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| Artist drawing of Eris and Dysnomia. Remember, the camera adds ten pounds. Copyright Robert Hurt, IPAC |
If you’re in the "Pluto is a planet" crowd, then you might consider selling all your 9th planet merchandise on eBay while you still can: astronomers have found that Eris is more massive than Pluto.
Eris was discovered a few years back, and observations indicated it might be bigger (that is, have a larger diameter) than Pluto. This is pretty hard to do, because it didn’t look like much more than a dot in telescopes; the diameter had to be inferred by its known distance and its brightness. If it’s made of something dark (like organic chemicals, common on distant objects), it must be big to look as bright as it does; if it’s made of something reflective (like snow or ice) then it’s smaller. Subsequent Hubble observations indicated it was indeed bigger than Pluto, and the former ninth planet took one more body blow.
Now it’s known that not only is Eris bigger, it’s more massive. About 30% more massive, in fact.
The mass is derived by observing the orbit of its moon Dysnomia (remember when they were called Xena and Gabrielle? Man, I’m glad they changed the names). By seeing how long it takes the moon to orbit Eris, the mass of Eris can be found. Mike Brown from Caltech (that evul librul who discovered Eris), and his grad student Emily Schaller, determined Eris to have a mass of 1.66 x 1022 kilograms. Pluto’s mass is 1.27 x 1022 kilograms.
Eris wins.
(Not that it’s all that big in the first place: Earth, for comparison, is 6×1024 kg, about 200 times more massive than both Pluto and Eris combined.)
First off, let me comment on how cool it is that we can determine the mass of an object that is currently 14 billion kilometers away. That’s amazing.
Second, of course, this news will probably mildly rekindle the "Is Pluto a planet?" debate. As I have said many times, there is only one answer to this: it doesn’t matter. Why? Because the word "planet" is ill-defined; the group of astronomers who tried to define it last year did an OK job, but the scientific definition left the public rather cold. People want to define the word viscerally, emotionally… I might even use the word unreasonably. In the case of the public, most people want Pluto to be a planet, and won’t be open to all the reasons why it shouldn’t be.
And in the end, we’re arguing over semantics. Pluto doesn’t care whether it’s a planet or not. Worse, lumping it into a category where it might not fit — and one with arbitrary boundaries — may make it easier to miss important facts about it. It’s a sort of mental illusion. If you think of it as a planet, you might miss a bit of data about it that fits better with Pluto being a big Kuiper Belt Object, or comet nucleus, or some other weird bit of flotsam.
Words have impact. They shape our thoughts.
Call it a planet if you want. It doesn’t matter that much to me — and less to Pluto itself — but you’ll be doing the object, and yourself, a disservice. And Eris is bigger anyway. Nyah nyah.
June 14th, 2007 at 10:58 am
Fnord!
June 14th, 2007 at 11:05 am
“Planet” is so vague. It’s mostly historical and emotional.
I don’t care if Pluto is tiny or not a planet anymore. I’m still very excited over New Horizons going to Pluto. I wondered all my life what this place looked like and I’ll know in a few years! But what saddens me is that a few people I know told me that they don’t care anymore because Pluto isn’t a planet.
Sad sad sad. It’s still the same object.
June 14th, 2007 at 11:10 am
Huzzah for the 10th planet!
June 14th, 2007 at 11:11 am
Now we’ll have to come up with a more current mnemonic than Stephen Colbert’s “My Very Educated Mother Just Said ‘Uh-oh! No Pluto!’.”
June 14th, 2007 at 11:25 am
Timothy:
“My Very Educated Mother Just Said ‘Uh-oh! No Pluto!’ Egads!”
June 14th, 2007 at 11:25 am
Yeah, but that’s gonna be pretty hard to beat on the humor scale…
June 14th, 2007 at 11:38 am
Hmmmph. I liked Xena/Gabrielle.
June 14th, 2007 at 11:57 am
Its’ not the SIZE that counts
at least that’s what I keep telling my girlfriend
J/P=?
June 14th, 2007 at 12:15 pm
You could take the Canadian pneumonic:
“My Very Educated Mother Just Said ‘Uh-oh! No Pluto, eh!’â€
June 14th, 2007 at 12:16 pm
Pity, I so liked this graphic:
http://mathiaspedersen.com/3dportfolio_poor_pluto.html
Maybe the artist can add Eris & Disnomia giving conspiratorial glances from the background.
And just for the record, I’m with Jessica — they should have stuck with Xena & Gabrielle.
June 14th, 2007 at 12:23 pm
My favorite term for Pluto (and also for Eris, Quaoarr, etc) is ‘dwarf planet’ .
June 14th, 2007 at 12:33 pm
> First off, let me comment on how cool it is that we can determine the mass
> of an object that is currently 14 billion kilometers away. That’s amazing.
What is (in my opinion) even more amazing is that this determination is made using laws of physics that were figured out three hundred years ago. The only thing that’s new for Eris & Dysnomia is having the means to collect the input data accurately.
June 14th, 2007 at 1:18 pm
And for all the highschool physics teachers (like me) out there. This makes a great test problem, give the students radius and period for the orbit and they can calculate the mass.
I would love to have two pictures with dates where you can see how big part of a circle Gabrielle have moved around Xena (I like the old names better to…) to really make it a hard problem where they need to think. Add the distance to the sun, period around the sun and some more data they don’t need and you will have a really hard problem.
June 14th, 2007 at 1:30 pm
Hmph! What’s wrong with “Round thing that goes around a star” as a definition of “planet”
June 14th, 2007 at 2:08 pm
I don’t like Pluto not being a planet myself, but we can’t set the definition of “Planet” with the small set of worlds we had at hand – so I stand by the “dwarf planet” definition.
I call Mercury through Pluto the “Nine traditional planets” now.
“My Very Educated Mother Just Said ‘Uh-oh! No Pluto” – Hadn’t heard that.
June 14th, 2007 at 2:32 pm
1) To John Paradox: Just hope that’s what she’s telling you (heh heh).
2) XENA! XENA! XENA!
June 14th, 2007 at 2:49 pm
As a wise person once said a long time ago, in a galaxy far, far away, “Size matters not!” Eris should be classified as a planet. Sure, a dwarf planet, like Earth is a terrestrial planet and Jupiter a gas giant. I’m sure some of the 200+ planet-like objects we know orbiting other stars won’t even fall in these categories. The IAU fell down on the job and created more ambiguity with their decision than they cleared up. Time for round two. Otherwise, this is cool news, and a good example about how we can know so much about something without ever actually touching it or bringing it into a lab.
June 14th, 2007 at 4:08 pm
“Round non-star going around a star” is too easy. It makes too much sense. As for Mike Brown, does he seriously think New Horizons ever would have been funded if Pluto hadn’t been a planet for 60 years?
June 14th, 2007 at 4:24 pm
Eh, I’m in the planet = round non-fusor category. I’ve always thought of it as more of a broad based term like “animal” or “plant”. We don’t fuss over the definition of a plant every time we discover a new species. It’s not “too vague” for biologists to deal with.
I mean, so what if no one wants to memorize the names of 50 planets. We don’t make school children, or anyone else, memorize the names of all of the species on Earth.
June 14th, 2007 at 5:31 pm
“I mean, so what if no one wants to memorize the names of 50 planets. We don’t make school children, or anyone else, memorize the names of all of the species on Earth.”
Heck, we hardly make them memorize the names of 50 STATES anymore, and a shocking percentage of high-school graduates cannot find the USA on a globe, much less any of the countries in the M.E.
Not to mention, a similar percentage cannot name more than 2-3 Presidents in the 20th century, the name of the first human to walk on another world, any of the ammendments that comprise the ‘Bill of Rights’, or more than 3 Chemical elements – - – - – but they ‘know’ that the world is ~ 10,000 years old, and humans (and most animals) were created pretty much in their current form.
(For evidence, see Jeff Foxworthy’s new TV show….)
June 14th, 2007 at 6:00 pm
I would like to remind everyone that the closest thing we have to a 10th planet, orbitally, is Quaoar.
Circular orbit, on the ecliptic. It may be the largest object in the Kuiper Belt that has not been dislodged from it’s formative orbit.
June 14th, 2007 at 6:31 pm
Quaoar,,,?HuH? How do you pronounce that and where is it?
I rather prefer Ceres as a tenth planet. Big and massive but unfortunately rather oblong. Ah well, at least it has a solar orbit.
Yeah, I also prefer Xena and Gabrielle. Of course, since they were “hot”, maybe we should look closer in to Sol for objects which can be so named,,,
,,,maybe we should pray on it,,,(grinning as he ducks the rocks),,,
Gary 7
June 14th, 2007 at 7:58 pm
Unlike the Good Doctor (or is that the “Bad†Doctor-he’s still cool!) I think that names are important, especially when dealing with the public. This is why Dr. Plait and numerous others, myself included, have expended considerable intellectual energy in correcting the oftentimes maliciously deceitful confabulation by anti-science types of the way a scientist uses the word “theory†and the way J.Q. Public uses the term. In all this back and forth about Pluto being/not being a “planet,†I have yet to hear anyone suggest that there may be some quantitative way to determine what is or is not considered a “planet.†Just as the word “theory†to a scientist, or even an informed layperson, suggests something substantive about the particular idea being discussed, I am of the position that the word “planet†can do the same thing for bodies in this, and other, solar systems.
What was it about Pluto that set it apart from the other major Solar system bodies when it was still considered a “planet?†In everything I ever remember reading, it was always pointed out that its orbital inclination and the eccentricity of its orbit set it apart from all the other known bodies in the solar system and that this likely indicates that Pluto’s origin and history were different from the other 8 planets. From what I, as an informed layperson, understand of current theories of planetary formation, it seems that Pluto was NOT born in the same disk of dust and gas that eventually gave rise to the planetesimals that formed the terrestrial planets, Mercury, Venus, Earth, and Mars, and perhaps the tiny rocky seeds that grew to become the gas giants Jupiter, Saturn, Uranus, and Neptune.
The IAU’s 2006 redefinition of a planet entailed a tripartite test for “planethood,†specifically: (I will paraphrase)
1. The object must be in orbit around the sun.
2. The object must be massive enough to have formed a self-gravitating sphere.
3. The object must have cleared the immediate vicinity of its orbit.
I think that is a good list as far as it goes, but there is something missing. That something needs to be a blurb about how and where the body formed . Perhaps a similar case of “what’s in a name?†is how birds, dinosaurs, and reptiles all fit together and where each branch belongs on the “bush of life.†Right now there is an ongoing discussion among paleontologists and biologists as to what are properly considered birds and what ought to called dinosaurs. The lesson being is that when applying labels to the parts of a system, the labels used ought to be such that they can be reasonably applied to future discoveries of similar objects/items.
Can not a way be found to make part of the definition of “planet†refer to the condition that it orbits in roughly the same plane as its primary goes around its axis? If it is likely, based on things like orbital eccentricity, orbital inclination relative to the primary star, and the properties of other bodies in the system, that the body in question did NOT form in a stars’ “protoplanetary disk†then it would not be a planet by the above conditions.
Looked at another way, as we are starting to find ever increasing numbers of objects orbiting other stars, I think it prudent to have a name that encompasses those objects that form around a star, in roughly the same plane, orbiting the same direction, and of roughly the same eccentricity, to distinguish them from objects that just formed higgledy-piggledy on the outskirts. These are precisely the sort of objects we are most interested in finding; we are not, at this point, interested in “plutos†orbiting other stars, but rather in objects the likes of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Just as the word “comet” even to a layperson, says something quite specific about the objects origin, properties, and history, we, professionals and interested laypeople alike, deserve a name that is both specific and informative. What name would you recommend for such bodies Dr. Plait?
June 14th, 2007 at 9:13 pm
I’ve wondered ever since the IAU definition concerning a “planet” having “cleared out it’s orbit”. Since the orbits of Neptune and Pluto cross, why is Neptune not given the astronomical dismissal from planethood?
June 14th, 2007 at 10:33 pm
There is a drawback in all this “dwarf planets” saga.
Size DOES matter to space agencies, for whatever reason.
If it had not launched already, New Horizons would have been scrapped or seriously postponed. And it risked that for years even when Pluto was acknowledged as a “planet”.
Check the problems with the Dawn probe for another example.
========
I really don’t understand this “we can’t have too many planets” business at all. There are more than 9 stars and more than 9 galaxies.
June 14th, 2007 at 10:57 pm
“From what I, as an informed layperson, understand of current theories of planetary formation, it seems that Pluto was NOT born in the same disk of dust and gas that eventually gave rise to the planetesimals that formed the terrestrial planets, Mercury, Venus, Earth, and Mars, and perhaps the tiny rocky seeds that grew to become the gas giants Jupiter, Saturn, Uranus, and Neptune.”
Pluto almost certainly formed from the same disk as the rest of the planets. Its inclination can be explained rather readily by the large number of other objects found near-by. Taken as a whole, I would wager, dollars to doughnuts, that the average orbital inclination of the Kuiper belt is rather small. They simply didn’t get swept up into one or two much more massive bodies. If they had, those few but larger “uncontroversial” planets would orbit in pretty much the same plane as everything else.
June 14th, 2007 at 11:56 pm
As a matter of fact plant IS a vague term not really used for classification at all. Fine for Aristotle, if it’s alive and moves it’s an animal if it doesn’t it’s a plant. Rather like the sky really, if it’s a moving point of light it’s a planet if isn’t it’s a star.
June 15th, 2007 at 1:17 am
Xena? Eris? I preferred 2003UB313. At least that was systematic.
June 15th, 2007 at 3:20 am
Somewhere, wispy residents of a gas giant’s upper atmosphere are defining a planet as ‘a non-star thing orbiting a star that has no solid surface.’ Everything else was obviously just not lucky enough to soak up Jupiter loads of gas. There’s really no way you can take perception out of it, and still have a scientifically precise definition. Right now, according to the IAU, and to my memory, a planet has to have enough mass to gravitationally compress into a somewhat spherical shape, directly orbit the Sun (what about other stars?), and have cleared its orbit of miscellaneous other debris.
I think the last is the point I find myself hanging on the most; we all get the idea — you can’t orbit in a debris family and still be a planet, but there’s no real limit here. I believe even Jupiter has some ‘co-orbital’ objects, though I beg pardon if my knowledge is incorrect. Joe and Jane Average Person want an easy definition, and to leave the nitpicking to the professionals, for example the plant vs. animal definition above.
June 15th, 2007 at 5:23 am
Wow!
these are great words: “Words have impact. They shape our thoughts.”
Makes me think of Earl Nightingale’s famous words: “You become what you think about”. On should look that up in google….
June 15th, 2007 at 8:01 am
Next thing they’ll be telling us that Brontosaurus is not a dinosaur!
June 15th, 2007 at 10:34 am
“I think the last is the point I find myself hanging on the most; we all get the idea — you can’t orbit in a debris family and still be a planet, but there’s no real limit here. I believe even Jupiter has some ‘co-orbital’ objects, though I beg pardon if my knowledge is incorrect.”
Some more specific suggestions have been made. They’re really just quantitative re-definitions of “cleared the orbit”. Check the Wikipedia link in my name for a couple.
“Joe and Jane Average Person want an easy definition, and to leave the nitpicking to the professionals, for example the plant vs. animal definition above.”
This is really the heart and soul of the “planet kingdom” idea. “Planet”, ultimately, is a marketing word. It never has had a real scientifically significant meaning, and there’s really no reason why, now, in the 7th year since the second coming, we need one.
Pluto is to Earth as Earth is to Jupiter. You [i]really[/i] can’t lump Earth and Jupiter into the same category and have it [i]mean[/i] anything, so why bother? I have no problem with “dwarf planet”, any more than I do with “gas giant planet”. But I, and seemingly the majority of people out there, [i]do[/i] have a problem with “not a planet”.
It’s a PR nightmare, and a needless one since the word planet has never really meant much.
June 15th, 2007 at 2:32 pm
Definition of “planet” for the Average Joe and Jane: “round-ish object that orbits around a star and does not orbit around another round-ish object”.
Who can get simpler than that? And what would be “soooooooooooooo wrong” with it?
June 15th, 2007 at 2:53 pm
[...] The more time passes, the more unbelievable the whole thing is. Now Eris has been discovered to be larger than Pluto. [...]
June 15th, 2007 at 3:12 pm
Let me end my contributions to this blog by saying that for once I really do not understand Phil Plait’s careless attitude towards the definition of “planet”.
Of all people, the Bad Astronomer should be the one pushing forward the idea that every rock in the Solar System, no matter how big or small, can reveal a treasure of information about the past evolution of the Sun and all its planets, satellites, asteroids, etc, including Earth of course.
By choosing to stick to 8 “planets” instead, we would stifle further research.
Imagine going to convince Congress that you need a couple of billion dollars to go study a few dwarves in the Kuiper Belt (See? It does not even _sound_ PC).
More likely than not, people (and Congress people especially) will find it absurd to throw money into yet more space probes: after all, by the end of the decade every “planet” will have been visited at least once, so why bother with more exploration?
“Those planets will still be there in 10 or 100 years”, anti-space naysayers will state, “so we can just send a probe a decade and it’ll be fine”.
All of this, when we are learning that satellites that looked liked boring pieces of ice are in reality extremely interesting objects? (Check http://www.spaceref.com/news/viewpr.html?pid=22840 “Cassini Finds Saturn Moons Are Active”)
Oh please!!!!
June 15th, 2007 at 5:38 pm
[...] Inicialmente descoberto em 2005, Eris — conhecido então como “Xena” — já havia sido notado no ano passado como fisicamente maior que o velho Plutão. Agora, novas pesquisas confirmam que Eris possui também mais massa que Plutão, jogando mais água fria nas esperanças de reabilitar o antigo nono planeta do sistema solar. Eris possui uma órbita um pouco mais distante, mas é quase um terço maior, o que significa que desde o início da história humana tivemos oito “planetas” no sistema solar, e uma grande série de outros objetos além de Netuno, sendo Plutão na melhor das hipóteses o segundo maior deles (ainda está aberta a possibilidade de encontrar objetos maiores que Plutão e mesmo Eris). Astrólogos e mesmo charlatões como Däniken, Sitchin e companhia, que adoravam um sistema solar com nove (ou dez…) planetas, continuam a reescrever suas idéias baseados no que a ciência de verdade descobre sobre o mundo real que nos cerca. [via BAblog] [...]
June 16th, 2007 at 10:25 am
[...] BadAstronomy has a good post on how the mass was measured. [...]
June 16th, 2007 at 7:29 pm
I could go on and on about the problems with the “orbital bully” definition of planet, but instead I’ll address your statement “Words have impact.”.
We have a planet walk in my town. And such is the impact of the word planet, there no pictures or descriptions of satellites at any of the planet stations. So planet walkers don’t learn of the varied moons of Jupiter ( huge volcanic plumes on Io) or cryovolcanoes on Triton. The planet walk is pre-Huygens so I can’t blame them for not telling how amazing Titan is. I know educated people with an interest in science who don’t know know anything about these little worlds. Such is the impact of the word planet.
I suppose an intelligent reasonable person could know about these little worlds and not admit that they are planets. But, such is the impact of the word planet, many never get the chance to decide.
May 16th, 2008 at 12:19 pm
your website rocks
March 13th, 2009 at 9:00 am
[...] Eris. It’s about 2500 km (1500 miles) across, making it comfortably larger than Pluto. A moon (named Dysnomia) orbiting Eris was discovered, and its orbit gave the mass of Eris as being 27% bigger than Pluto [...]