Earth & Sky

A tenth planet?

Not so fast, says Harvard astronomer Brian Marsden.

Brian Marsden is Director of the International Astronomical Union’s Central Bureau for Astronomical Telegrams and of the Minor Planet Center at the Harvard–Smithsonian Center for Astrophysics. He once proposed that Pluto should be cross–listed as both a planet and a minor planet and assigned the asteroid number 10,000. But this proposal was not accepted. Jorge Salazar spoke to him in July, 2005 on whether 2003 UB313, which some have called the “10th planet,” should receive that official designation. In his answer to Earth & Sky, Marsden provides deep background on the planet question.

Salazar: So right off the bat, is the object 2003 UB313 a planet?

Marsden: Well, yes, there is the question of what do we mean by the word “planet?” I think the word planet really has problems. It’s been applied to many different things over the last couple of thousand of years.

Originally, planets were wandering things in the sky. The word included the sun and moon and did not include the Earth. That didn’t become a serious consideration until the 16th century, when Copernicus suggested that the planets go around the sun, and that the Earth is one of them.

In the late 18th century, Uranus was discovered and became the seventh planet. The object we call Ceres was advertised as the eighth planet 20 years later. And then there were three more, moving around the sun, in much the same way as Ceres, so that by 1845, there were said to be 11 planets if you look in a textbook of that time.

Now, the last 4 Ceres and the others were really very small, under 1,000 km across, although the sizes were not known too well at that time. Beginning in 1845, astronomers started finding a lot of very small objects, and then it was decided to call them minor planets, or asteroids.

At around the same time, we indeed had what became the eighth major planet, Neptune, certainly well worth calling a planet. Then Pluto was added, for better or for worse, in the 1930s when it was discovered. It was initially thought to be quite large. The initial estimates were that it was comparable to the Earth both in diameter and in mass. And the idea that its mass was comparable to that of the Earth continued well into the 1960s, although by that time the feeling was that its diameter was a lot less than that of the Earth. We now know that Pluto does not have a mass comparable to the mass of the Earth, but more like 1/400th the mass of the Earth, and among the bona fide planets, Mercury has the least mass, and even that is 1/20th the mass of the Earth. Pluto is 1/20th the mass of Mercury.

With that in mind, I think we must seriously consider whether we want Pluto to be considered as a planet or not. Unfortunately, or maybe fortunately, the public does tend to have a special attachment particularly school children, I’ve found to Pluto as the ninth planet. When my grandson was 2 years old, somehow he learned the names of the planets, and was talking to me on the phone one day reciting them, and very proudly at the end said, “Pluto.” And I corrected him on that, even though he was only two years old.

Salazar: Are you suggesting that Pluto should not be classified as a planet?

Marsden: Quite clearly, Pluto is one out of many members of the trans–Neptunian region. Beginning in 1992, we’ve had lots of objects discovered out there. In fact, there are a couple of hundred objects almost exactly like Pluto, dynamically indistinguishable from Pluto in having orbits that cross the orbit of Neptune but don’t allow close approaches to Neptune. There’s long–term stability to their motions, what we call a resonance with Neptune.

So, there’s nothing really very special, dynamically about Pluto. And over the last couple of years in particular, we’ve discovered several rather large objects out there in the outer solar system, including 2003 UB313.

Salazar: So, in your opinion, Pluto is not a planet?

Marsden: I think the point is, “What is Pluto?” It’s a member of the trans–Neptunian belt or region. We’ve got nearly a thousand objects in the same region that have been discovered during the last 13 years. We’re finding some of those objects comparable to Pluto in size, and 2003 UB313 seems clearly to be larger than Pluto.

So what do we do? Do we want to call them all planets? Do we want to add them to the list of nine? From an astronomical point of view, at least, we need to catalog Pluto where it belongs, together with all of these other objects in the trans–Neptunian region. We have a system for doing that. Pluto has been left out until now, and I think that it is the time to put it in there, together with 2003 UB313. This will certainly help us to advance our real understanding of small bodies.

But there is the cultural problem. As I said, the meaning of the word “planet,” has changed over the last 2,000 years. For the last 75 years, Pluto has been considered by most people as the ninth planet. And the question is, do we want to change that?

The point of the problem is that the public speaks just of plain “planets.” It’s a troublesome term to say “planet” by itself. You need some qualifier major or minor, or inferior or superior, or trans–Neptunian, or extrasolar all kinds of adjectives in front of the word planet are needed before you can speak of them in any meaningful way. We’re left with a situation that Pluto is considered by so many members of the public as the “ninth planet,” whereas, astronomically, it doesn’t have any entitlement to such a special nomination. It is a little hard, I suppose, after 75 years to remove it from that position.

There’s a compromise. Yes, we can continue to call it the ninth planet if we want to, but at the same time, I think that we should seriously say that it is a minor body, catalog it with the minor bodies, including 2003 UB313. That would be much more meaningful, scientifically. But for cultural reasons, we can also call it the ninth planet.

Salazar: And what about 2003 UB313? Should it be called a planet?

Marsden: It wouldn’t follow, from what I am saying, that 2003 UB313 is automatically considered to be the 10th planet. Obviously, there’s not an enormous amount of culture about this object yet, like the 75 years that we have for Pluto. So just because Pluto may be continued to be called the ninth planet, and this object is larger, it doesn’t necessarily follow that we would want to call this the 10th planet.

I think we should stick with our 9 planets.

And then the question arises of under what circumstance would we even consider, in the future, to have a 10th planet? If you could look out far enough into the solar system, you might find a fairly massive object at a great distance. If it’s a lot more massive than Pluto, you might want to consider that as a 10th planet. Suppose we find a Mercury–sized object out there? You might want to do that. I would tend not to. But I think I’d go for something larger, maybe Mars–sized. I don’t know, actually. I do think that if we find something way out there that is Earth–sized the Earth, of course, having a very special significance to us I would be prepared to say that that could be the 10th planet.

For the present, I would stick with Pluto as the ninth planet for cultural reasons. I would not add 2003 UB313 as a planet. And I would categorize both Pluto and 2003 UB313 with all of the other objects that we’ve been finding out there beyond Neptune.

Salazar: What would it take for a newly found object to be considered a planet?

Marsden: Well there were several different issues, really. Some people have said that in order to decide what to do about this new object, we really have to address what astronomers mean by the word “planet”. Several astronomers have been arguing along those lines that we really need to define what we mean by “planet.” I can understand that point of view. I’m a bit bothered by it, because I think the word has run out of steam, you might say, because it has meant so many different things in the past. We can define star, and galaxy, and things like that quite well. But astronomers have really been stymied by trying to define the word “planet.”

Obviously, a star isn’t a planet. A star has enough mass to be self–luminous. We all can agree on that. There are also these things called brown dwarfs, that are between planets and stars, and it’s been calculated that if you put together something 13 times the mass of Jupiter, then you’ll have a brown dwarf. So that sets an upper limit to what an astronomer might want to mean by the word “planet.”

But we’re really interested here in what is the lower limit. You need some other criterion to set the lower end. One kind of limit that some people have been using is to say, well, the planets are round. The Earth is round, Jupiter’s round, even Pluto is round. Why are they round? Objects that are massive enough have enough self–gravity to collapse into a roughly spherical shape. That’s in contrast to some of the very small minor planets, which are very irregular in shape. So, you can try a definition at the lower end, saying, well, if objects are roughly spherical, then we might consider calling them planets. This immediately means that Ceres, the largest asteroid, counts as a planet, according to this view. It’s round. And there may be a half dozen to a dozen other asteroids in the belt between Mars and Jupiter that one might want to consider planets under this circumstance. And there are quite a few more out in the trans–Nuptunian belt. You could in principle come up with several dozen additional planets to the 8 or 9 that we’re accepting. The point about this definition is that it’s based on physical principles. If there’s too much mass, you have a brown dwarf. If there’s too little mass, than it cannot be round due to the inward pull of its own gravity.

You also would want to add that it’s going directly around a star. After all, we also have satellites, the moon being a wonderful example going around the Earth. And then the Earth goes around the sun. Actually it’s the center of mass of the Earth–moon system that goes around the sun. Jupiter has four rather large satellites, and the center of mass of Jupiter and its satellites is well inside near the center of Jupiter. Those satellites of Jupiter and Titan, Saturn’s largest satellite and Triton, Neptune’s largest satellite and the moon these would all qualify as rather beautiful planets if they were just orbiting the sun directly. Some of them are comparable to Mercury in size, so we call them satellites because they’re not going around the sun directly. But in terms of the physical bodies, there’s no reason why we really shouldn’t call them planets

And as we look out beyond the solar system, we do see planets going around other stars. A lot of these are being discovered, maybe 100 or so being discovered over the last decade or so. They are bona fide planets. They go around stars. There’s at least one, if not a couple, out there that don’t seem to be going around stars. They’ve perhaps escaped from a stellar system. And the feeling of the International Astronomical Union is actually, if they’re not currently orbiting a star, then they’re not planets. And that’s a little odd. Because they’re the same kind of body. What if one of the planets in the solar system escaped? It’s rather difficult to do that, but it could happen. Would it then no longer be a planet?

What if, among these small objects I was indicating might be planets, just a couple of hundred kilometers across, what if something comes crashing into them and knocks a big piece off so that it’s no longer round. If it’s no longer round, is it still a planet?

So, all sorts of practical questions like that that come into play if you try to define the meaning of the word planet. So, this is where the IAU has gotten a little tied up at the moment. The IAU is talking about this sort of thing.

But then the question is, do you really need to talk about it in order to establish, or describe, or define the state of Pluto and this new object? The important thing about Pluto and this new object is that they are among many objects in that region. They all go around the sun. It’s a rather complex structure, but there are belts of similar objects out there. And the same with the asteroid belt, it isn’t quite so complex as what goes on out there. And this is the most important characteristic of these bodies, and that’s why we call them minor bodies. You wouldn’t have a major body in a situation like that because it would drive everything else away.

This is sort of where we stand. From a logical point of view, the traditional 19th century 8 planets, Mercury through Neptune, are special. If you want to include Pluto in there, you can. Maybe sometime in the future it will go out of the popular culture. But at the present time, I don’t think I’d go to extreme lengths to “demote” it. That’s a horrible word to use.

Salazar: Whether 2003 UB313 is ultimately considered a planet, or a minor planet, it’s eventually going to receive an offcial name. Any word on the name yet?

Marsden: The discoverer did suggest a name, which is in keeping with names of other objects out there in that region. I don’t know whether this will be accepted. I won’t say much about it, except to say that it is a mythological name, but it is not in the Greek–Roman tradition. One committee in the IAU works with small bodies in the solar system, and we do tend to pay at least some attention to what the discoverer proposes. There’s another committee that’s involved with larger bodies, satellites of the major planets, and naming features on planets. They seem more to be taking the view that, well, after all, with previous planets, it wasn’t the discoverer who named them. Planet Uranus was certainly not named by the discoverer William Herschel. He wanted to name it Georgiamsitus, after King George III.

Some years ago, right around 1993–1994, the IAU did decide, with regard to minor bodies in the trans–Neptunian belt, that these objects would receive the names of creation gods in various different cultures all around the world. Not just Greek–Roman, all over the place. So, that has been done with the names like Varuna. And then the objects that are dynamically just like Pluto, it was decided that they would have the names of underworld gods, maybe even underworld characters. One can imagine Al Capone or something like that, but I don’t think that that’s going to be used for this purpose. But Pluto is like that, of course, and there have been a few other cases.

But out there there are a lot of complex types of orbits, Sedna named for an Inuit goddess of the sea has a very different kind of orbit to the Pluto group, Plutinos we sometimes call them. It doesn’t come any closer than 75 times the distance from the Earth’s distance to the sun. It takes 11,000 years to go around. It goes way out, more than 1,000 times the Earth’s distance. So that’s a different kind of thing.

This new object, 2003 UB313, is sort of like that, but not exactly. It’s currently out near 100 times the Earth’s distance from the sun. It’s about at it’s farthest point from the sun, and it comes as close as a little beyond Neptune. And its period is about 560 years.

So, we’ve got a variety of different orbits out there, different types of orbits, and actually we haven’t decided whether some kinds of objects, with some kinds of orbits would be named in some particular way, and others differently, except the groups that I mentioned: the underworld characters and the creation gods.

Salazar: Is there anything else you’d like to share with our listeners?

Marsden: I think the important thing is what we’ve managed to learn about the outer solar system during the last 13 years. It was an awfully long time after Pluto was discovered before there was any action out there, you might say.

Even months after Pluto was discovered, there were people thinking that there would be other bodies out there. There was a very nice article published in August, 1930, already saying that the solar system consists of the terrestrial planets Mercury, Venus, Earth, and Mars the minor planets or asteroids, the giant planets Jupiter through Neptune. And this article suggested that, perhaps in Pluto, was the first in a new zone of ultra–Neptunian bodies out there. And then, in various times throughout the 20th century people pursued that idea. But it wasn’t until 1992 that we actually had additional objects discovered out there.

The objects being found were so faint, and it’s so difficult getting the telescope time to observe them. I thought quite frankly that we just wouldn’t get enough time on them to be able to follow them up properly. As it is, we’ve lost about half of them, because we just don’t have enough telescope time. They may have just been observed for a couple of days. But we have managed to hang on to quite a few of them, and we can see the variety of dynamics that they follow. We’re getting orbital information, as well as physical properties. Some of them are reddish in color, for example. During these past 13 years, we’ve really learned much more about the outer solar system than I would have thought possible.

Salazar: Dr. Marsden, thank you.