San Pedro de Atacama Celestial Explorations

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Differences between planets and asteroids

Published on Tuesday 17 January 2006

This page is a first in a series written to answer some of the questions I hear during my tours. Since a lot of publicity has been done on the discovery of new planets in our solar system, here is a different point of view.

What is a planet ?

There is no official definition or even a concensual definition of what a planet is. Instinctively, a planet is a large body, orbiting around the sun (or more generally around a star), in the plane of the ecliptic. Large objects are planets, small bodies are asteroids. Planets are confined to this plane, while asteroids can have quite high inclinations compared to this plane. While for a long time there was a huge gap between the smallest planet and the largest asteroid, Mercury has a diameter of 5000km while the largest asteroid of the time, Ceres, is about 900km of diameter, this gap is now smaller but still real and easily visible, giving us a solar system with 8 planets, millions of asteroids, and a lot of unneeded publicity.

Visually, a planet looks like a star, except it does not twinkle, and it changes position in the sky with time. The stars are fixed, or almost so, the stars, do move, but very slowly. The fastest star moves something like the diameter of the moon in 500 years. If you were to see the sky as it was 2000 years ago, you would recognize the same constellations, apart from a few stars which would be slightly off. Planets on the contrary have a motion which can be appreciated in a few days or a few weeks for the slowest ones. Morevoer, it was seen quickly that these "planets" move all in the same plane, in a band of the sky we call the ecliptic, or the zodiac, which is divided in 13 constellations: the 12 "classical" zodiacal constellations, plus Ophiuchus, were planets spend in fact more time than in neighboring scorpion.

The etymological origin of planets comes from the greek word Planes (planetos in the genitive form) which is the adjective "errant". "Plane" on the other hand is planis (like in Chrysia planitia on Mars). Planets in modern greek is Planitis.

Pluto and the transneptunian asteroids :

There has been a lot of discussion concerning planets and asteroids, mainly after the discovery of transneptunian asteroids. The first one of course was Pluto, but with the excuse that for a very long time, nobody had a precise idea of its size. Taking its magnitude, and an average asteroid albedo (percentage of light reflected by the asteroid) led to a diameter in the same range as Mercury. The famous planetary astronomer Gerard Kuiper tried to estimate its apparent diameter, found 0.23 seconds of arc, which led to a diameter of 5800km. Later, infrared measurements, and stellar occultation led to a precise diameter of 2280 or so km. The discovery of its first satellite in 1975 led to a correct estimate of its mass. More recently some larger transneptunian asteroids have been discovered, and no years has passed since the discovery of "the tenth planet". More about this later.

Here is a view of the solar system, as seen from above, with the 4 largest planets (Jupiter Saturn Uranus and Neptune) and all asteroids with a semi major axis larger than 6 AU. Said otherwise all asteroids which have an orbit larger than 6 times the sun to earth distance.


Pluton is included in these asteroids. Which one is it, I know it, but difficult to know otherwise. It looks like any other of these asteroids. Sedna and 2003UB313 are outside of this field of view.

The definition at the beginning of this text points to the right parameters. The mass, and the inclination of the orbit. The mass, because a planet is supposed to rule dynamically over the smaller objects around it. For example, Pluto is rotating in 3/2 the time of Neptune because Neptune, with its large mass forces Pluto's orbit (and many other asteroids in the transneptunian belt) to be so.

So here is the solar system as we knew it in the past, making a large error on Pluto's real mass :


At that time, Pluto supposedly having the same order of mass as Mercury, being far from the asteroid pack, but with a similar mass as the smallest planet could be called a planet. In this diagram, I extrapolated the known mass of Pluto, but with a diameter of 5800km instead of 2280. There was a 2 order of magnitudes of difference between a small planet and a large asteroid, things were easy back then... :)

If one plots a diagram of mass versus inclination with the current data, I think it is rather clear which group a given object belongs to. I plotted here the log of the mass in order to be able to better see such a wide range of masses. I used either published mass estimates, and for 3 large TNAs having the same type of surface property as Pluto, I used the same density as Pluto extrapolated to the best estimated size.


which I would separate in two groups, planets in the lower right, and asteroid in the left of the diagram. Like this :


There is still a factor of 13 between the mass of Mercury and that of 2003UB313 (at least the mass I estimated here of 2.6 10+22 kg) and clearly it is out of the ecliptic plane... Now of course, we could divide in two groups like this, not very logical, but if it pleases some people, why not...


The fact is that while people believe we could still discover planets (i.e. large objects in the plane of the ecliptic, governing the motion of their neighbours, the ecliptic zone has been fully surveyed to a quite faint limit (magnitude 22 or 23 ?), they would have to be quite far to have escaped to these surveys. On the other hand quite far from the ecliptic, there can still be some large pluto style asteroids to be discovered. We will know I believe in the coming ten years.

Transneptunian objects or transneptunian asteroids ?

One can already argue about an interesting fact. While the small bodies between Mars and Jupiter are normally called asteroids, the small bodies outside of Neptune orbit are generally called transneptunian "objects" while they are physically and dynamically asteroids. The reason for this is not very scientific, but related to the fact that there are fashions in astronomy. One of them is that discovering asteroids is an old timer activity, and that it does not produce real science. Clearly using valuable telescope time finding asteroids is not fashionable. There was a time when asteroids would live long streaks on the long photographic exposures done by the astronomers. Back then they were called the vermin of the sky, which tells you how much they are considered by the "real astronomers". I remember seeing a paper on how the IRAS satellite images were processed. Something was said which read like "we then perform a post treatment in order to remove all the unwanted features in the images : cosmic rays, asteroids, etc...".

Said otherwise, unless dedicated telescopes are built for TNOs, people believe (maybe rightly so) that discovering one or ten new asteroids will not change basically our knowledge of the Solar System. There is a lot of pressure now to use large telescope time, and one has to explain that your research is really fundamental. You may have read this type of "information" without knowing such things. A good example is the recent return of the Stardust capsule. It has captured comet dust, and is going to allow astronomers to study comets. Is it going to teach us profound things on the creation of the solar system as was stated ? I doubt it ! About the formation of the Universe, clearly not, eventhough this is what I heard on the TV... So because of this very strong pressure to perform, astronomers have to show that their research is important, eventhough it leads to some stupid assertions. It is therefore important to be able to read between the line, and try to figure out what is true, and what is useless superlative.

A fact is nevertheless that if TNOs (objects) had been called TNAs (asteroids) , we would know less than one hundred by now.

Is the current decrease of discovered transneptunian asteroids a direct effect of the fact that the astrophysicists leading the time allocations committees have understood they had been lured into giving precious telescope time to discover ridiculous asteroids ?.

If we wanted to not take this effect into account we could say that in 2006, we should discover around 25 TNA, and reach the false conclusions that there are only 1000 TNA in the TNA belt. :) The fact is that the heydays of TNO discovery is gone, and we will have to wait for larger dedicated telescopes to come on line to know more about the transneptunian belt.


These asteroids have mainly been discovered at a few major observatories, like Mauna Kea, Kitt Peak and Cerro Tololo, as can be seen in the following figure. The most important program, the Deep Ecliptic Survey conducted at Cerro Tololo and Kitt Peak has discovered most TNAs by now.


Another important point to understand is that basically half of the transneptunian objects are currently lost. They were observed over a very short arc, a very long time ago. Some people need dedicated telescopes...

Cultural planets ?

There has been a lot of funny ideas expressed about what a planet is. A very interesting one is that of a cultural planet. I.e. "Pluto should be called a planet because it has been called a planet for 70 years and is known as such by the general public". The problem with this is that astronomy is not a religion, but a science. No dogma there, things are bound to evolve as we understand the universe better. If we have the right understanding, things will not change, but if our knowledge is partial (and we can never know how good or realistic our explanation is) things could change. A rough idea is that things which are closer to us are normally better known than things which are further away. We will very likely not revise the value of the Earth Moon distance (we know it at a given moment to a few mm), but nobody would bet too much on the distance of far away galaxies.

The first asteroids were called planets, and later when it became clear that there were many small objects between Mars and Jupiter, they were called asteroids. The first transneptunian asteroid was called a planet, and we know there must be zillionth of similar objects in this zone of the solar system, it is now called an asteroid, no big deal. A more profound change was when we discovered that the Earth was not the center of the solar system, and that planets were rotating around the sun following elliptic orbits. We admitted that for more than 1400 years astronomers had been wrong, and forgot about epicycles. We didn't continue to pretend planets were rotating around the Earth making epicycles because it was a cultural thing, or that because for more than 1400 years childrens at school had been taught that planets were making epicycles (or were they taught ?). This is a very profound thing, and this stupid argument should not be used, in fact we should take the opportunity to explain to the public that in science, there is progress, and sometimes we change our point of view, and sometimes, very profoundly so.

Take for example the expansion of the universe. A direct consequence of the expansion of the universe, which is an observed fact, is that in the past the universe was smaller than now, and eventually that it may have been concentrated a single "point". It is not exactly so, but let's simplify things here. Then, after admitting the expansion of the universe as a fact, theorists worked on this and made models about what happened in the early seconds of the universe, describing the evolution of the universe till 10-43 seconds "after the Big Bang". Now, after 1998, we understood that the expansion of the universe is accelerating with time. It means there must be a repulsive force, which is called the dark energy, because most of what we should know about this force is still in the dark :). It is not really so, it is called the dark energy because we also know that there must be a component to the mass of the universe which is not emitting any radiation, which we call the dark matter. Together with the dark energy, the dark matter account for 95% of the mass (or the energy) of the universe. Now, what is the probability that the 1970s models of the Big Bang describing the evolution of the universe using only 5% of its content of the universe are still correct ?. Should we forget about possible new discoveries for "cultural reasons" ? I don't know if the Big Bang will still be valid for us in 50 years, we just know it is the best theory we have right now to describe the universe, but we also know this could change. It is very important to convey this idea that nothing is definitive in Science. If we can not do it for an obvious thing like Pluto being now considered an asteroid, I think we will have a much harder time when it comes to more profound things such as the origin of the Universe. This cultural argument is really a lame one.

Brown planets

We know a lot of different stars, from large red supergiants to small brown dwarf stars, which are so cold that they emit mostly in the infrared light. There are no such "color" differences for planets. When looked up close, each planet is different from the others. We normally divide planets in gas giants (eventhough Uranus and Neptune are different from Jupiter and Saturn), in telluric planets, and the smaller objects are normally called asteroids. There is a main belt of asteroids, between Mars and Jupiter, Earth Crossing asteroids, Centaurs (between Jupiter and Neptune) and transneptunian objects. Recently we have had 3 planets of a different type. They are called Brown planets, not because of their temperature like brown dwarf, but because of their discoverer.

When an asteroid is discovered, the normal procedure is to report two nights of observations to the Minor Planet Center, which checks the data (to see among other things if it is not already known, if the positions looks correct, etc...), and gives it a temporary designation, publishes it, so that other observers can follow it, and eventually make some physical observations. Usually, much later, when the orbit is determined with a good quality, the object is given a number, and the discoverer (or discovery team) has the privilege to submit a name and a citation for the object to the committee for small body nomenclature (CSBN) of the IAU which is then normally accepted within a few months. If you read on a web page something like : "Thus 2003 UB313 can be decoded to tell you that the data from which the object was discovered was obtained in the second half of October 2003. Next, the discoverers propose a permanent name" you can understand that either the writter of the page does not know anything about the current procedure (which is explained here), or that he wants you to believe a different thing from the normal procedure. "Damned my planet still does not have a real name... :)."

When Uranus was discovered, William Hershell thought he had discovered a comet and he announced it as such. Only when the orbit was calculated did it become clear that the object was a planet (see above, a large object orbiting in the plane of the planets).

When the good doctor Brown and his team discovered the TNO now called 50000 Quaoar, which now nobody would call a planet (it is only the 8th largest transneptunian object), it was not announced as an asteroid by the Minor Planet Center, but through a press release of Caltech, we learned that the object already had a name, and only several months later, when the orbit was sufficiently well known, did the IAU give it a number and accepted the discoverer's suggestion of Quaoar. I, like many other astronomers, was puzzled by the non respect of the admitted procedures for announcing a discovery or naming it from people who should know...

A few months later the same team discovered Sedna, a very interesting asteroid since it is our actual limit in the solar system. Again, instead of following the normal procedure, we had a press conference at Caltech (eventhough there had been leaks about the discovery of a "planetary sized object", well, 1500km, everybody will decide if this is planetary size...), then several months later, a number, then the CSBN admitted Caltech's name again. At this occasion, we started to see press releases by the large news agencies talking about the discovery of a tenth planet in the solar system.

Then last year, in July 2005, 2003UB313, which looks a lot like an asteroid name, but which was announced as the tenth planet (well the second tenth planet ? or is it ?). The announcement of 2003UB313 is linked to the announcement of another rather large transneptunian asteroid called 2003EL61. Brown's group discovered 3 large transneptunian asteroids, and kept then secret in order to make follow up observations, and announce the discoveries in due time. However 2003EL61 was discovered by a group of spanish astronomers, and reported to the MPC before Brown's group. So pressed by the spanish discovery, and a stupid story of "hacking" which was not (well, when you are able to make and even create a confusion between an asteroid and a planet, you can also make a confusion between internet access to a site referenced in Google and hacking). Anyway, 2003UB313 was announced like and asteroid, as it should have long before, but later a press campaign was organized to announce the discovery of the tenth planet... again. So here we clearly have a new definition of a planet, a planet is any asteroid discovered by Brown's group.

The funny part being that on Dr Brown's web page, we read both the title : "The discovery of 2003 UB313, the 10th planet." Like if HE decided what was a planet for the rest of us (Caltech's arrogance ?), then later we read "it is quite clear that Pluto should certainly not be put in the same category as the other planets" or even "There is no good scientific way to keep Pluto a planet without doing serious disservice to the remainder of the solar system". It took me some time to understand this. Then I understood: forget about Pluto, the 9th and 10th planets are Sedna and 2003UB313... :)

To conclude, taking about scientific ways, I think that bending science in order to make a publicity stunt is a poor thing. These discoveries, whatever we call them are wonderful in themselves. The objects that Brown's team have discovered have profoundly changed our knowledge of the transneptunian belt. What does a person gain of using the media to his advantage ?. One should not make confusion between its own publicity (is it really needed ?) and scientific facts.A scientific fact is built on peer reviewed publications, and important discoveries talk by themselves, without having to measure the thickness of a pressbook. Proximity of Pasadena to Hollywood ? New ways of doing science ?
More seriously, I think that a look on the diagram above gives a clear clue of what should be called a planet, and what should be called an asteroid. What is relayed by the press may not always be the truth...

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