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TheAstronomer
2006-Sep-10, 04:38 PM
Pluto. It's a planet, then it's not. This week we review Pluto's history, from discovery to demotion by the International Astronomical Union. Learn the 3 characteristics that make up a planet, and why Pluto now fails to make the grade. ...

Read the full blog entry (http://www.astronomycast.com/?p=3)

LtCommanderData
2006-Sep-14, 12:39 AM
I am rather dissapointed that pluto didn't make the cut. So what is it now? My Very Energetic Mother Just Served Us Nine! Nine what? I guess I understand they needed to identify what a planet is but I dont like it very much... oh well... what can you do?
I do have a question though. What are the requirements for a planet to be a planet?

kheider
2006-Sep-14, 01:59 AM
Pluto. It's a planet, then it's not. This week we review Pluto's history, from discovery to demotion by the International Astronomical Union. Learn the 3 characteristics that make up a planet, and why Pluto now fails to make the grade. ...

Read the full blog entry (http://www.astronomycast.com/?p=3)

It was a good show. But Pamela was incorrect when she was commenting on 'Dwarf Planet' Ceres (8:43 into the show).

Ceres has been classified as a Spheroid (in hydrostatic equilibrium) or it would not qualify as a 'Dwarf Planet'.

Ceres is not a (major) Planet because it has not "dominated the neighborhood around its orbit clearing it of comparable objects".

Asteroid Vesta fails as a 'Dwarf Planet' because it is not spherical.

I did enjoy listening to the show.:clap:

-- Kevin Heider

StarStryder
2006-Sep-14, 02:07 AM
How about

"My very errant mathematician just solved us nothing."

or

"May victorious Eggbert make justice safe under nightfall."

or, simple and back to basics...

"My very earnest mother just served us nachos."

What ideas do you have?

kheider
2006-Sep-14, 02:24 AM
Pluto. It's a planet, then it's not.

Poor Pluto?:rolleyes: Ceres was stripped of the title Planet and demoted all the way back to mere asteroid over a hundred years ago. I am glad to see Ceres finally getting some respect. :dance:

One of our follow Bautforum members, Vilkata, put up a Ceres website at: Ceres: The Dwarf Planet (http://home.comcast.net/~eliws/ceres/)

You can read about the "Ceres website Bautforum thread" at http://www.bautforum.com/showthread.php?t=46154

-- Kevin Heider

Jakenorrish
2006-Sep-14, 03:13 PM
Nice site that. Well worth a visit.

hhEb09'1
2006-Sep-14, 03:22 PM
[FONT="Lucida Sans Unicode"][SIZE="3"][CENTER][COLOR="Olive"]I am rather dissapointed that pluto didn't make the cut. So what is it now? My Very Energetic Mother Just Served Us Nine! Nine what? My daughter came up with "My Very Evil Mother Just Served Us Nothing" :)

Galaxy
2006-Sep-14, 10:48 PM
So, how come Neptune is a planet when it has all these little Plutinos hanging out in its orbit? They can't all be living in Neptune's lagrange points.

hhEb09'1
2006-Sep-14, 11:39 PM
So, how come Neptune is a planet when it has all these little Plutinos hanging out in its orbit? They can't all be living in Neptune's lagrange points.I thought that was the definition of Plutinos (http://www.bautforum.com/showthread.php?p=453939#post453939)? They're in some 3:2 resonance with Neptune, so that when they're close to the orbit, Neptune is far away.

Kevn
2006-Sep-15, 03:55 PM
Just wanted to jump in here to say I loved the first podcast. Can't wait for the next one.

Galaxy
2006-Sep-16, 06:25 AM
I thought that was the definition of Plutinos (http://www.bautforum.com/showthread.php?p=453939#post453939)? They're in some 3:2 resonance with Neptune, so that when they're close to the orbit, Neptune is far away.

Right, but aren't they still in Neptune's orbit? Or does it not matter because Neptune is so far away when they are close? When are you far enough from the local planet to not be counted when astronomers are trying to determine if it's cleared its orbit?

hhEb09'1
2006-Sep-16, 02:43 PM
Right, but aren't they still in Neptune's orbit? Or does it not matter because Neptune is so far away when they are close? When are you far enough from the local planet to not be counted when astronomers are trying to determine if it's cleared its orbit?In this sense, they are like objects at lagrange points--they are in positions in space/time (as opposed to space or spacetime) such that the planet cannot be expected to have cleared them

Galaxy
2006-Sep-16, 03:59 PM
In this sense, they are like objects at lagrange points--they are in positions in space/time (as opposed to space or spacetime) such that the planet cannot be expected to have cleared them

That makes sense. Thanks.

Rkaloger
2006-Sep-16, 05:41 PM
I thought saying teachers had a "vested interest" in keeping Pluto a planet was a bit disingenuous. Imagine the windfall text book writers and education materials suppliers are set to reap thanks to this. Or the researchers who can now use Pluto as an easy target to study the "new" field of dwarf planets. Yet it's teachers who are defending Pluto just so they don't have to come up with a new neumonic? This whole Pluto thing just reinforces the stereotype of the cold, uncaring, inflexible scientist that cares more about creating and following a rigid set of rules than educating the general public.

BobK

tuross
2006-Sep-16, 06:54 PM
Thanks for the show, Fraser and Pamela.

I enjoyed being invited to share the friendly, informative conversation.

tuross
2006-Sep-16, 06:55 PM
I thought saying teachers had a "vested interest" in keeping Pluto a planet was a bit disingenuous. Imagine the windfall text book writers and education materials suppliers are set to reap thanks to this. Or the researchers who can now use Pluto as an easy target to study the "new" field of dwarf planets. Yet it's teachers who are defending Pluto just so they don't have to come up with a new neumonic? This whole Pluto thing just reinforces the stereotype of the cold, uncaring, inflexible scientist that cares more about creating and following a rigid set of rules than educating the general public.

BobK

BobK, I took the comment of <i>"vested interest"</i> to mean that my teachers are warm, caring and flexible scientists. This is what I've discovered in my last 6 years of being a student. As an online and distant education student, I'm hit by the double whammy of costs - the textbooks plus postage. Thank you to my warm and caring teachers.

MattB
2006-Sep-17, 12:27 AM
In all the old 1950s sci-fi movies, the astronauts find an "alternate Earth" in the same orbit as Earth, moving at the same speed, but always on the opposite side of the Sun, so no one ever saw it until the astronauts went out into space.

Under the new definition of "Planet" would a system with these two planets no longer be called "planets" just because one of them didn't clear the other out.

kheider
2006-Sep-17, 09:20 PM
In all the old 1950s sci-fi movies, the astronauts find an "alternate Earth" in the same orbit as Earth, moving at the same speed, but always on the opposite side of the Sun. Under the new definition of "Planet" would a system with these two planets no longer be called "planets" just because one of them didn't clear the other out.

In reality that is impossible. Even if the (2) Earths were exact copies of one another, the gravitional attraction of the other Planets would cause one Earth to wander into a larger & longer orbit thus allowing the other Earth to over take it. This would result in a collision between the (2) Earths.

You might be able to briefly have (2) identical Jupiters at L3 Lagrange points (http://en.wikipedia.org/wiki/Lagrange_point) because Saturn is not really massive enough to have a major affect on a very well balanced "Jupiter(1)-Sun-Jupiter(2)" pendulum.

-- Kevin Heider

hhEb09'1
2006-Sep-17, 10:09 PM
In reality that is impossible. Even if the (2) Earths were exact copies of one another, the gravitional attraction of the other Planets would cause one Earth to wander into a larger & longer orbit thus allowing the other Earth to over take it. This would result in a collision between the (2) Earths.Not just the other planets--that particular lagrange point is unstable, any perturbation will knock it away

MattB
2006-Sep-18, 01:52 PM
Okay, so I'll buy that everything I saw in 1950s sci-fi movies might not be physically possible.

But what about a "binary planet" (like Pluto and Charon) that orbit a central point -- external to both -- as they both orbit around a star, but are the only two items in the orbit, and are spherical.

What would be the grounds for excluding a "binary planet" from being considered a planet? "We're sorry, but your moon is too big"?

Or is there a reason that two Earth-like planets couldn't be in orbit around each other?

kheider
2006-Sep-18, 10:34 PM
But what about a "binary planet" (like Pluto and Charon) that orbit a central point -- external to both -- as they both orbit around a star, but are the only two items in the orbit, and are spherical

What would be the grounds for excluding a "binary planet" from being considered a planet? "We're sorry, but your moon is too big"?.

Binary planets will occur. Pluto and Charon are Binary 'Dwarf Planets' since their barycenter (center of mass) is external to both of them. They are not binary 'Planets' because their mass is not great enough to clear their orbit of comparable objects.

Daphnis (http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2262) (a 7 kilometer-wide shepard moonlet; 136,500 km from Saturn) is orbiting in the Saturn Ring System. Since it is one of the largest "ice cubes", "it is clearing it's orbit of debris."

But Daphnis will never get big because the rings are inside the orbits of Saturn's moons and the tidal forces (Roche limit (http://www.knowledgehunter.info/wiki/Roche_limit)) will prevent the material in the rings from coalescing gravitationally to form moons.

Getting back to your question, Planetesimals (and then later Planets) form from large clumps that are orbiting inside a relatively flat gas disk that is orbiting a young star.

Theory suggests that the Earth-moon system was created when a Mars sized Planetesimal collided (http://www.psi.edu/projects/moon/moon.html) with the Earth. It is believed that the Pluto-Charon system was also created when a large object hit Pluto.


Or is there a reason that two Earth-like planets couldn't be in orbit around each other?

It might be possible. It would require two similiar sized Planetesimals colliding off-center (a near miss) or one planetesimal trapping another roughly equal sized planetesimal. Either way it would require one of the planetesimals to have been knocked out of its original orbit by a larger object. The math here is well beyond my dynamics understanding and may even require a 3rd planetesimal making a one-time pass through the system to help establish equal masses and a stable orbit.

-- Kevin Heider

emarksmi
2006-Oct-15, 04:56 AM
Sorry to jump into this thread late. I just now discovered this message board and caught up with my PodCasts.

This new definition of planet bothers me on several levels. First, it doesn't actually say "Dominates its orbit", it says "has cleared its orbit". Nothing in the solar system has truly cleared its orbit. In fact, there are NEA's that very nearly share Earth's orbit that periodically overtake it. If you want to try to use this definition, then the ONLY planet would be Jupiter because every other planet in the solar system is where it is primarily becasue that is where Jupiter forced it.

Second, it does not attempt to address what the upper limit of a planet is.

Lastly, and most importantly, it applies to our own solar system only. Now we have the search for extrasolar-something's-that-used-to-be-planets-but-are-now-something-else. Sorry, it just doesn't make sense to me.

Ronald Brak
2006-Oct-15, 05:21 AM
Lastly, and most importantly, it applies to our own solar system only. Now we have the search for extrasolar-something's-that-used-to-be-planets-but-are-now-something-else. Sorry, it just doesn't make sense to me.

I was watching Battlestar Galactica last night and they reffered to a large object that wasn't orbiting the sun as a planet. If it's Battlestar Galactica versus the IAU then Battlestar Galactica is going to win.

DAVEK
2006-Oct-30, 09:15 PM
As an astronomy teacher, I would say I have a vested interest in seeing this kind of debate happen again next September. It was wonderful to start the term with a group of students who really did want to know what the definition of a planet is and why some things did count and other didn't. Maybe next year we can start calling Jupiter a brown dwarf? (Yes, I know why it's not).

Also, I like the basics of the new IAU definition (dynamicist that I am), but I also understand that we also need a term that is based on the physical characteristics of the body rather than its orbital characteristics. For many years now I have reserved the term "world" to apply to any body in the solar system big enough to be in hydrostatic equilibrium (and too small to be a star) whether or not it is orbiting the Sun or a planet, and whatever else is orbiting nearby. That way I can treat Pluto, Ceres, Io, and Titan as well Mars and Earth as worlds and then go on to talk about their geologic processes, atmospheres (or lack thereoff), etc. That is a distinction that seems to work pretty well in my experience.

Lastly, we will probably have to come up with a better definition of a binary planet. Basing it on the location of the barycenter can get you into some odd situations.

Example 1: Eventually the Earth-Moon system will become a binary planet system. As the Moon's orbit moves away from the Earth due to tidal evolution, the barycenter of the Earth-moon system will move out of the Earth. All we have to do is wait a little to return the Moon to its status as a planet.

Example 2: Triton-Neptune. Doug Hamilton pointed out to me a couple of weeks ago that when Triton was originally captured it would have been in a highly eccentric orbit. At the outer parts of that orbit the barycenter was outside of Neptune, while in the inner parts it was inside. So for a while Triton alternated between being a planet and not being a planet each orbit.

DK

suitti
2006-Nov-20, 06:17 PM
Study of Earth's moon, The Moon, is considered planetary science. That's right, The Moon, the only planet in the solar system with a first and last name. (Smokey the Bear has a middle name.)

One might say that there are seven planets. The Sun, the Moon, Jupiter, Venus, Saturn, Mars and Mercury. That's right - the Earth is not a planet.

SingleDad
2006-Dec-12, 04:29 AM
:boohoo:

My daughter and I have come up with a simple solution to a complex problem.

1) Place a boundary on the "Known Solar System" and stop at Neptune, since there seems to be so many questions on what is beyond. As we learn more, expand the boundary.

2) Objects will be called what they are. Big round things are planets. Big not round things aren't planets. If something is in orbit around something else it's a satellite. If that satellite is in orbit around a planet it is a moon.

3) If there is a ring such as the asteroid belt. It can not have a planet, unless like in the case of Jupiter it drag the junk in it's orbit. BUT no classification can be made until this is known!

4) No more jumping the gun! just because you see something move in your telescope doesn't mean it's a planet :p

Sorry little Pluto you're outta there. From what I understand, it sounds like we have another belt like the asteroid belt only of snowballs. It also seems to me people are making this a lot harder than it has to be.

hehehe sorry I posted this so late... had a 15 year-old run a red light a total my car. This one kinda slipped my mind until tonight when I didn't get my daily pod cast.

All Love
Brian and Megan

laurele
2007-Dec-21, 11:32 PM
:boohoo:

My daughter and I have come up with a simple solution to a complex problem.

1) Place a boundary on the "Known Solar System" and stop at Neptune, since there seems to be so many questions on what is beyond. As we learn more, expand the boundary.

2) Objects will be called what they are. Big round things are planets. Big not round things aren't planets. If something is in orbit around something else it's a satellite. If that satellite is in orbit around a planet it is a moon.

3) If there is a ring such as the asteroid belt. It can not have a planet, unless like in the case of Jupiter it drag the junk in it's orbit. BUT no classification can be made until this is known!

4) No more jumping the gun! just because you see something move in your telescope doesn't mean it's a planet :p

Sorry little Pluto you're outta there. From what I understand, it sounds like we have another belt like the asteroid belt only of snowballs. It also seems to me people are making this a lot harder than it has to be.

hehehe sorry I posted this so late... had a 15 year-old run a red light a total my car. This one kinda slipped my mind until tonight when I didn't get my daily pod cast.

All Love
Brian and Megan

So who would determine what is "big" enough and what is not regarding round objects? No matter who decides, the boundary is arbitrary.

Placing a boundary on the "known solar system" at Neptune doesn't make sense. We know that the Kuiper Belt and Oort Cloud are part of the solar system. The sun's gravitational influence does not end at Neptune.

No one has made a sound argument for why an object located in a belt cannot be considered a planet if that object is in hydrostatic equilibrium. The great majority of objects in the Kuiper Belt are not in hydrostatic equilibrium; therefore, they are asteroids. The same is true of all objects other than Ceres in the asteroid belt between Mars and Jupiter.

"Clearing its orbit" was never an issue associated with the definition of a planet before the IAU came up with it. It is highly problematic because none of the planets has completely cleared its orbit of asteroids. And Neptune has not cleared its orbit of Pluto.

In my astronomy class, run by an amateur astronomy club in existence since 1949, the instructor defines a planet as "a non-self luminous spheroidal object orbiting a star." That's it. Dr. Alan Stern and over 300 professional astronomers agree with that definition and have signed a petition saying they will not use the one created by the IAU.

KaiYeves
2007-Dec-23, 06:22 PM
I am rather dissapointed that pluto didn't make the cut. So what is it now? My Very Energetic Mother Just Served Us Nine! Nine what?
Many Very Early Mornings Jason Saw Ultimately Nothing.

Or, if we can count Pluto:
My! Very Educated Idiots Just Screwed Up Numerous Planetariums!

laurele
2007-Dec-23, 10:06 PM
Many Very Educated Morons Just Screwed Up Nine Planets (I did not make this one up).