Planets are round
The International Astronomical Union has (finally) come up with a proposal for the definition of a planet: it's round. If accepted, it means our Solar System has at least 12, not 9, planets.
NEWS Planets are round. Will that do?
EDITORIAL Round objects
Here's an amusing excerpt from the IAU questions and answers factsheet:
Q: Is a "pluton" a planet?
A: Yes.
Q: Is Pluto a "pluton"?
A: Yes.
Q: Is Pluto a planet?
A: Yes.
....so... that clears that up then.
Do you agree with the definition? Disagree? Let us know.
And do you have a handy mnemonic to remember the new planets by? (That's Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, UB313/Xena). Here's one popular one used to remember the more traditional 9 planets: "My Very Educated Mother Just Served Us Nine Pizzas"
Comments
This guy, who's blooging away like mad at the conference, finds the whole issue incredibly unimportant. His commenters disagree.
Posted by: Oliver | August 16, 2006 02:54 PM
Technically Earth is not - and has never officially been - a planet.
[editor's note: a link to a website has been deleted here as it was irrelevant and possibly spam... please let me know if this website was linked to in error]
Posted by: Dr. Martin Kilpatrick | August 16, 2006 03:26 PM
Patrick Nielsen Hayden at Making Light has a post pointing to more comment from all quarters on this vital issue. And there are letters in the New York Times
Posted by: Oliver | August 17, 2006 11:52 AM
Your comments are boring. Why do you bother with these blog things?
Posted by: Freddie Cheeseman | August 17, 2006 12:18 PM
Here's a news update from Jenny, one of the reporters who worked on this story.
The planetary scientists of the American Astronomical Society have decided to support the new planet definition.
Meanwhile, I've also spotted a blog where people are having fun with mnenomics. Check it out for inspiration.
Posted by: Jenny | August 17, 2006 04:18 PM
I disagree with the barycenter definition for a double planet. If Charon was closer to Pluto, the barycenter would be within Pluto, and yet the two objects would still be of the same relative masses.
My suggestion is to use the magic number {[sqrt(5) - 1] /2 ~ 0.618} as the factor of secondary:primary above which a pair will be called a double object.
Posted by: Edward Schaefer | August 17, 2006 05:38 PM
Big props to Yvette over at Asymptotia for the best mnemonic yet: “My Very Educated Mother Can’t Justify Someone Using New Planetary Conventions… oh no, they haven’t named the last one yet!!!”
Posted by: Oliver | August 17, 2006 06:35 PM
Meanwhile, Mike Brown, discoverer of UB313, has an op-ed in the New York Times. He makes the point that planet is a sort of arbitrary term, like continent. But his example -- that no one really minds that Australia is seen as a small continent and Greenland as a large island -- doesn't really satisfy. Greenland is firmly part of the North American plate. Australia has a plate of its own. That makes a difference. The continent that has the most questionable claim on the term is surely Europe. The Urals are more a palaeoboundary than a boundary, and it might make more sense to call Europe a subcontinent, as we do India (where the boundary between plates is rather more active...). But that's another barrel of monkeys.
Brown's conclusion is effectively an arbitrary cut off somewhere below the diameter of Pluto, letting UB313 in, but keeping Charon and Ceres out.
Posted by: Oliver | August 18, 2006 12:25 PM
Umm.. if 12 planet gets affirmated, what mnemonic are the students going to use to memories the 12 planets?
Posted by: Mariam Taylor | August 22, 2006 10:23 AM
Greetings
I think the definitions proposed are slightly too simple. Here is a counter proposal.
Planet: Any material body that exhibits ALL of the following characteristics.
1. Too small to fuse Deuterium under self-gravity.
2. Spherical under self-gravity in the absence of an annual freeze-thaw cycle (as modified by rotational effects and recent impact history).
3. Uniformly differentiated (as modified by rotational effects and recent impact history).
4. Any baricenter is outside any co-orbital, non-stellar object.
All values shall permit of small non-uniformities. Earth is a planet even though the surface layer (oceans and continents) is not uniformly differentiated, but if the crust is included to a deeper depth it becomes effectively so.
The first defines the high end. The second two define the low end. The fourth distinguishes between a planet and a moon.
This definition does not require that a planet orbit a star. Rogue planets are permitted.
Posted by: Dana Johansen | August 22, 2006 08:01 PM
I have always stipulated that if a body is round, and orbits the Sun...it's a planet. If the body has some sort of atmosphere hanging about...even more. My detractors have become mysteriously silent of recent since the IAU made it's definition known.
But, I do have an issue with naming Charon a planet. Barycenter or not..it's a satellite of Pluto.
Respectfully,
Chicago Astronomer Joe
Administrator
http://astronomer.proboards23.com
Posted by: Chicago Astronomer Joe | August 23, 2006 08:54 AM
Speaking as a geologist, I'm perfectly happy with the roundness criterion, and comfortable with the separation/ double planet criteria. I'm a little teed-off about the gaffe over re-using the term "pluton", but worse disasters of nomenclature have occurred. A few years ago "plutino" was suggested in this context, and I don't know why it was discarded this time.
But the really big problem is going to be - what's the order of these outer "pl"&("anet"||"utino")"s" ? Even if the "smudges" are calculated to have a longer semi-major axis, at these long periods a "smudge" could spend centuries within the orbit of of apparently closer "smudge". That's going to keep the mnemonic-writers in business!
Posted by: Aidan Karley | August 23, 2006 10:15 PM
I was so happy to hear the decision. Because here in Kerala, South India several Modern astrologers started predictions based on calculations including Pluto. Atleast some of the believers are convinced that these predictions are meaningless!
Posted by: Dr.A.Rajagopal Kamath, Popular Science Author in Malayalam | August 30, 2006 06:26 PM
Hello everybody I have come back just to warn you about this new planet definition
I disagree with the new definition.
Apparently the astronomers failed to realize that Earth, Mars, Jupiter and Neptune are also no longer planets according to this new definition.
Earth has not cleared its orbit of all Asteroids, and neither have the other planets. Jupiter and Neptune also have trojan asteroids orbiting in their orbit and Neptune also has Pluto, along with half the Kuiperbelt in its orbit.
Posted by: Albert Einstein | January 17, 2007 03:24 AM
It is good that someone writes articles which really matters something. Thank you for this article, it's full of knowledge which is hard to find in tons of rubbish in our famous world wide web. Regards and good luck!
Posted by: tapety | November 29, 2007 12:39 AM
Technically Earth is not - and has never officially been - a planet.
Posted by: Otel | June 2, 2008 05:18 AM