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A. Schaller / STScI
New observations suggest that the dwarf planet Eris, shown in this artist's conception, isn't as big as scientists thought it was.
Everyone knows that the solar system is no longer seen as a nine-planet set. When you count the dwarfs, there could be scores or hundreds of planets out there -- as I write in my book, "The Case for Pluto." But a case could be made that Pluto is once again the ninth-largest planet orbiting the sun, based on observations reported over the weekend.
Pluto was ousted from the No. 9 spot five years ago, after the discovery of another dwarf planet on the solar system's icy frontier, known today as Eris. It was Eris' apparent status as an object slightly bigger than Pluto that brought the controversy over the definition of a planet to a head. If Pluto was an honest-to-goodness planet, shouldn't Eris be one as well?
In 2006, the International Astronomical Union approved a definition that established a new class of objects, called dwarf planets, which were big enough to be basically round but not gravitationally dominant enough to "clear out the neighborhood of their orbit." What's more, the IAU ruled that dwarf planets were not really planets.
My book delves into the questions raised about that definition, particularly in light of what we've been learning about planetary systems since then. Now there's a new question: Is Eris bigger than Pluto after all? Based on observations of Eris' occultation of a faraway star in the constellation Cetus, the answer could well be no.
Sky & Telescope's Kelly Beatty reports that the latest observations suggest Eris is actually slightly smaller than Pluto. He quotes the Paris Observatory's Bruno Sicardy as saying Eris is "almost certainly" no wider than 1,454 miles (2,340 kilometers), compared with Pluto's estimated width of 1,456.5 miles, plus or minus 6.5 miles (2,344 kilometers, plus or minus 10 kilometers).
"If the early results hold up, this time it's the dwarf planet Eris' turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt," Beatty writes. The Kuiper Belt is the broad zone of icy objects that lie beyond Neptune's orbit.
Gathering the data for the measurements was a grand astronomical feat: Three teams of scientists watched the distant star disappear when Eris crossed in front of it. By analyzing how long the star was covered over, as seen from three vantage points in Chile, the astronomers could calculate how wide Eris' round disk was. Previous estimates were based on indirect data, such as Eris' brightness.
Further observations will be required to reduce the uncertainties surrounding the two worlds' widths. The current estimates are so close that Caltech astronomer Mike Brown, whose team discovered Eris and two other dwarf planets, can justifiably say Eris and Pluto are "more or less the same size." And when you rank the two by mass rather than size, Eris clearly comes out on top. That implies that Eris' interior is denser and thus rockier than Pluto's.
"How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?" Brown writes. "As of today I have absolutely no idea. ... Something is going on in the outer solar system, and I don’t know what."
Whether Pluto is bigger or smaller than Eris really doesn't affect its status as a dwarf planet. But it does illustrate that small celestial objects can deliver some big scientific surprises -- and that it's a huge mistake to write off the little guys of the solar system.
Correction for 6:50 a.m. ET Nov. 8: I originally wrote that Pluto might be the ninth-widest object in the solar system, neglecting to take into account that some moons are wider than Pluto (and in fact wider than Mercury). Thanks to Stevesliva for pointing that out.
More on dwarf planets:
- Pluto debate is about more than one little world
- Interactive: Guide to the new solar system
- Pluto maps raise new questions
- 80 years of Pluto
Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also check out "The Case for Pluto," Alan's book about the controversial dwarf planet and the search for new worlds.
Leave a Comment:
As a Scorpio, I demand the reinstatement of Pluto as a planet!
I never understood the need to come up with the classification of "dwarf planet". If it's round and orbits a star, let it be called a planet... what's wrong with that? Are we afraid we will ultimately find too many of these objects and run out of names? Or that it would make our traditional planet feel less special? Maybe that we'd have to spend too much time updating elementary school solar system models and re-inventing achronyms for children to remember the names of the planets...
So then if some of the asteroids in the asteroid belt are shaped like a ball should they be called planets?
What I dont understand is why people are getting upset when we reclassify something because there is a need based on the number of small spherical bodies that are out in the Kuiper belt.
What is the big deal, call them dwarf planets. The more we find out about our solar system and the universe the more we will have to revisit the classifying names of things.
Why are people afraid of change, does changing pluto into a dwarf planet lessen that value of Pluto, the answer is no, the only place it changes is in your head.
Even our own moon is much larger than Pluto or Eris, and perhaps some of the asteroids may be circular and orbit the Sun.
In fact, we know that there's an asteroid that is circular and orbits the sun ... Ceres ... which is why Ceres is now known as a dwarf planet (as well as a member of the asteroid belt).
@ Pirate C: It's not the re-classification that is the issue it's the way it was done. For starters, a basic understanding of the English languages reveals that something called a dwarf planet is in fact a planet, much like a sports car is a car. So announcing that Pluto is no longer a planet but a dwarf planet is linguistic nonsense. Further more it was declared that although it's big enough to be a planet it did not "clear" its neighborhood" because its orbit intersects with Neptune's. Well then Neptune's should also therefore no longer qualify as a planet. The biggest objection is that the re-classification did not seem to stem from a need for clarification but rather some strange conservative mind set that would not allow for more than 9 official planets. I agree that the old definitions were rather vague, but instead of taking the opportunity to come up with precise specifications, they just came up with even more absurd definitions. "Big enough to be round" The concept being that the object's gravitational force would force it to be spherical. What if we encounter a small spherical object? Definitions are by nature arbitrary so why not have settled on a specific mass? And is "clearing its neighborhood" really the best scientific phrase the authorities could come up with?
The whole process was exemplary of committee driven compromised stupidity. And that's what's objectionable.
I agree with sarcastoid's assertions here. How the committee could come to call it a dwarf planet even though the committee feels it's not a planet is beyond me. It's a horrible way of labeling things.
As to why this all happened I don't know, but I'm sure someone said it'd be tough to include all the names of this stuff in a textbook for kids in physical science, and for me that is just unacceptably lazy. Either you include everything or you don't teach it. Simple.
I personally have no problem calling Pluto a dwarf planet as long as I don't get people saying it's not planet. If it's not a planet then that word should not be included in it's label.
In astronomy we are clearly very far from a consensus as to what call all this stuff but I for will continue to call Pluto a planet. We are in fact allowed to make choices like that. If you want to you can call jumbo jets "flying buses", it's up to you.
The human mind finds the universe so much easier to deal with if everything in it can be categorized, cataloged, and named. All the matter revolving around our Sun -- gas behemoths, rocky globes, icy boulders, and far-flung dust -- they're just stellar system bodies. Some small, some large. Just stuff. Every star has it. But we're so entrenched in our history, and unwilling to let go. It's a mortal sentimentality that, in my opinion, just doesn't have a place in dispassionate science. Someday, I trust we'll wise up and generalize/homogenize our nomenclature, and keep the politics out of it. Until then, let the sniping continue unabated!
Tiz,
Categorizing things is what we humans do. We organize, sort, pigeon-hole and collect things into groups and sub-groups. We're the only animals on this planet, to my knowledge, that do that.
Even you did it in your summation.
"Some small, some large." Then you further categorized it all as "Just stuff." You differentiated between "the gas behemoths, rocky globes, icy boulders and far-flung dust." Why, if it's just stuff?
Categorizing and DISCRIMINATING between different flora, fauna and other HUMANS is, for better or worse, what we do.
I agree in certain circumstances it would be better if we did not do this. But frankly I don't think we can stop.
It's who we are.
"9th or 10th widest object?" No. Seven moons are larger, including our own.
Aha, good point, will fix.
long live Pluto!!!!!!! give us our PLANET BACK!!!!!!!!!!!!!!!!!!!!
I second that! Also Eris should be number 10. Dwarf planet still is a planet. It is just a small planet. Sorry for the analogy, but we don't go around telling dwarf people they aren't actually people.
No, we need more.
Pluto and Eris, dwarf rocky planet. Earth, Venus - rocky planets. Neptune and Uranus - dwarf gas giants. Jupiter and Saturn - gas Giants.
Then, after we get visuals around other stars, we can add giant rocky planets and giant gas giants.
lol, "giant gas giants". That's awesome.
we need to use words like "super" and "extreme" more often when we classify things.. ;-P
Funny that no one has a problem with the notion of dwarf stars...oh, that's right, you were brought up with the idea.
Not all stars are alike, not all planets are alike.
Also know that we originally called 'sub-dwarf planet' objects 'planetoids,' ('planet-like') which is really more appropriate. ('Asteroid' means 'star-like')
The only thing I wonder, at the low end of the scale (and I'm okay with an arbitrary definition, as long as there is one we can agree on): At what point is something too small to be called an 'asteroid,' and is just a 'meteoroid?'
(BTW, it's a 'meteoroid' in space, a 'meteor' as it moves through the atmosphere, a 'meteorite' if it makes it to the ground...)
Interesting..........also in 2006 NASA launched new horizons to explore a planet ? or a moon ? or a drawf planet ?......what were they thinking ?
I still say its a planet, Disney thinks its a dog
Neither Pluto, nor New Horizons really cares, do they?
Re-classifying it makes it no less an object worthy of investigation, and probes have gone to various moons (starting with our own), planets, asteroids and comets, so NASA is hardly size-chauvinistic, is it?
In my opinion, the IAU is discredited and has no standing. The reason in my opinion is that they waited till there were just 400 people left before voteing on Pluto (BBC had a good article on this a while back). Pluto is the 9th planet. Apparently, Even NASA thinks so.
I thought that the demotion of Pluto was stunningly ridiculous. How can Pluto *not* be a planet when it has enough gravity to have a moon in orbit?
Three moons (that we know about) - Charon, as well as the recently discovered Nix and Hydra. Nix and Hydra are pretty tiny.
There are several asteroids with natural satellites .
Wow, people are still griping about Pluto's reclassification? There are more differences than similarities to the 8 planets to justify the dwarf planet moniker, including inclination, orbital eccentricity and vast distance from the sun. And just because it's been reclassified, that doesn't make it any less interesting or important as a target of study. I, for one, am looking forward to the New Horizons fly-by so we can finally replace those fuzzy purple images from our textbooks and astronomy sites. Besides, as one of the largest of the Kuiper Belt denizens, Pluto is now the biggest fish in that pond, as opposed to being the oddball of the "original 9" planets. I'd consider that a promotion! What sounds better... Freak Planet or Kuiper Belt King?
Actually, Pluto isn't that freaky when you consider some of the truly odd (and Saturn-sized) planets detected in other star systems. I don't think it's an either / or situation. Pluto can be a big Kuiper Belt object as well as a small planet (dwarf-class), just as Ceres is an asteroid (due to its location) as well as a dwarf planet (due to its geophysical characteristics).
The original definition of planets was the wandering stars that early astronomers noticed moving through the heavens and not fixed in their relationship to other stars. Going with that and only counting "planets" that were visible with earth based telescopes by humans without computer assisted scanning capabilities, that just leaves the decision at whether to include Pluto or not and certainly does not not include any additional objects as "planets" in the classical sense. Then we can get into cataloging all of the dwarf planets, asteroids, and anything else that we find out there.
Poor little Pluto... we should hurry up and throw our space exploration suits on and get out there to see everything up close and personal!
Well , Pluto's orbit is so elliptical that it crosses Neptune's , Pluto does not obit within the planetary plane , Pluto and it's largest moon, Charon , are sometimes treated together as a binary system because the barycentre of there obits does not lie within either body .
Hence ;
We are left with two choices , Pluto is either the strangest planet in our solar system , or it'sa prosaic all be it large Kiper belt object .
Well, if that's how you judge strangeness, Eris is significantly stranger. And there are even stranger Saturn-size planets in extrasolar systems. Or are they not planets because they're so strange?
Dang, the term "strangelet" is already taken.
...Let's use "spookylet" or "weirdorb".
Well, if that's how you judge strangeness, Eris is significantly stranger. And there are even stranger Saturn-size planets in extrasolar systems. Or are they not planets because they're so strange?
The I A U needed to refine the definition of the word " planet " as a result of the mounting sub-planet / exo-planet discoveries you mentioned . The fact that Pluto acts in a way contrary to that definition is why i chose the word strange .
Okay, so Pluto is a dwarf planet. but it's not a planet. Am I the only one that doesn't understand that? If it's not a planet then why the hell would someone call it a dwarf planet?! Is this the case for all dwarf planets? None of them are planets? Really, in terms of simply classifying things that is a poor use of a word.
What if we called a dwarf person not a person? that just doesn't make sense at all!! They are still people!
If it's not a planet and there is a group of people sitting around deciding on what to call the damn thing they should not include the word "planet"! If it's a binary planet then it's a planet. If it's a dwarf planet then it's a planet. If it has the word planet in the lousy name then it's a dang ol' planet! It's just a word so we should at the very least use the correct word. If it's not a planet, and it's not a moon, and it's not a comet, and it's not and asteroid or meteor, and it's certainly not space dust, then we should come up with a word for what it IS!!
so, "the IAU ruled that dwarf planets were not really planets." ...Well, if they ruled what they are not they surely must have ruled what they ARE. I am going to look for the answer to that question. If anyone could answer that for me in the mean time I'd sure appreciate it.
I have a question. Let's say something is about Mars size, spherical, but orbits the star in a highly eliptical orbit like a comet. Is it a planet or comet? What if it also has a satelite? An atmosphere?
In the case of Pluto , it is classified as a Kiper belt object .
So, by definition, there are no planets in the Kuiper Belt? Does Pluto have dual classification as a "Kuiper Belt object" and a "dwarf planet"?
I never proposed there could not be planets in the Kiper belt . If they sweep there orbits and behave like good planets , im all for letting them join the club. As far as dual classification , when the question was posed " what is Pluto ?" my answer was simply , "a Kiper belt object ".
Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term "dwarf planet," which was first coined by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, to indicate a third class of planets which are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for "dwarf planets" to be classed as not planets at all. The IAU did not "have" to do anything other than allow Eris's discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems.
Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto's, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets!
Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common?
It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become.
Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don't ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?
I have the idea of a "kuiper planet" category, myself. I agree, it's a planet, but I don't think it belongs in the same class as the other 8 "classical" planets. Pluto's composition and orbit point to it not being formed in the same manner as Mercury, Earth, Jupiter, Neptune, et al. Ceres on the other hand, by dint of its origination, may well be a much more "classic" planet. I don't know enough about Ceres to make an argument there. "Planar planet", for planets more aligned to the solar equatorial?
Kuiper Planet is a good name in my opinion. But then again I have no problem calling it a dwarf planet, as long as everyone recognizes it as a planet. I think you and the others that have mentioned it are correct that we will not be able to have a good reliable definition until more is understood about the origins of all the planets. I for one would not group the gas giants in "being formed in the same manner as" the inner planets, even if they did accrete material in a similar way. We just don't know enough for sure about the formation of the solar system. We have a very good idea of how and when it all happened but these are relatively sound theories that could be proven wrong at any time. I doubt they will be proven wrong though. More likely they will be clarified as more data is found. For instance, the giant impact that is believed to have resulted in our Moon is only a theory, and it is generally accepted as the best fit, but it is by no means a matter of fact in a strict sense of the word.
What's wrong with "planetoid"? Or is that a planet like object that is not a dwarf planet?
Give him back his title now you crazy scientists, you.
Sorry Alan, I'm in the camp of "It's not a planet, It's a kuiper belt object." To me the tell-tale, critical, factor is it's eccentric orbit. That's mho.....by the way ,I'm also here as Stephen(Bunch of #s)..from my work computer.