Copernicus Would Not Be Proud
August 24, 2006 Last weekend I attended the Astronomer’s Conjunction, an annual amateur event held in western Massachusetts. After listening to two teams of well-informed amateurs debate the Pluto-planet question, I asked the following hypothetical question: Let’s pretend that we wake up on Monday morning to the following exciting news. Astronomers using the Keck Telescope announce that they took a series of deep images of Proxima Centauri, the nearest star to the Sun. The pictures revealed an orbiting body that is well above the threshold to be spherical, it has three moons, and a spectrum shows that it has an atmosphere. Should we call it a planet? After nobody in the audience objected to this designation, I added, “Actually, I was just describing Pluto.”
We heard the news today that after 76 years, less than 400 members of the International Astronomical Union decided to boot Pluto out of the family of planets. This was an unfortunate decision for a number of reasons, not the least being the bad PR that such a tiny minority of self-appointed elites is ramming its arbitrary decision down the throats of the entire astronomical community. I predict that some of the members who voted for the new definition of “planet” will come to regret it, because it will fail to accommodate future discoveries. But I’m pleased that the IAU made a decision, and that the new official definition of “planet” is well motivated and based on sound scientific reasoning. Nobody is going to die or lose their life savings because Pluto will no longer be considered a full-fledged planet.
But there are several gaping holes in the IAU’s definition, including one that is so immediately obvious that it boggles my mind that intelligent people could actually vote for it. The definition is anti-Copernican in the sense that it defines planets as objects orbiting the Sun. In other words, only solar-system objects can be considered true planets. Excuse me, but even under the most liberal definition for “planet” that one could seriously propose, we know of about 50 planets in our solar system right now. We already know of more than 200 planets outside the solar system. And if the official definition were extended to include planets around other stars, the total in our Milky Way Galaxy alone must be something on the order of 100 billion to a trillion. In other words, the IAU definition completely ignores 99.99999999999 percent of the planets that exist in our galaxy. Huh?
Maybe this was prudent, because adding extrasolar planets to the resolution would have made it even harder to reach a verdict. Maybe a bunch of astronomers thought, “We need to learn more about planets outside the solar system before we can come up with a sensible definition.” But something inside of me says that the IAU wimped out, and that it might end up sweeping this problem under the rug for a very long time, just as it did with the Pluto question.
Perhaps certain members of the IAU leadership, who were responsible for the exoplanet omission, felt concern that future discoveries could throw a monkey wrench into the official definition. If that’s what they thought, their concern was justified. For example, one of the criteria is that a planet “has cleared the neighborhood around its orbit.” Not only is this statement rather vague when applied to our solar system, but I predict that in the next few years, we’ll find systems around other stars where it fails miserably. Astronomers who simulate the evolution of planetary systems on computers have shown that in some circumstances, interacting planets can leave behind a system with two Jupiter-mass planets that share the same orbit, with one 60 degrees ahead of the other. Such a “Trojan” configuration is stable for billions of years, so it almost certainly exists in nature. It’s only a matter of time until planet hunters such as Geoff Marcy and Paul Butler uncover this kind of system (it’s possible that several have already been found!). So if the IAU definition is applied to other stars, two very massive planets sharing an orbit would not be true planets, because neither one has cleared out its neighborhood.
Astronomers have also found that at least half of very young stars are surrounded by disks, and there’s good reason to think that many of these disks will spawn planets. It will take tens of millions of years for these planets to clear out their regions of space. Does that mean that a newborn 5-Earth-mass object that has not had time to clear out its zone is not a planet?
I also suspect we’ll find systems that through some kind of freak dynamical evolution, have left planet-mass objects in zones of rubble. Heck, even Earth shares its region of space with tens of thousands of asteroids of various sizes. One could counter that these bodies are in unstable orbits. But both Jupiter and Neptune harbor vast populations of Trojan asteroids that will remain in their orbits for billions of years. And even many trans-Neptunian bodies, such as Pluto, cross Neptune’s orbit. Under the IAU’s vague new rules, we could rule out Earth, Jupiter, and Neptune, and perhaps other planets as well.
And by not setting a specific size or mass requirement, the IAU has left itself wide open for criticism as soon as astronomers find Mercury-, Mars-, and perhaps even Earth-sized bodies in the distant solar system. Such discoveries are only a matter of time, given the limited nature of surveys to date.
I know it was really, really hard to come up with the definition that the IAU accepted today, and I also know it was impossible to come up with one that would satisfy everyone. The definition that was accepted could have been much worse. And while I hope the passage of this new definition calms down the debate for awhile, we have not heard the end of this story.
Time to Make a Decision
August 22, 2006 The latest news from the International Astronomical Union’s General Assembly in Prague is that a revised proposed definition for “planet” is under consideration, and that the debate has been rancorous. If Oliver Stone were to direct a movie about the deliberations, he might give it the title Astronomers Behaving Badly. The IAU is scheduled to take an official vote on Thursday, and at this point, I would not want to hazard a prediction on what the members will decide, if they decide anything at all.
I have made my views on Pluto crystal clear on this blog: its physical characteristics (spherical shape, three moons, atmosphere, etc.) clearly place it within the family of planets. I also call on the IAU members to accept an official definition that will accommodate new discoveries both inside and outside the solar system, although that now seems unlikely. But right now, my main hope is that the IAU decides something. This Pluto/planet-definition controversy has been simmering for years, and the IAU fiasco has turned into an embarrassing spectacle. For decades, the public has been told that the solar system has 9 planets. Then people heard last week that the total will be upped to 12. And later this week, they might wake up to the news that it has dropped to 8. Surely, this affair could have been handled more discreetly, generating less confusion among the public at large.
Yes, the debate has given the public a glimpse into the scientific process, and layfolk have received a demonstration that science is a dynamic process in which conclusions change with new discoveries. And it’s almost always good for astronomy when it makes the news. But there are several important reasons to resolve this Pluto/planet issue. First, astronomical researchers need to know what to call Pluto and other borderline objects in their communications to one another. Perhaps more important, how are we to convince people that they should abandon outmoded concepts such as creationism and intelligent design if astronomers can’t even reach a consensus on something as seemingly basic as the number of planets in our solar system? Science opponents are always looking for cracks in the scientific establishment, and astronomers have handed them a dream example on a silver platter. It’s time to resolve this issue and move on, and that’s more important than what astronomers decide to call Pluto.
So I hope that the IAU this week accepts one of the proposed definitions, even if it means Pluto is booted from the family of planets. Future discoveries may force revisions to whatever planet definition is accepted, so it won’t necessarily be the end of the story. And of course, whatever definition is accepted won’t change our understanding of Pluto’s origin and context within the solar system. But sweeping the definition problem under a rug until the next IAU General Assembly convenes in 2009 is hardly an acceptable outcome. Once the IAU renders its verdict, then it’s the task of researchers, educators, amateur astronomers, and media outlets like S&T to communicate the result to the public, and explain the rationale for the decision. At that point, astronomy aficionados of all types will have an opportunity to turn a negative into a positive.
The Great Planet Controversy
August 17, 2006 Gosh, am I glad I was not a member of the International Astronomical Union committee tasked with the thankless job of proposing a definition for “planet.” These seven unfortunate souls were inserted between the proverbial rock and hard place. Given the wide range of objects in our solar system, and the many different opinions within the astronomical community and general public, there was no chance that they could come up with a definition that would satisfy everybody.
So I don’t want to sound overly critical of the proposal, which has since been strongly endorsed by the American Astronomical Society’s Division for Planetary Sciences. The committee’s proposal essentially defines “planets” as nearly round objects that orbit stars. I don’t have a strong opinion as to whether the IAU General Assembly should vote “yes” or “no” to this proposal on August 24th (I bet it will pass by a large margin). The committee deserves credit for coming up with a definition based on physical principles that can be applied to objects inside and outside the solar system. But there are several glaring inconsistencies in the proposed definition that will open up a can of worms if approved.
For example, what exactly is meant by “nearly round?” The committee defines it as an object in “hydrostatic equilibrium” (i.e. its mass is sufficient for gravitational compression to overcome its material strength and force it into a nearly round shape). But where does one draw the line between an object that is in hydrostatic equilibrium and one that is just slightly out of hydrostatic equilibrium? If they haven't done so already, astronomers will find borderline cases, so the decision whether or not to include certain objects as planets will be arbitrary.
Making matters worse, the committee is including Pluto’s largest satellite Charon as a planet, because the system’s center of gravity is located in the space between the two objects. In other words, the committee members are saying that Pluto and Charon form a double-planet system.
That sounds clean cut, but it’s not, because the location of a system’s center of gravity depends both on the objects’ masses and their physical separation. At the moment, the Earth–Moon system’s center of gravity is inside Earth. But tidal interactions cause the Moon to recede from Earth by about 4 centimeters per year. In many billions of years, the center of mass will reside in outer space. Does this mean that future astronomers will suddenly have to change the Moon’s status from satellite to planet? This flies in the face of common sense. Given that Pluto is 7 times more massive than Charon (which means the system’s center of gravity is much closer to Pluto), a lot of folks will think it’s patently obvious that Charon should be considered a satellite rather than a planet.
For those who bemoan this new definition because it includes Pluto, I ask the following: what should we do when astronomers find a body, either in the outer solar system or around another star, that is halfway or two-thirds of the way between Pluto and Mercury in size? Such a discovery is just a matter of time. As I wrote in my essay last Thursday, wherever one draws the line that distinguishes planets from non-planets, it will be arbitrary.
And for astronomers who want to divide planets into various subclasses, like giant planets, terrestrial planets, and ice dwarfs (and the proposed definition recognizes Pluto-like objects as “plutons”), Mother Nature will always create objects that don’t fit cleanly into the categories. We already know of several extrasolar planets that would not fit into any classification scheme based on our solar system, and there are tens of billions of planets in our galaxy alone. These planets are going to display a bewildering variety of sizes, masses, orbits, physical characteristics, and so forth. And even if you count all the known round asteroids and Kuiper Belt objects in the solar system, we still know of many more planets outside our solar system than within it. Any sensible definition of “planet” must take them into account.
So despite all the arguments I have heard over the past few days, my position remains unchanged. The simplest way to define “planet” is to use Pluto as the minimum size of a planet, and state that any body found orbiting a star (or brown dwarf!) the size of Pluto or larger is a planet. And despite the fact that astronomy does not operate in a cultural vacuum, my conclusion is not based on sentiment or history, or the desire to prevent kids from having to memorize the names of dozens of planets. It’s based on the fact that Pluto has many characteristics that we commonly associate with planets: a respectable diameter that’s well above the minimum size to be spherical, an atmosphere, a multiplicity of moons, and probably rings. I freely admit that my definition is arbitrary, but I challenge anyone to come up with a less-arbitrary scheme.
Finally, most of the public’s attention has been focused on the low end of the planetary size regime. But the controversy extends to the upper end as well, and the proposed definition fails to address this problem in a satisfying manner. Basically, this issue boils down to the question of how astronomers should draw the line between planets and brown dwarfs. Currently, objects between about 13 and 75 Jupiter masses are generally considered brown dwarfs, because they briefly fuse deuterium in their cores (anything above 75 Jupiters is a star). But there are many gray areas. Should it matter whether an object orbits a star or another brown dwarf, or how it formed?
For example, Geoff Marcy and Paul Butler’s group found a star that has 7- and 17-Jupiter mass objects that are coplanar, meaning they probably formed in a disk. Should the 17-Jupiter-mass object be considered a planet because of its origin, or since it's above the deuterium-fusion threshold, should it be termed a brown dwarf? What about the dozens of known free-floating objects (not bound to stars) that have less than 13 Jupiter masses? Should we call them planets because of their low mass, do we call them sub-brown dwarfs since they probably formed in stellar-like processes, or do we have to adopt the horrible acronym PMOs or planemos, short for “planetary-mass objects”? What about the 5-Jupiter-mass object that orbits at a very far distance from the 25-Jupiter-mass brown dwarf 2M 1207? That system probably formed like a very-low-mass binary star, but the 5-Jupiter-mass bugger is well below the deuterium threshold. The proposed planet definition either fails to clarify many of these ambiguities, or it leaves us with unpleasant outcomes.
I want to make it clear that I don’t have a strong disagreement with the proposed definition, but it’s an imperfect solution to a complex problem. Don’t be surprised if there are modifications down the road.
A New Type of Cosmic Explosion
August 15, 2006 I want to thank everyone who has responded to my postings. I’m new at this game, and I’m very pleased to see people writing to the blog with such interesting and insightful comments. Writing magazine articles is essentially a one-way street, with the flow of information and interpretation going from the author to the reader. I really enjoy this blog format, since I can express myself more freely and less formally, but more important, I am receiving quick and valuable feedback from readers.
My original plan for today’s blog installment was to discuss your Pluto comments, and point out that the committee’s proposed definition of a “planet” will be released to the public tomorrow morning. I have seen the press release announcing the definition, but it is embargoed by the International Astronomical Union until 2:00am EDT Wednesday, so I’m not supposed to say anything about it. I can tell you, however, that Pluto aficionados will probably be happy. But check our Web site tomorrow morning for more details.
My plans for today’s posting changed when I saw the new papers posted on Astro-ph last night. Astro-ph is an Internet site where astronomers post their research papers before they actually appear in printed journals. This Web site is a great way for astronomers to disseminate their work quickly to the entire professional community. Last night, I saw a paper that totally blew me away, and it’s so exciting that I want to tell you about it. It suggests that there is a new type of mega-explosion process operating in our universe. Scientifically, this is much, much, much more important than the Pluto/planet debate.
The paper, written by a large group of distinguished astronomers with Avishay Gal-Yam of Caltech as lead author, reports the detection and subsequent observation of a relatively nearby gamma-ray burst (GRB) on June 14, 2006. GRBs are extraordinarily violent and bizarre events. For those of you who have been following the GRB story, including my feature in the August 2006 S&T, you probably recall that GRBs come in two classes: long-duration bursts (lasting several seconds to several minutes) that come from exploding massive stars, and short bursts (lasting a fraction of a second to perhaps 2 or 3 seconds) that originate from a variety of processes, including merging neutron stars. As my August 2006 article explains, astronomers have accumulated compelling evidence that long bursts are powered by massive stars collapsing to form black holes or neutron stars, and in the process they generate high-speed jets that punch through the dying star. Colliding blobs of material within these jets probably trigger the actual burst of gamma rays.
One of the key pieces of evidence in favor of this scenario is the fact that whenever astronomers have pinpointed the location of a long GRB relatively close to Earth, astronomers have always seen a supernova. This association unambiguously links long GRBs to the explosion of massive stars, in line with theoretical predictions. But in this new paper, Gal-Yam and his colleagues report that the relatively nearby long GRB detected by NASA’s Swift satellite on June14th is not associated with a supernova. In other words, the GRB was close enough to Earth (about 1.7 billion light-years) that a supernova should have been visible. Yet when astronomers looked for a supernova with the Hubble Space Telescope and other instruments, they only saw evidence for an afterglow — the interaction of the jets with the surrounding interstellar gas. Either there was no supernova, or it was too faint to be detected, which means it was the lowest-luminosity supernova on record.
Another group, led by Johan Fynbo (University of Copenhagen, Denmark), is just about to come out with a paper reporting a second long GRB without an underlying supernova. This burst was detected by Swift on May 5, 2006. Fynbo's group also failed to find any evidence of a supernova accompanying the June 14th burst.
Theorists are just coming to grips with these discoveries, and it might take months or years for a consensus to emerge. Right now, my speculation as an armchair theorist is that astronomers might have seen two events in which the core of a dying star collapsed to form a black hole, and a supernova either failed to occur or was smothered by inrushing stellar material. Many theorists have predicted that such “failed supernovae” should exist.
But it’s also possible that there is a range of supernova luminosities, and these two events just happen to fall at the low end. In this scenario, the collapsing stellar core initially formed a neutron star, but it later accreted enough infalling stellar gas to form a black hole. Computer models suggest that the subsequent explosion will fail to eject large quantities of nickel-56, the radioactive isotope that is responsible for most of a supernova's early luminosity.
A third possibility is that these events are an extreme version of short bursts, which are not associated with supernovae. But that raises the question of why these bursts lasted so long — about 2 minutes for the June 14th GRB — which is much longer than other short GRBs. Perhaps we’re seeing a powerful and previously unknown explosion process.
Right now, astronomers have more questions than answers. Just when scientists thought they were beginning to understand GRBs, Mother Nature found a way to stay one step ahead of the curve. No doubt about it, the planet controversy will dominate the media spotlight over the next few days. But ultimately, these GRB observations will tell us a lot more about how the universe works than what a committee of astronomers decides to call Pluto.
Pluto: 9 + 1 = 10, not 8
August 10, 2006 I grew up in Hershey, Pennsylvania, just a few miles downwind from Three Mile Island. When I was studying math in the 1970s, Hershey was not known for the quality of its public schools. The local school board seemed to think that the kids needed to know just enough to work in the chocolate factory. But when I learned that 9 + 1 = 10, I was apparently taught correctly. I confirmed this fact when I went to college.
Yet amazingly, some of the most learned professional astronomers, and even some of my esteemed S&T colleagues, think that 9 + 1 = 8. They argue that because of last summer's announcement of a new Kuiper Belt object (KBO) larger than Pluto (2003 UB313), the solar system now has 8 major planets. In other words, astronomers have been saying since 1930 that the solar system has 9 planets, then they find another planet, then all of a sudden we have 8 planets. Am I missing something here?
The campaign to drum Pluto out of the category of "major planets" has actually been gathering steam since the early 1990s, when David Jewitt and Jane Luu discovered the first KBO besides Pluto and its moon Charon. The argument goes something like this: With a diameter of just 2,300 kilometers (1,400 miles), Pluto is just too much of a pipsqueak to deserve membership in the exclusive club of major planets. It's merely the largest known member of the Kuiper Belt, several of whose members are half to two-thirds Pluto's diameter. If we count Pluto as a major planet, we should also count all of these other KBOs, and pretty soon we’ll have more planets than Elvis impersonators.
Admittedly, the critics' contentions have considerable merit. Until last year, Pluto really was just the largest known member of the Kuiper Belt. I also have to concede their point that if Pluto had been discovered in 2006 instead of 1930, few astronomers would call it a major planet.
But that's not the whole story.
For every argument against Pluto being a planet, Pluto's defenders can summon an equally legitimate counterargument. When critics say Pluto is small, defenders can point out that it's easily big enough so that gravity can pull it into a sphere, and that astronomers from Jupiter might consider Earth to be an overgrown asteroid. When critics say Pluto has a highly elongated orbit, defenders can point out that many of the 200 known extrasolar planets have even more eccentric orbits. When critics say Pluto shares its region of space with zillions of other KBOs, defenders can point out that Earth shares its orbit with thousands of asteroids (and even mighty Jupiter and Neptune share their orbits with swarms of Trojan asteroids).
The bottom line is that Pluto is big enough to be round, it has an atmosphere for at least part of its orbit, and it has at least three satellites. It might even have rings. While asteroids and moons share some of these characteristics, we don't know of any that have the whole kit and kaboodle. And since these are all traits one normally associates with "planets," it's not at all obvious that Pluto should be booted from the realm of planethood.
What is clear is that because Mother Nature makes objects in a continuum of sizes, wherever you draw the line that distinguishes a major planet from a minor planet, your boundary will be arbitrary. Astronomers have been calling Pluto a major planet since 1930, and they haven't been saying that just to kids, they've been saying it to each other in their books and research papers. The simplest thing to do is to simply grandfather Pluto into the planet club, and set its 2,300-km diameter as the minimum size for a major planet.
The debate has festered because there is no official definition of what constitutes a "planet." For years, the organization that resolves these nomenclature matters, the International Astronomical Union (IAU), has sidestepped the controversy. But Mike Brown's announcement last summer of 2003 UB313 forced the issue. The IAU has to give this object an official name, and that name will depend on whether it's a major planet, or just another KBO. And since 2003 UB313 is only slightly larger than Pluto, once the IAU decides its official status, it will resolve Pluto's as well.
The IAU has appointed a committee, chaired by the eminent astronomy historian Owen Gingerich, to come up with a definition of "planet." Gingerich's committee, which includes members on all sides of the Pluto debate, has reached a consensus after a period of intense discussion, and National Public Radio reported today that the definition will include Pluto. However, the report was based on what 5 committee members thought before the committee even met, and S&T has learned that some of the "facts" that NPR reported are wrong — but we don't know which ones! According to the NPR report, the definition will establish several classes of planets, such as terrestrial planets (Mercury, Venus, Earth, and Mars), giant planets (Jupiter, Saturn, Uranus, and Neptune), and dwarf planets (Pluto and the larger KBOs, and perhaps several main-belt asteroids). In other words, if the IAU adopts this classification scheme, we might have dozens of objects that fall under the term "planets."
This scheme makes sense scientifically in terms of our solar system, but I predict that as we learn more about planets around other stars, we'll find plenty that don't fit neatly into these categories. The history of science tells us time and time again that Mother Nature refuses to conform to human expectations and nomenclature systems. I also predict that a lot of people in the general public would find such a classification scheme confusing and dissatisfying.
The committee will present its definition at the IAU's General Assembly in Prague next week, and a vote by the full membership is expected around August 25th. S&T editor in chief Rick Fienberg will be at the conference to cover this unfolding story, and SkyandTelescope.com will keep you posted with accurate and authoritative coverage. I have great confidence that whatever it decides, the IAU will eventually come to a sensible conclusion that will lay the controversy to rest. While I enjoy discussing the Pluto debate, ultimately the science is more important than the semantics, and it's time to move on. When it comes to the solar system, the world needs to know whether 9 + 1 = 10, or whether we need to learn New Math.
Greetings Astronomy Aficionados!
August 8, 2006 I am the senior editor at Sky & Telescope with the last name that nobody can pronounce (it's NOY-uh, like "paranoia"). I've been working at S&T for nearly 3 years, although I also did a 6-month internship at the magazine back in 1991, while attending the science journalism graduate program at Boston University. I later worked on the editorial staffs of Discover and Astronomy magazines, and I served as editor of Mercury magazine for three years.
My job at S&T is to write and edit science articles, both news stories and features. I previously authored two books on astronomy and have won several awards for my astronomy journalism and outreach activities. I am the proud owner of 5 telescopes, although my amateur activities are limited to eyeball-to-eyepiece astronomy.
My interests in astronomy are diverse. I've covered stories ranging from planetary science to cosmology, and everything in between. Since I started my career in science journalism in the early 1990s, I have written about topics such as extrasolar planets, the various missions to Mars and the other planets, the Kuiper Belt, the shocking realization that our universe's expansion is accelerating, and the breakthroughs in our understanding of gamma-ray bursts (check out my feature article on GRBs in the August 2006 S&T). If it's above our atmosphere, chances are that it's interesting. The really boring and depressing things seem to be here on Earth.
My main goal with this blog is to inform you of interesting new developments in astronomy and space, particularly stories that the mainstream press ignores. I'll also help you separate truth from fiction, and reality from myth. I will highlight the truly important results and provide an antidote for the bogus and overhyped claims that permeate Cyberspace.
For example, a particularly egregious example occurred yesterday with all the articles claiming that the distance scale of the universe might have to be revised because of one group's measurement of the distance to the galaxy M33. Many groups of distinguished astronomers working with independent techniques over many years have painstakingly established the extragalactic distance scale, and they have come up with consistent, reliable results. To convey the impression that all of this work might have to be overthrown because of one group's distance measurement to a single galaxy is an example of the irresponsible journalism that is all too common these days.
Besides highlighting good science while debunking the bad, I'll discuss interesting and controversial subjects, such as NASA funding, the possibility of extraterrestrial life and alien visitations, sending humans to the Moon and Mars, alternative cosmology, "intelligent design," and so on. I've been covering this stuff for years, and I care deeply about it. I'll probably ruffle a few feathers, but I won't be bashful about my opinions. When you write back, don't be bashful about yours. If there's something you want me to write about, let me know. If you agree with me, let me know. If you disagree, let me know (but also tell me why). This blog is a two-way street, and I want it to be fun and stimulating. I'll be back on Thursday to weigh in on the issue of whether Pluto should be considered a planet, and I'll explain why a resolution to this debate may soon be at hand.