies of these mutual occultations permitted astronomers to finally accurately measure the size and mass of Pluto. Pluto was found to be tiny (under 1200 km in diameter). You could fit over 150 Plutos inside of Earth. In fact, more than three Plutos would even fit inside the Earth’s Moon! Pluto has only about 0.2% of Earth’s mass! It is tiny, indeed! Had that been known when Pluto was discovered, there may have been some more debate about where it should be considered a planet or not. But, tiny as Pluto is, it was still much larger than the largest asteroid, and it was the only body of its size known. So, there was no compelling reason to boot it from the list of planets.
Pluto’s orbit, though, is also a bit odd. All of the rest of the planets have orbits that are almost in a plane. The rest of the planet deviate no more than about 7 degrees from that plane. Pluto’s orbit, however, is inclined 17 degrees. That puts it well outside the range of the rest of the planets. What’s more, Pluto’s orbit is far more elliptical than other planets. It has an eccentricity (a measure of how far from a circle an orbit is) of more than 0.25. For comparison, Earth’s orbital eccentricity is 0.017. But, orbital eccentricity itself is not enough to kick it off the list. Mercury also has a very elliptical orbit, with an eccentricity of 0.21, and it is the one with the 7 degree inclination orbit. One could argue that if orbital eccentricity and inclination alone disqualify a planet, then Mercury is also not a planet. Incidentally, I actually have heard that argument given by those so vehemently opposed to Pluto’s planetary status that they are willing to sacrifice Mercury from the list just to get rid of Pluto!
Pluto’s orbit has another oddity. It has an orbit that is elliptical enough that it crosses that of Neptune. The two never run into each other, though, because the orbits are inclined so that when Pluto is at the same distance as Neptune from the Sun, it is not at the same level as Neptune’s orbit, so they always miss. But, still, if Pluto came close enough to Neptune, then the vastly large planet’s gravity should hurl Pluto off into the cosmos. That, too, won’t happen because Pluto’s orbit has a resonance with Neptune’s that keeps the two from ever doing that. This is much the same concept as Cruithne and Earth’s orbital relationship, or Earth’s co-orbital asteroids. Now, that is something that people often latch onto as a reason to say that Pluto is not a planet. It is small enough that its orbit can be dominated by a larger planet. This, too, has long been a reason to debate Pluto’s planetary standing.
However, despite Pluto’s tiny size and odd orbit, all of the talk of de-listing it from the list of nine planets was just that: talk. It was something that astronomers would talk about, but mostly out of curiosity. Pluto was still the only thing like it out there, so it was an oddity. When I taught my planetary astronomy class, we had nine planets: four terrestrial ones, four gas giants, and Pluto. It did not conform. It was the odd one. It did not fit with the other bodies. It might should have not gotten onto the list, but it was there. Astronomers are sometimes a pretty conservative lot. So, once on the list, it stays unless there is a really compelling reason to remove it.
Then, new discoveries at the end of the Twentieth Century brought the whole matter to a head. Other small icy bodies were found beyond Neptune. The astronomer Gerald Kuiper had proposed that some comets come from objects orbiting the Sun in a giant belt beyond the farthest planet. These icy bodies, when their orbits are disturbed enough, dive in close to the Sun and act as the comets that we have known from antiquity. All of the comets were tiny. But, astronomers proposed that some of these things would be much larger than others, just as many asteroids in the asteroid belt are much larger than the average near Earth crossing ones. Finally, these large objects started to be found. At first, they were still much smaller than Pluto. Then, some were found that were close to Pluto’s size. This triggered a debate about where the cutoff should be for being big enough to be a planet.
Then, in 2003, the object that came to be known as Eris was discovered. It was larger than Pluto. It also was of the same basic composition as Pluto. It had a similar orbit. If Pluto was a planet, then clearly this nearly identical but larger body was also a planet. Now, there was really serious debate. What to do with Pluto now that there are more things like Pluto.
At first, you’d think that Pluto having been kept on the list because it was the only thing like it in the Solar System would mean that when more things like Pluto were found that Pluto would be instantly booted. Indeed, that is the argument that many have for Pluto not being a planet. But, ironically, that there are more things like Pluto out there also has others arguing that this is a compelling reason for Pluto and these other bodies to be planets. Now, Pluto is no longer an anomaly. There are several planets like Pluto. Clearly these are two polar opposite opinions. Which is right? Or, are neither position right? So, while we have had a long standing water-cooler debate on whether Pluto should really be on the list of planets or not, the discovery of other bodies like Pluto meant that there needed to be serious discussion on just what the definition of planet should be.
And, I will continue with this in my next post.
-Astroprof
Image courtesy NASA, HST
Astronomy Link List on February 5, 2009 at 4:28 am: 1
This article has been added to the Astronomy Link List
Sili on February 5, 2009 at 2:07 pm: 2
Thank you. This is so admirably clear and wellreasoned. I might even end up changing my mind at the end of the series.
This is indeed a subject that causes much emotion. /gratuitous friendsspamming (I know there are more amusing takes on it out there.)
As you may recall I’m a descriptivist when it comes to language, so please don’t take this to be a game of gothca:
I know asking people about their usage is pointless, since most are unaware of how they speak (compared to how they think they speak), but is this a turn of phrase that’s natural to you?
I’m just curious, since I think this is the first time I’ve seen a double modal ‘in the wild’.
Astroprof Defining Planets « AstroGeek on February 6, 2009 at 3:55 pm: 3
[…] discussion about what defines a ‘planet’. Read Defining Planets (Part I) and Defining Planets (Part II). Possibly related posts: (automatically generated)Lots going on Posted in Astronomy Blogroll, […]
Astroprof on defining planets, parts III and IV « AstroGeek on February 11, 2009 at 11:36 am: 4
[…] continuing his excellent series of posts on defining planets. I previously posted about parts I and II, here are parts III and IV. Possibly related posts: (automatically generated)Astroprof Defining […]
Lilly on March 2, 2009 at 11:16 am: 5
Earth has been the only planet since millions of years with the proof of life.All other planets are merely assumptions by astrologers that earth-like planets exist beyond our Milky way. Recently an earth like planet was discovered by astrologers which was supposed to have life, fire,water and Oxygen like gas.But that is all a foolish assumption.
Astroprof on March 2, 2009 at 12:04 pm: 6
Pretty much everything that astrologers say is foolishness. If you want to learn about the science of what is in the sky, stick with astronomers.
Link list – 5th February 2009 | Astronomy Link List on April 6, 2009 at 10:01 am: 7
[…] Defining Planets (Part II) Astroprof’s Page The second helping on the discussion of planets covering the discovery of Pluto and how it never really fitted in to our idea of the planet. […]