Here are notes from the "What's in Your Planet" presentation at the American Astronomical Society's summer meeting in Pasadena, Calif., on June 9, 2009, based primarily on Twitter postings:
OK, we're getting ready for the forum on planethood (and #pluto) at the Am. Astronomical Society meeting.
Steve Maran of the #aas calls the speakers the "seven samurai" ... ranging from Neil Tyson to Alan Stern. More details during introductions.
Bill Nye (the Science Guy) comes up from the audience, shakes hands with the speakers.
Steve Maran notes that last year's Great Planet Debate came to the conclusion that the "existing definition was not very good."
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Neil deGrasse Tyson, host of "Nova ScienceNow," director of Hayden Planetarium, author of "Pluto Files" is first speaker.
Tyson reviews history of Pluto, named by English girl ... in America at the time, Pluto Water was known as a powerful laxative.
Initially, Pluto was thought to be big planet (size of Neptune); over decades, size shrunk.
A 1978 paper, published jokingly in Eos, noted shrinkage and projected that Pluto would disappear completely in 1983.
Tyson: "I prefer to think of Pluto not as the ninth planet, but as the first object in a new swath of real estate."
Tyson joshes Planetary Scientist Institute's Mark Sykes (Plutophile) with picture of Sykes "wrestling" with Tyson.
By the way, Tyson was on the wrestling team in high school, so I wouldn't advise trying to take him on.
Tyson recaps IAU vote and the ensuing controversy.
We're revisiting the jokes, the cartoons and the Calif. Assembly bill dissing "mean-spirited" astronomers & sticking up for Pluto.
Tyson says "clearing out orbit" is definable, cites analysis showing Pluto, etc., are far less able to scatter objects in same orbit.
Tyson also cites quotes from Herschel and Frederick Leonard questioning whether members of groups of objects can be seen as planets.
Tyson says it's trivial to continue debating planethood definition because "there's too much else to talk about."
Tyson touts scads of ways to classify objects, by gravity, density, shape, composition, chemistry, magnetic field, etc.
Tyson: "This is what we should be thinking about now, not arguing over the fricking definition of a planet."
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Now comes Alan Stern, principal investigator for NASA's New Horizons mission to Pluto, co-author of "Pluto and Charon."
Stern says objects can be classified by location or by attributes. "I'm not much for location, that's for Realtors."
Stern says that the issue of planets is of paramount issue to planetary scientists like him.
Planet definition should capture "essence of what it means to be the central object in planetary science," Stern says.
"It's not that the dwarf planets are the misfits, it's the four terrestrial planets and four giant planets that are misfits."
Stern notes that there may be as many as 1,000 dwarf planets. They formed like other planets, have characteristics of other planets.
Stern says the only thing that distinguishes them as attributes is their size.
IAU resolution recognized the significance of the attribute that self-gravity crushes an object into a round shape.
"Clearing the neighborhood is broadly recognized as flawed concept," Stern says. It says nothing abt. nature of object.
Stern says formula for "clearing" is dependent on age of star, distance of orbit; reordering objects would change classification.
Stern says any test that does not qualify Earth as a planet in any circumstance won't pass muster.
Stern lays out his "Star Trek test" ... Kirk and Spock should be able to look out the window and know what a planet is.
Stern urges simpler definition: big enough to crush itself into equilibrium shape (round) but not big enough to spark fusion (star).
Stern calls this GPD (geophysical planet definition). Would be easier to teach. Our solar sys. would have about 30 planets.
Stern sees an extension of Copernican revolution, with "our" type of planet in the minority. "Somewhere, Copernicus is smiling."
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Chas Beichman, NASA ExoPlanet Science Institute researcher, says he'll try to stay out of the crossfire.
How define planet? By mass? Location? Method of formation? Must a definition have observable consequences to have meaning?
Beichman outlines mass definitions for stars (>70 Jupiter masses), brown dwarfs (13-70 Mj) ... and planets? (<13 Mj)
Is a very low-mass, free-floating object a sub-brown dwarf or a planet? Beichman turns to exoplanet search.
Exoplanets show a lot of diversity ...
Some planets have been found way far away from host star, beyond 10AU ... that's beyond Saturn's orbit.
It's hard to make planets through traditional core-accretion method that far out. So maybe core-accretion definition is problematic.
If many young gas giants are found far out from host stars, that means there would have to be whole new mechanism for planet-building.
Beichman: there's a difficult classification ahead, and we shouldn't ask IAU or AAS to solve it... instead, ask Supreme Court...
Planethood may turn out to be like porn, as defined by Supreme Court: "I know it when I see it."
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Univ. of Ariz.'s Renu Malhotra on planetary migration ... lessons from Pluto.
Malhotra says Pluto is like a loose end that you pick on until a whole garment unravels.
Malhotra: "Pluto being an outsider was the beginning of a revolution."
Malhotra notes the 3:2 ratio between Pluto's and Neptune's orbital period that keeps Pluto absolutely safe from collision.
Malhotra says that worlds like Pluto could've been "phase locked" in the 3:2 orbit, just as pendulum clocks experience phase locking.
This "phase locking" would have happened as Neptune migrated outward; data indicate that's just what happened.
Bottom line? "The planets did not form where we find them today" ... which suggests we might have to rethink planet formation process.
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Now comes Jean-Luc Margot (UCLA), who proposes another word for planets, dwarfs, satellites and free-floaters: "WORLDS."
If you move Earth into Jupiter's orbit, or 150 AU or 1500 AU, dynamical definition changes. Geophysical definition doesn't.
But that's not unique. If you think of magma vs. lava, meteoroid vs. meteorite ... they're the same objects but in different contexts.
Planetary taxonomy can be based on dynamics, the taxonomy for "worlds" can be based on geophysics (roundness).
MargoT speaks up for the "clearing orbit" formula ... Lambda = Mass ^2 X Period. That provides clear line, as Tyson noted.
Margot says roundness definition is problematic, cuz there's big uncertainty due to density.
Margot notes that Saturnian moon Mimas is smaller than asteroid Vesta, yet Mimas is round and Vesta is not.
Margot also notes that all dynamically dominant bodies are round, but the reverse is not true.
Thus, Margot recommends that IAU adopt Lambda threshold for "clearing orbit," and dropping roundness requirement for planet.
Margot says round things, ranging from Mimas to the giant planets, should be called worlds. "Some worlds are planets, others are not."
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Now comes Mark Sykes, director of Planetary Science Institute, member of Dawn science team (Ceres mission), lawyer and sometime opera singer.
Sykes wants to get away from "dots floating in the sky," notes that geophysical processes occur on round things.
Gets into water activity and its relation to habitability. If there are subsurface oceans on dwarf planets, as some think, those oceans may be livable. Interesting counterexample: Water activity in honey is too low to sustain life, and that's why honey doesn't go bad.
Sykes talks about Dawn and New Horizons missions (to Ceres and Pluto, respectively). He talks up Ceres (perhaps in part because he's on the science team for that mission). Ceres is "the smallest planet," he says.
If you look at planetary characteristics, such as potential for life, geology and atmosphere, you can construct a Venn diagram with Earth, Mars, and perhaps Ceres and Pluto in the center patch. Asteroids, irregular trans-Neptunian objects and comets are completely outside the Venn diagram. The three circles of the Venn diagram fall into the planet pigeonhole.
We don't need to come up with a different term, Sykes says. Geophysical def. is easier to teach, easier to understand. Dynamically dominant planets are a logical subset, but the reverse is not true. It logically groups objects together that motivate a broader range of common scientific investigations. It is more useful. So saith Sykes.
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Now comes the final speaker of the "seven samurai": David Weintraub of Vanderbilt University, author of "Is Pluto a Planet?"
Weintraub listened to all speakers, thought all of them were really smart. But he has a [historical] perspective that says perhaps none of us are as smart as we think we are. He goes back to 1780, just before discovery of Uranus. At that time we were pretty sure we knew what planets were. They were objects that orbited the sun. Also had to define them by what they were not: comets, for example.
1781: Uranus discovered. Orbits sun, not a comet, must be a planet.
Then a mysterious piece of math came into play: A planet had to follow a distance rule called the Titius-Bode rule. The rule fit Uranus and also predicted a trans-Mars planet. In 1800, collaboration set out to find Trans-Mars, but someone else, Giovanni Piazzi, discovered Ceres. That fit the Titius-Bode rule. Thus, the Titius-Bode rule was accepted as a condition of planethood. "We, of course, were wrong."
Then Herschel complained that Ceres was too small and faint. In effect, he was adding another "what they are not" criterion. They shouldn't be too small or faint. Then, at 1802, Pallas was found, close to Ceres. Herschel suggested the name "asteroid" for these objects and suggested there might be more of them.
Over the rest of the 19th century, the asteroid belt became more populated. Now we know what an asteroid is, but we don't know what a planet is.
Then, in 1846, Neptune is found. It didn't follow Titius-Bode rule, so the rule was tossed out.
In 1850, planet defined as something that orbits the sun, is not a comet, is not too small (?), and is not part of asteroid belt (?).
Now, looking back, why were Uranus and Neptune accepted as planets? Just because they were big and bright and not asteroids? "I think astronomers at the time should have thought a little bit differently." Really, they didn't fit: Completely different ... much bigger than terrestrial planets, much smaller than Jupiter or Saturn, much farther than other planets. Maybe they should have been considered "icy outer-solar-system objects."
"They were really very different objects from any other objects known at the time."
There's an analogy to the current debate over Pluto and the other dwarfs. The current rethinking is "really a very appropriate thing for us to be doing."
Weintraub says he'd like to have a sensible planet definition, "but I'm not sure we're anywhere close to being able to do that."
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BEGINNING OF QUESTIONS:
QUESTION: Is it important that Pluto has been accepted as ninth planet? Shouldn't we just grandfather Pluto in?
SYKES: Now kids are learning that there are 8 planets. And they're not learning about Pluto or dwarfs or asteroids. In fact, the kids (wrongly) think that Pluto is like an asteroid.
STERN: Most important thing is that we have a scientific system that reacts to new data. Number of planets is uncountable, from his perspective.
TYSON: Hidden travesty here. People think that there's some significance to knowing a number of planets. I get letters now from kids who never knew they had nine planets. They don't have angst about it as some adults might.
BEICHMAN: I'm all for teaching that there's 359 planets.
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QUESTION: Difference-in-scattering parameter is telling us something physical.
STERN: It's telling you something important to dynamicists, but it doesn't tell you anything about their intrinsic attributes.
SYKES: Number is important to dynamics crowd. We are in a paradigm shift, from geophysical perspective, we're shaking off the last of the mythological aspect of planets ... planets as special objects.
MARGOT: Dynamical dominance tells us something fundamental, probably related to the formation of planets.
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QUESTION: Observation, not really question. Particular definition is not practical ...
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QUESTION: Dwarf stars are stars, but dwarf planets are not planets? If you have a modifier to the word "planet," wouldn't you call it a planet?
MARGOT: We've had "minor planets" [aka asteroids] for a long time, and no one has had a problem with that.
MALHOTRA: We have neutron stars which are not stars. The problem of dwarf planets not being planets is a non-problem.
TYSON; This calls for much more nuanced repertoire of modifiers.
WEINTRAUB: The adjectives are very important, but we still need to know what the noun means.
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QUESTION FROM BILL NYE: What about the word "plutoid," and what about "trans-Neptunian" vs. "ultra-Neptunian."
TYSON: What might work is if IAU brought in a Latin language expert. (Nye: Trans = across ... ultra = beyond) A bit of contretemps over Latin here.
MARGOT: Some objects cross Neptunian orbit (Nye: "I'm diggin' that.")
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QUESTION: What about moon? Is Earth-moon a double-planet system?
STERN: Of course. If stars can orbit stars, planets can orbit planets. Someday we could look upon Jupiter, for example, as a multiple-planet system.
TYSON: It'd be interesting if you had a distinction for co-orbiting objects where the center of mass is outside the objects. And about location, location, location, there are many objects that tell you about location. E.g., a cloud that is on the ground is called fog, and you'd like to know that if you're going to drive through it. A droplet of water on a leaf is called dew. We should account for nuances in planetary science.
SYKES: IAU definition counts on there not being embarrassing discoveries (e.g., object bigger than Mercury on solar system's edge). People would rebel about an object the size of Earth not being considered planet.
STERN: "Andy Griffith Show": Barney Fife considered so dangerous he was given just one bullet. He cites example of IAU's definition of north. "I think the IAU has used its one bullet and should stay away from this stuff." Plutoid good example of IAU making things worse.
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QUESTION: Why not use term "dom" planet? (for dominant). Why don't we make distinctions based on how bright the planets are as seen from Earth?
WEINTRAUB: Our perspective of what we can see from Earth would not be a good criterion for deciding what to call something.
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QUESTION FROM JAY PASACHOFF: No real definitions of clearing orbit, don't even know what round is (Earth is ellipsoid). Why having this discussion? Because of IAU administration, naming of Mike Brown's new objects had to go to some committee. We didn't really have to define planets, we just had to define committees. We could have just combined committees. This is premature, we don't really need a definition and we should discover a few thousand more planets. Is anyone going to bring a formal resolution to IAU in August to change the definition.
STERN: I think they should quit while they're behind.
WEINTRAUB: Decision should be made by people in the trenches, not by the IAU.
MARGOT: I don't think it was about the naming. It was similar to the asteroid issue in the 1800s. KBOs just reached a point where something had to be done.
STERN: I have a hard time excluding objects based not on what they are but what they're with. "To a guy with a hammer everything looks like a nail." Dynamicists see everything in dynamical terms.
SYKES: IAU process motivated because of the way the committees' deliberations turned out. First committee was deadlocked, second committee raised such ire that a definition was improvised hastily. "Really the brouhaha at the iau was much more a political or religious debacle than any kind of measured scientific discussion."
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QUESTION FROM INTERNET: In light of all adjectives, is there is a specific type of planet that could harbor life?
BEICHMAN: Lot of time spent trying to quantify what is a habitable planet. Foresees "food fight" over that kind of definition. "I think the food fights are just beginning, and that's what makes the science so exciting."
TYSON: Idea of "Goldilocks zone" is not that useful anymore, now that we're finding tidal stresses could create habitable environment far from sun. "We're looking with a bigger and bigger net than we ever thought we'd use."
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QUESTION: Any thought about asking public what they thought about planethood?
TYSON: Public was not outside the debate. That's what sparked the IAU action, because it started looking like the astronomers didn't know what the hell they were doing.
STERN: We're living in an open-source world now.
STEVE MARAN: Sees analog to requirement for tax on fish oil. "There is a retirement community in N.M., where if you buy in you have to agree that Pluto is a planet." [I think he's referring to a story I wrote, which was basically a developer's joke!]
SYKES: What is useful will survive.
MARGOT: I would encourage people who disagree to take part in the process.
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QUESTION: What is cutoff between brown dwarfs and planets?
BEICHMAN: Brown dwarf classification runs from 13 to 75 Jupiter masses. Where deuterium fusion can be sustained.
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QUESTION: How does discovery of hot Jupiters affect our view?
MALHOTRA: It was a surprise to find these planets. Prevailing view is that these planets did not form there but migrated inward. It certainly has changed our views about how planets formed. They're quite mobile after formation.
STERN: Discovery has widened our perspective. "Nature got us beat." No one predicted pulsar planets, or the prevalence of subsurface oceans on other planets. "Nature has done what nature will do, and we are always catching up ... You should fasten your seatbelt."
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Steve Maran mentions that he's co-written a book titled "Pluto Confidential."
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Now here are notes from Caltech astronomer Mike Brown's AAS presentation on June 10, 2009, adapted from Twitter updates:
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I'm sitting in on the talk by dwarf-planet finder Mike Brown of Caltech.
Mike Brown says he won't be talking about whether Pluto should or shouldn't be a planet.
Brown lists eight biggest transNeptunian objects: Eris, Pluto, Makemake, Haumea, Sedna, Orcus, Quaoar and Snow White (not yet ofcly named).
Planet-hunter Brown will focus today on Sedna (way far out ice world) and Snow White (most recently found big trans-Neptunian object)
Worlds can be scattered out of solar system by giant planets, but to bend orbit back, you need another interaction, say with a passing star
Sedna has a 12,000-year orbit, a track that can't be affected by giant planets or passing stars. How did it get there???
Was Sedna just formed there? No, you need things to start in circular orbits. Scattered by Planet X? No, nothing seen. "Very sad" about that
The answer Brown likes most: "Sedna is really the signature of a dense stellar birth environment."
Sedna's existence suggests that the sun formed in a dense star cluster, with Sedna serving as the "fossil evidence."
Cluster birth hypothesis could be nailed down if more Sednas are found. That's what Brown and colleagues have been looking for.
Now they're "redoing the sky" with new survey, looking for things that move over the course of 24 hours. That extends range to 1500 AU.
They found about 40 things, but Sedna was the farthest-out thing they saw. The second-farthest-out thing was Snow White.
Snow White is at about its farthest part of its orbit, so it's not like Sedna.
Statistical analysis suggests that there are about 40 things on the edge of the solar system as big as Sedna or larger.
The analysis suggests that there's something out there *much* larger (though perhaps intrinsically dimmer) than Sedna.
Hundreds of Kuiper Belt objects found with the Subaru Telescope, but still not enough data to resolve the cluster-birth question.
Brown believe Sedna is a "lighthouse" in the far solar system, just as Pluto was a lighthouse for discoveries in Kuiper Belt.
Now Brown turns to Haumea, the football-shaped dwarf planet. It spins rapidly, and is as wide as Pluto in one dimension.
If "planet" were defined as the width of Pluto, Brown says he would have argued for Haumea "now ... now .... now." (as it spins on screen)
Brown also talks about the collision that probably set Haumea spinning and struck off its moons.
By running the calculations for how Haumea's orbit has changed, Brown found that the orbit became more eccentric, just as observed!
Brown says "we're very excited" about Hubble's new Wide Field Camera 3, which will look for more faint objects from Haumea family.
Brown says Haumea's satellites are the most strongly interacting moons in the solar system. Little Namaka may be ejected in 100 years (?)
Because of those interactions, Brown and team should be able to spot Namaka transits over Haumea. "Maybe one" has been spotted, he says.
Brown says "we have another seven years of trying" to catch Haumea/Namaka transits, which should help piece together what happened to Haumea
Computer modeling could put Haumea back together again. "We should have named it Humpty-Dumpty," Brown jokes.
Many more sky surveys ahead, which should turn up more about the edge of the solar system. Now there's time for a couple of questions.
Brown is asked about idea of finding far solar-system objects by looking for transits over more distant stars, called "blind occultations."
Space-based transit surveys could be "fantastic" for spotting far, dim objects. Ground-based surveys, not so much.
Brown says it's a mystery how the Haumea collision could have occurred, due to the small (10^-7) probability of that collision.
OK, I asked Mike Brown about a few things that may clear up some of the things I wrote about his dwarf-planet presentation.
Haumea's smaller moon, Namaka, would have been scattered in 100 years based on an orbital projection that is now known 2 b wrong.
Hubble's WFC3 will look at Kuiper Belt objects for which spectral signatures are unknown, in hopes of finding the Haumea signature.
Brown also said Hubble's revived STIS instrument will be used to watch for a particular transit involving Haumea's 2 moons. Stay tuned
Brown agreed that Hubble is a big deal for his far-solar-system observations. "It's the coolest one out there," he said.