A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

The death of the 10th planet

A remembrance of 5 years ago, today, excerpted from How I Killed Pluto and Why It Had It Coming

As an astronomer, I have long had a professional aversion to waking up before dawn, preferring instead to see sunrises not as an early morning treat, but as the signal that the end of a long night of work has come, and it is finally time for overdue sleep. But in the pre-dawn of August 25th, 2005, I awoke early and was up sneaking out the door, trying not to wake my wife Diane or our one-year-old daughter Lilah. I wasn’t quite quiet enough. As I was closing the front door behind me, Diane called out, “Good luck sweetie!”

I made the short drive downhill through the dark empty streets of Pasadena to the Caltech campus, where I found myself at 4:30 AM, freshly showered, partially awake, and uncharacteristically nicely dressed, unlocking my office building to let in news crews that had been waiting outside. All of the local news affiliates were there, as well as representatives of most of the national networks. Outside, a Japanese-speaking crew was pointing their TV camera up at the sky, their flood lamps disappearing into space. A glance at their TV monitors showed nothing but flood lamps disappearing into space.

The news crews would probably have preferred to be asleep, too, but today was the last day of the International Astronomical Union meeting in Prague, and the final item on the agenda at the end of two weeks' worth of discussion was a vote on what to do with Pluto. Everyone’s favorite ice ball was in eminent danger of being cast out of the pantheon of planets by the vote of astronomers assembled half a world away, and whatever happened would be big news around the whole globe.

I like planets. But I didn't care enough about Pluto to get up at 4:30am. A year earlier, it is true, I was waking up at 4:30am or 1am or 3:15am or whenever the then-newborn Lilah happened to need a late night bottle, but, after a year, I was pretty happy that these days Lilah almost never woke up in the middle of the night. I liked my sleep.

But this Pluto vote half a globe away mattered enough to me to drag me out of bed that morning. To me that vote had nothing to do with the ninth planet; it was all about the tenth. And I cared a lot about that tenth planet, because eighteen months earlier I had discovered it, a ball of ice and rock slightly larger than Pluto circling the sun every 580 years.  I had been scanning the skies night after night looking for such a thing for most of a decade, and then, one morning, there it finally was.

 At the time of the Pluto vote, my discovery was still officially called only by its license plate number of 2003 UB313, but to many it was known by the tongue-in-cheek nickname of Xena, and to even more it was known simply as the tenth planet. Or maybe, after today, not the tenth planet. Xena had precipitated the past year of intensive arguments about Pluto, but it was clear that Xena would also share whatever fate was dealt to Pluto. If Pluto was to be a planet, then so too Xena. If Pluto was to be kicked out, Xena would get the same boot. It was worth waking up early to find out the answer.

The previous two weeks in Prague had been perhaps the most contentious gathering in modern astronomical history. Usually the International Astronomical Union meeting is nothing but a once-every-three year's chance for astronomers to advertise their latest discovery or newest idea while spending some time in a nice international destination, having dinners with old friends and catching up on their celestial gossip. On the final day of each meeting, in a session attended by almost no one, resolutions are passed, usually all but unanimously, on such pressing topics as the precise definition, to the millisecond, of Barycentric Dynamical Time (I have no idea what this actually even means, but, presumably, it matters critically to someone).

This year was different. The usually placid astronomers had spent their time in Prague arguing and bickering day and night about Pluto and about planets and about how to reconcile the two. While several of the typically unintelligible resolutions were indeed to be voted on this last day, the final two resolutions would be all about Pluto. The usually sparsely attended final session was likely to be full of surly astronomers itching for a fight.

While the astronomers were gathering for their vote in Prague, the news crews and I were arriving in the early morning on the Caltech campus in Pasadena so that we could watch the excitement via a webcast from the other side of the world. My job was to provide commentary and analysis for the press and moral support and scientific cover for the astronomers who were – rightly, I thought – trying to take the bold move of ridding the solar system of the baggage of planet Pluto.  I found the webcast, projected it on the large screen, and we all sat back to watch.

Three mostly esoteric and tedious hours later it was all over. On the final vote, the air was filled with yellow cards with which the astronomers in Prague were voting “no” for Pluto planethood. There was no need to count; the vote was not even close. After hours of detailed explanation and analysis and discussion of the subtleties of all of different possible outcomes, I could finally just say: “Pluto is dead.”

The cameras whirred; correspondents talked into their microphones, and, on a screen on the other side of the room, I could see myself on some local television station repeating, like an echo, “Pluto is dead.”

Before anyone else could ask a question I quickly picked up the phone and called my wife Diane, who was now at work. I had made a similar phone call 18 months earlier, only minutes after I had discovered Xena. Back then, the moment she picked up the phone I said "I found a planet!"

Back then, her voice had risen, “Really?”

Yeah! Really!

This time, instead, the moment she picked up the phone, I said, “Pluto is no longer a planet!”

Her voice dropped, “Really?”

Yeah! Really! I was still excited from the vote and had not quite grasped her mood.

 She paused for a long time. “And Xena?” she finally asked quietly.

But Diane already knew the answer. Xena had indeed gotten the same boot as Pluto, and Diane was already mourning the little planet that we had gotten to know so well.

In the days that followed I would hear from many people sad about Pluto. And I understood. Pluto was part of their mental landscape, the one they had constructed to organize their thinking about the solar system and their own place within it. Pluto seemed like the edge of existence. Ripping Pluto out of that landscape caused what felt to be an inconceivably empty hole.

That first morning, though, Diane was having the same reaction, but for Xena instead of Pluto. For her Xena was more than just that thing that used to be called “the tenth planet.” She had listened to me enough over the previous 18 months that she had gotten to know all about the onetime 10th planet. She knew about the tiny moon, the incredibly shiny surface, and the atmosphere frozen in a thin layer all around the globe. Diane and I had discussed the excitement of the search, what to name the 10th planet, and how many more like it might be out there. Xena had become as much a part of our own mental landscapes as Pluto might have been for anyone else. And Xena would be forever tied in our minds to our daughter Lilah, who was only 3 weeks old when Xena was announced to the world. All of those memories of the first months of our Lilah's life -- the lack of sleep, the dazed confusion, the questions about what life would be like after this sudden change-- are tied up with all of those memories of what became10th planet mania -- the rush to learn more, the push to discover others, the questions about what life would be like after this sudden change. And now, just a little after Lilah’s first birthday, Xena was gone.

I had to tell Diane: The astronomers did the right thing. Xena is not really gone, of course. It is now actually the largest of the dwarf planets, where it rightfully deserves to be.

Lilah will probably not learn about Xena in school, but, someday, we'll tell her that when she was three weeks old the world first heard about the 10th planet, and we'll pull out our little box of Xena news clippings and talk about that year when Lilah and the 10th planet were both burning themselves into our lives as things that we could never again imagine the universe without.


  1. It seems your pronouncement of planetary death for Pluto and Eris is a bit premature. National Geographic has an article discussing how new developments in astronomy are leading many to question the decision made five years ago. Having large numbers of planets seems to be the norm rather than the exception, as we're finding from new exoplanet discoveries. And using one definition for our solar system and another for every other solar system is hard to justify.

  2. @anon I think that as we learn more our ideas about how to precisely define a planet will continue to evolve -- just like they have for the past 5000 years. But it seems really really really unlikely that they are going to evolve in a way that ever puts Pluto and Xena (Eris) back in the same category as the rest of the planets!

  3. the mad LOLscientistAugust 24, 2011 at 1:09 PM

    The only thing more disappointing than "losing" Pluto was not having the names Xena & Gabrielle stick. So what if they weren't kosher? =^..^=

  4. Why don't the people who provide most of the funds needed to operate many of our nations obseratories have a vote on the fate of Pluto and Xena? Maybe because they would be named planets again. We provide the cash we should make the final choice!!

  5. @mike I'm not sure the eight recognized planets all deserve to be in the same category! If you look at a gas giant like Jupiter, it seems to be intermediary between a terrestrial planet and a star. When we get around to colonizing Jupiter's neighborhood, we'll find its four biggest moons (one of which is larger than Mercury) meet our expectations as "planets" more than Jupiter itself does. I won't be happy until we see all as being part of the same dynamic, sliding scale. No committee vote can change what Pluto and Eris are intrinsically, or the sense of awe and wonder I feel about them. But I fear the IAU definition of "planet" did take the wind out of the sails of public interest in "dwarf planets," which is a shame. As a lifelong space enthusiast, I tell everyone, "They're ALL planets!" And I feel that's at least as good a definition as anything else I've heard.

  6. I get that definitions will evolve but I don't think that means Pluto and Eris will automatically be excluded from being considered planets. True, they won't be in the same category as "the rest of" the planets recognized by the IAU, but neither are Jupiter and Earth in the same categories. Both are considered planets, but they are very different from one another. In some ways, Jupiter is more like a brown dwarf than a planet. Maybe what we need is a system for planets like the Herszprung Russell Diagram we have for stars. That diagram encompasses a huge range of stars from supergiants to brown dwarfs. A system like this for planets might very well put Pluto and Eris is the lowest end planet category, equivalent to the position brown dwarfs occupy in the stellar classification system.

  7. @Raven I didn't mean to suggest that the exclusion of Pluto and Eris will be automatic. I think it will just be natural. But that is just a prediction of what future societies will decide. No way to know.

    @Bryan Totally agree that Jupiter and Mercury are very different beasts, for example. But they do have one thing in common, which is that they gravitationally dominate their orbital vicinities. So if you think of the word planet as describing behavior instead of some set of physical properties they look very similar.

    As I've said elsewhere, I think the real solution is to treat the word planet as a concept rather than a definition. Any time you try to define it someone can always make it slip out of your grasp. But the conceptual distinction between the 8 dominant guys and everyone else is pretty hard to miss.

  8. To whom it may concern:

    I believe that the International Astronomical Union simply got it wrong. Having so may complicated rules to describe what a planet must be, what it must not be, what a planet must do, what a planet must not do etc., is not the way to go.

    Planets needs to be classified according to just two things, density and size, that's it!

    As for size, we should apply a lower limit, based on what we are familiar with in the Solar System, which should be 1,000 miles in diameter. Less than 1,000 miles in diameter would have to be in a different category (dwarf planet, planetoid, planetesimal etc.).

    Worlds in our Solar System that I think should be referred to as planets (of one sort or another) include, but not limited to, Ganymede, Titan, Callisto, Io, Moon, Europa, Triton, Eris and yes, Pluto.

    As for density, we should apply a lower limit based on what we are familiar with in the Solar System, i.e. water at a density of A1 g/cu.cm (as a base point) can and does freeze (especially on other planets) or could be liquid or vaporous.

    In a liquid state, it would form an ocean (that could harbor life); in a solid or frozen form, it could be walked upon and may also harbor life as it does here on Earth In a vaporous state, the water could act as an atmosphere (that quite possibly harbor life, although I am unfamiliar with any such life form here on Earth); but in any case, sea level ocean water density at approximately A1 g/cu.cm would be a density starting point; anything less dense would need a sub-category (see Saturn, below).

    As far as Saturn is concerned, well it would need a special sub-classification for planets such as "light gaseous planet" or "low density planet." Other planets would also need sub-classifications such as "planet orbiting planet" or in some cases, "double planet." Planets could also be categorized by their location in the Solar System, i.e. scattered disk planet, detached planet, Oort cloud planet, etc.

    This does not need to be complicated folks, keep it simple...just back away from the planetary mumbo jumbo, so we can all get some sleep.

  9. I was talking to the Senior Eldercare person about math anxiety when she made a startling revelation: she viewed numbers as little people! She gave them personality traits and described them; they had no basis in reality, but for her, it made the numbers "cute".

    I've NEVER viewed numbers in this way. I've always viewed them as objects to be manipulated. Manipulating them was fun. Since they weren't human they didn't care.

    Later I thought of them as a component or description of something else. If you don't do the manipulation right, you don't properly describe the something else. It's like giving it the wrong color or some bizarre shape.

    I don't see how the girl saw 1,2,3,33,409, whatever, as people. It made me wonder if maybe sometimes my view of mathematics is wrong. If it was, how would I know?

    This has GOT to be the most confusing post I've ever made. I've erased and rewritten a bunch of it because I keep seeing logical fallacies. It's HARD to describe the personalization of inanimate objects. Yet people routinely do it all the time, it's easy!

    How do they get these ideas? How do I get these ideas? I'm glad Mike's a great writer because we can see the evolution of such ideas.

    Mike starts out looking for a planet. He finds one (+). His wife is very proud. He realizes it's not a good idea to call it a planet. His wife is very confused. Hadn't her husband reached The Pinnacle?

    We have to label things or language would not exist. Then we load up extra baggage on the labels to help us keep track of them. Then other people get hold of the labels and they don't have the same baggage. It's different baggage. They think the label is personally threatening or worse threatens Western Civilization.

    I've been trying to describe some of my own labels for motorcycles and have quickly found embarrassing contradictions; I myself have illogical baggage that has escaped me for years and years. Motorcycles should be just transportation but they never are. Harley Davidsons are a symbol of patriotism or rebellion, depending on the individual. Hondas are a symbol of engineering or shoddy cheapness, depending on the individual. There's a lot of nationalism involved.

    I would hope that the discussion about dwarf planets and KBO's will result in people understanding their labels of things. They don't change the things. They change the people.

  10. Someone is trying to make a definition for planet based on a diameter of 1000 miles. Why should we use miles and not meters? Why should we include just some of those smaller objects?

    I think it would just be more simple to make the borderline to 4000 km. It can also be asked why the borderline could not be just 400 km?

    And why the density should be in the definition? What makes it so important? And also I think that takin it to the definition makes it just too complicated to be a simple definition.

    Why not include number of satellites for definition of planet or why not iclude orbital period, mass, surface gravity, rotation period, colour or even the objects name...

    And ofcourse what about Stars? The Sun is much larget in diameter than 1000 miles. In your definition it would be a planet too, because the fusion things should be ruled out because they just make things too complicated.

    And I cant stant the fact that you are just trying to take Pluto to the list and trying to let Ceres out. I can tell you that Ceres was discovered much earlier than Pluto and Ceres was classified as a planet with Pallas, Juno and Vesta for decades, until so many more "asteroids" were found.

    I think including Ceres and maybe Vesta and Pallas andeven Juno as planets would be more important than just trying to iclude Pluto at any costs at the list of planets.

    I would rather call Juno a planet than follow your stupid definition. IAU definition is much beder (better) than yours.

    I dont like those who dismiss Ceres and try to make Pluto too important.

    PS: When I was youg I knew that Ceres was a planet a long time ago but was later reclassified as an "asteroid".

  11. "I was talking to the Senior Eldercare person about math anxiety" ....

    That really is very interesting. I have to wonder if the lady doesn't have a form of synesthesia.

    For those who haven't heard of this, synesthesia's basically a blending of the senses that some people are born with. A common variety is where someone actually perceives that individual digits and letters have colors. Thus, by extension, whole words and numbers can have distinct color signatures.

    It's also possible for someone with synesthesia to literally taste colors.

    In this lady's case, assuming I'm right, numbers have personalities instead of colors, but there's still that sensory crossfeed going on inside the cranium.

    The whole thing strikes me as oddly relevant to Mike's "The death of the 10th planet" post. There's a form of cultural synesthesia going on. I suspect that a lot of people associate personalities with the celestial objects currently or formerly known as "planets". There's a reason why these are typically named after deities in the various religious traditions. They're all people with personalities as well.

    Considering all the commotion Xena caused when she was discovered, it's perfectly apt that Mike gave her the official name of Eris, goddess of strife and discord.

    And thus, a large number of people take the reclassification of Pluto and Eris as "dwarf planets" rather seriously. It really feels like someone is heaping reproach on an old friend (in the case of Pluto). Pluto, of course, doesn't really care what we call it, but a lot of people still do.

    While I don't fully agree with the IAU's August 2006 definition of "planet", I suspect Mike is right: they're not likely to change their minds any time soon on how to deal with the dwarfs. It's all too emotionally charged.

    Lately, I've actually been shying away from using the term "demotion" to refer to the change in Pluto's classification. I prefer to just say "reclassification". It's a more neutral term.

    I don't think the dwarfs, in their own way, are any less important than the Big Eight. We need to study them all to fully understand how the solar system works.

    Bob Shepard of Denver

  12. Dear Anonymous:

    I would like to rebut your comments you made on August 29, 2011, at 4:35 a.m.

    Unlike people who fail to identify themselves and hide behind a ghostly "anonymous" label, I am not just someone, but identify myself in all of my recent comment postings.

    As all Americans use the Imperial measurement system, not the metric system, we use inches, feet, yards, miles, etc.; not meters, although it would be impractical and inappropriate to use meters in this case, which is about 0.91 of a yard, it would be appropriate to measure a body in kilometers (one mile is equivalent to approximately 1.61 kilometers), so lets not say one form of measurement is better than the other, more appropriate than the other or we should restrict such measurement to one or the other; they are just different forms of measurement used in different parts of the world.

    Using your own 4,000 kilometer example, any known object less than the size of Callisto would be something other than a planet. As for 400 kilometers, well that just goes too far, i.e. a world as small as Mimas would qualify as a planet, now that's a joke.

    Density is of importance in this case because otherwise a large ball of diffused, translucent gas and dust, such as a nebula, could qualify as a planet.

    As for the Sun, well this is where density comes into play, because the Earth is on average five times more dense than the average density of the Sun. You could say that no planet could be consuming itself.

    I have no problems with worlds such as Pallas, Vesta and Hygiea being classified as proto planets; not asteroids or dwarf planets. Ceres is the only main asteroid belt world that looks like a miniature version of a planet; it may even have a liquid water ocean below its icy crust, and certainly, of all main belt asteroids, be considered a dwarf planet.

    I am not sure how Juno fits in, if you actually meant to include this as a planet, now I am laughing aloud.

    All of the asteroids that make up the main asteroid belt constitute less than 4% of the mass of the Moon, which makes them pretty insignificant, except Ceres.

    I am sorry that you dislike my "definition," as I was not offering, nor suggesting one. All I really wanted to convey was that a planet should be defined by its physical attributes, rather than other conditions such as its shape, mass, location or ability to clear its neighborhood, for example.

    Thank you for your mature, insightful criticism and wonderfully entertaining use of English. Next time, try to get your parents to proofread your postings.

  13. Correction to above comment;

    Shape and mass are physical attributes and were included erroneously.

    What I meant to say was that a world should be categorized by only its physical attributes, such as mass, volume, density, size, lack of self-consumption (as a star) etc.

    A body should NOT be categorized based on its orbital status, inability to clear its neighborhood, etc.

  14. Dear George Smith

    I am sorry if I hurt your feelings at my last post. I was just so angry about that your definition for a "planet" was made for including Pluto as a planet and not Ceres, even though Ceres was called a planet before it was defined as a largest of objects in a "belt" (just like Pluto).

    And English is not my native language so I can't write it as well as those who are native speakers of it.

    And im writing as "anonymous" because I have not bothered to register a profile. And yes you identify yourself and I do not. I say again that I am really sorry.

    And as for Juno I was just saying that it was a planet with Ceres, Pallas and Vesta for decades until it lost its status. So it has just as good reason to be a "planet" as Pluto does if we think the concept of a planet historically.

    I think you just made your definition to include Pluto as a planet. It seems that you chose the size limit to include Pluto. I think it is not nice to try include Pluto at any costs.

    And as I said I didnt like that you didn't include Ceres. It deserves to be called a planet if Pluto does. Thats my opinion.

    And why couldn't a planet be somethig that consumes itself (like the Sun or Sirius). What is wrong about that if there is nothing wrong about icluding some small icy trans-Neptunian bodies.

    I think its wrong to make the borderline thinking about how to include one object (Pluto), and not much about anything else.

    Maybe you Americans just love that icy rock too much to think rationally. That might be Disney's fault like Neil deGrasse Tyson says.

    My opinion is that there is nothing wrong about just to include the "big things" whit diameter more than 4000 km as "planets", even though I think that the IAU definition is good enough, but if you really want to make a definition based on diameter then I would say 4000 km.

  15. I understand why people get sentimental about terms, but really it doesn't erase a place, or make it less interesting. It's semantics.

    I mean Vesta was called a planet for decades before being demoted to an asteroid. It is still there and still as interesting, with its huge blasted out bottom. We are visiting is now, via Dawn space probe and its amazing whether it’s called a planet or an asteroid.

    Why get so protective over the term planet. We just had planets, then found out the Sun was different and the other planets orbited it. Galileo also found moons orbiting Jupiter; our moon got demoted. Ceres was found and called a planet, so were Pallas, Juno and Vesta; then we found a whole lot more out there and then had asteroids. Things change when we find out more. New terms just make it easier catalog them. We should keep definitions simple and not try convolute things so we can shoehorn our favorites into the term plant.

  16. Two ideas: First when we as human make classifications we often make very definite boundaries. For Instance In track and Field for instance a Junior Athlete is one on 19 years or younger on the 31st Dec. of the year of the competition. Thus if he's born on the 31st dec 1992 he is considered a junior for any competition held in 2011 but an adult in the yr 2012. However if he/she is born a few hours later, or on a different time zone, and it's birthday is jan 1st 1993 he is elegibble to compete as a Junior for all 2012.
    The point I am trying to make is that a cutting point could be made arbitrarily as to Mass, Volume or Maximum Diameter for a body to be consdered no longer a plantet but a Dwarf Planet.

    The other thing I wanted to mention is a possible classification (Mimicking Resolution 5A of IUA) of the Satellites of Planets as in a) Co-planet: baricenter off the larger Body b) "Classic" Moon: when it has achieved hydrostatic balance and has cleared it's neighborhood c) Dwarf Moons when have hydrostatic balance but have not cleared its neghborhood and d)Irregular moons (as in captured/ non round/high inclination/retrograde)


  17. satellite trackingJanuary 10, 2012 at 1:30 AM

    Nice post and mine opinion is that planets needs to be classified according to just two things, density and size.

  18. oh oh no pluto

  19. And to think some will argue (like Anonymous here) that hot dogs have dog meat in them.

    -Mr Chili Cheese Coney from Sonic®

  20. Great article, & some heated comments!! I hadnt realised classification of planets was so complicated